Physicochemical properties of L-α-amino acids

The acid–base properties of L-α-amino acids in aqueous media are governed by their amphoteric nature and the specific contributions of the amino acid residues (R-groups). Free amino acids are defined by the presence of a minimum of two ionizable groups, each possessing a distinct pK _a value (pK _a = − log K _a). The α-carboxyl group, with a pK _a value ranging from approximately 2.0 to 2.5, and the α-amino group, with a pK _a value ranging from approximately 9.0 to 10.0, are characteristic of this class of compounds. Inductive and resonance effects within the residues significantly modulate these constants; for instance, the side-chain pK _a of aspartic acid is 3.70, whereas the guanidinium moiety of the arginine residue maintains a pK _a exceeding 12.00. The charge of an amino acid is subject to variation in accordance with the pH value of the environment. At the isoelectric point (pI), the amino acid becomes neutral, whereas below the pI, it is acidic, and above the pI, it is basic. This property plays a crucial role in reactivity and biochemical interactions (Pinho et al., Reference Pinho1994; Ninni and Meirelles, Reference Ninni and Meirelles2001; Zhang et al., Reference Zhang2024).

The hydration shell architecture of amino acids is a direct consequence of the interplay between hydrophilic hydration (electrorestriction around the zwitterionic termini) and hydrophobic hydration (surrounding the R-group). In the domain of coordination chemistry, the solvation shell properties are a pivotal factor in determining binding affinity. The process of forming a coordination bond is accompanied by the partial desolvation of the ligand. The process is governed by the release of ordered water molecules into the bulk phase, a phenomenon driven by entropy. It has been demonstrated that this entropic gain frequently compensates for the endothermic nature of desolvation, thereby stabilizing the resulting complex species (Marcus, Reference Marcus2010).

The solubility of amino acids is a highly variable parameter, resulting from the nature of the side chain (R) and the energy of the crystal lattice. At a temperature of 25 °C, these values range from a few grams to several hundred grams per liter of water (from 0.045 g/100 g H₂O for tyrosine to 162 g/100 g H₂O for proline). Small aliphatic amino acids, for example, glycine and alanine, exhibit high aqueous solubility. Conversely, those possessing hydrophobic moieties, such as the aromatic amino acids tryptophan and phenylalanine, demonstrate significantly reduced solubility profiles. Proline is distinguished by its exceptional solubility, which is notably high at room temperature. This property is indicative of its classification as one of the most hydrophilic amino acids. It is imperative to emphasize that the solubility of amino acids is contingent on pH. As the isoelectric point is approached, the positive and negative charges within a molecule are balanced, and amino acids characteristically attain a minimum level of solubility. However, as the pH shifts toward an acidic or basic state, there is an increase in solubility. These tendencies are central to processes such as protein folding, where hydrophobic side chains aggregate in the protein core, while polar residues are exposed to the aqueous environment. Ionizable side chains, such as those found in aspartic acid, glutamic acid, lysine, and arginine, have been shown to directly influence acid–base catalysis and salt bridge formation (Tseng et al., Reference Tseng2009; Do et al., Reference Do2020).

The energetics of amino acid solvation involve enthalpic (ΔH) and entropic (ΔS) contributions that vary with amino acid type and local environment. Charged and polar amino acids characteristically manifest substantial negative hydration free energies, a consequence of their pronounced ion-dipole interactions. For instance, arginine and lysine, with their positively charged side chains, exhibit strong binding with water molecules, contributing to protein solubility and stability. Conversely, hydrophobic amino acids exhibit less favorable interaction energetics, and their propensity to aggregate in aqueous solutions serves as a driving force in protein folding. The field of solvation energetics, as defined by substantial negative hydration enthalpies (e.g., approximately −40 kJ/mol for glycine), is pivotal to protein conformational stability and the rational design of amino acid–based therapeutic agents (Chang et al., Reference Chang2007; Hajari and van der Vegt, Reference Hajari and van der Vegt2015; Dragan et al., Reference Dragan2017; Hossain et al., Reference Hossain2017).

The physicochemical properties of amino acid solutions are central to a wide range of experimental and industrial applications. Techniques such as electrophoresis, chromatography, and crystallization rely on differences in charge, solubility, and interaction potential of amino acids under controlled aqueous conditions. Moreover, a comprehensive understanding of amino acids’ behavior in water is imperative for the field of drug design, wherein modified amino acids are employed to enhance the solubility, stability, and targeting properties of therapeutic compounds (Tripathy et al., Reference Tripathy2018).

Environmental factors affecting L-α-amino acids

The behavior of amino acids in an aqueous solution is governed by a combination of physicochemical properties and external environmental factors. These include pH, temperature T, ionic strength I, the presence of co-solutes, and pressure (Zamyatnin, Reference Zamyatnin1984; Privalov, Reference Privalov1990).

One of the most critical environmental factors is pH, which directly impacts the ionization state of amino acids. Amino acids behave like acids and bases because they have many ionizable groups (Zhou et al., Reference Zhou2018). In dilute aqueous solutions, that is to say, in solutions in which water is present in large excess, the acid–base equilibrium of these groups can be described by dissociation constants (K _a). K _a is defined as the process of proton dissociation in successive steps.

For the fully protonated form $ {\mathrm{NH}}_3^{+}-\mathrm{CH}\left(\mathrm{R}\right)-\mathrm{COOH} $ , two alternative, equilibrium processes are possible:

dissociation of the carboxyl group:

$$ {\mathrm{NH}}_3^{+}-\mathrm{CH}\left(\mathrm{R}\right)-\mathrm{COOH}\hskip0.4em \rightleftarrows \hskip0.4em {\mathrm{NH}}_3^{+}-\mathrm{CH}\left(\mathrm{R}\right)-{\mathrm{COO}}^{-} $$

dissociation of the amino group:

$$ {\mathrm{NH}}_3^{+}-\mathrm{CH}\left(\mathrm{R}\right)-\mathrm{COOH}\hskip0.4em \rightleftarrows \hskip0.4em {\mathrm{NH}}_2-\mathrm{CH}\left(\mathrm{R}\right)-\mathrm{COOH} $$

Consequently, intermediate forms can undergo further dissociation, ultimately resulting in a completely dissociated form:

$$ {\mathrm{NH}}_2-\mathrm{CH}\left(\mathrm{R}\right)-{\mathrm{COO}}^{-}. $$

As illustrated in Table 1, a comprehensive compendium of data on the dissociation constants of protein amino acids is provided. When extrapolated to the infinite dilution limit, these constants provide the most accurate fit to the experimental data. While the results were obtained from experiments conducted in NaCl solutions, it is important to note that the true thermodynamic equilibrium constants are independent of the properties of NaCl. Consequently, these results can be utilized to determine the charge of amino acids under different electrolyte conditions. The activity coefficients demonstrated a correlation with a range of theoretical models (Gallegos and Wu, Reference Gallegos and Wu2020).

Table 1.

The dissociation constants of L-α-amino acids at room temperature when extrapolated to the infinite dilution limit (Gallegos and Wu, Reference Gallegos and Wu2020)

Diagrams illustrating the variation in the concentrations of individual amino acid species with respect to solution pH are designated ‘speciation diagrams’ (Figure 2).

Figure 2.

Graphical simulations of speciation diagrams for L-α-amino acids (alanine, lysine, aspartic acid, and histidine) were generated using the HySS 2009 program, with the program’s development being based on experimental data (potentiometric titration, Hyperquad 2013 software, t = 25 °C, I = 0.1 M NaClO₄).

The above speciation curves represent, respectively:

• • monoamino monocarboxylic acid L-α-alanine (A⁺ – the protonated α-carboxyl and α-amino group; A^± – the deprotonated α-carboxyl group, the protonated α-amino group; A⁻ – the deprotonated α-carboxyl and α-amino group);

• • diamino monocarboxylic acid L-α-lysine (A²⁺ – the protonated α-carboxyl, α-amino and additional amino group; A⁺ – the deprotonated α-carboxyl group, the protonated α-amino and additional amino group; A^± – the deprotonated α-carboxyl and α-amino group, the protonated additional amino group; A⁻ – the deprotonated α-carboxyl, α-amino group, and additional amino group);

• • monoamino dicarboxylic acid L-α-aspartic acid (A⁺ – the protonated α-carboxyl, additional carboxyl, and α-amino group; A^± – the deprotonated α-carboxyl group, the protonated additional carboxyl and α-amino group; A⁻ – the deprotonated α-carboxyl and additional carboxyl group, the protonated α-amino group; A²⁻ – the deprotonated α-carboxyl, additional carboxyl and α-amino group);

• • amino acid with a heterocyclic ring L-α-histidine (A²⁺ – the protonated α-carboxyl, α-amino and imidazole group; A⁺ – the deprotonated α-carboxyl group, the protonated α-amino and imidazole group; A^± – the deprotonated α-carboxyl and imidazole group, the protonated α-amino group; A⁻ – the deprotonated α-carboxyl, α-amino group and imidazole group).

For amino acids with two ionizable groups, the speciation curve usually shows two sigmoidal transitions that match their pK _a values. For those with ionizable side chains, an extra inflection point shows the third pK _a. In the area around each pK _a value, adding a small amount of acid or base only causes a small change in pH. This makes amino acids good biological buffers. The diagrams also illustrate isoelectric points (pI), defined as the pH at which an amino acid has no charge. It is evident that at the pH of the maximum concentration of the neutral species A^±, the amino acid reaches its isoelectric point. It is imperative to understand the significance of these diagrams in the context of protein purification, the estimation of the proportions of protonated and deprotonated species at any given pH, and the comprehension of the mechanisms through which enzymes and buffers function (Rawlings et al., Reference Rawlings2004; Barrett and Rawlings, Reference Barrett and Rawlings2007; Zentner and Henikoff, Reference Zentner and Henikoff2013).

The R substituent exerts its influence on acid–base equilibria principally through the mechanisms of inductive and mesomeric effects. The inductive effect is characterized by the redistribution of electron density along σ bonds, whereby electronegative groups exert an attraction force on electrons, thereby stabilizing the negative charge in the conjugate base and decreasing the pK _a. Conversely, electron-donating groups have been shown to increase electron density at the dissociation center, resulting in the destabilization of the anion and an increase in the pK _a. This phenomenon is particularly pronounced for the α-carboxyl group, whose pK _a is lower than in simple carboxylic acids precisely because of the proximity of the positively charged α-amino group. The mesomeric effect is predicated on charge delocalization in conjugated systems: acceptor groups can additionally stabilize the anion, lowering the pK _a, while donor groups transfer electron density and increase the pK _a. In aromatic amino acids, such as tyrosine or histidine, the acidity of the phenolic or imidazole group is significantly influenced by the side-chain resonance. Consequently, the pK _a of amino acid functional groups exhibits significant variation: acidic side groups (Asp, Glu) have pK _a in the range of 3.5–4.5, histidine around 6, tyrosine around 10, and basic groups of lysine and arginine around 10.5 and 12, respectively. It is important to note that these values can also be subject to variation in proteins, contingent on the prevailing electrostatic environment (Warshel et al., Reference Warshel2006; Lara-Popoca et al., Reference Lara-Popoca2020; Samadov et al., Reference Samadov2025).

The reciprocal influence of cationic, anionic, and hydrophobic moieties mediated by the solvent is fundamentally a manifestation of the ‘co-sphere overlap’ phenomenon. In accordance with the principles of the McMillan–Mayer theory, the structural reorganization of the aqueous environment in the proximity of functional groups is the determining factor in the excess thermodynamic properties of the system. It is evident that cation–anion interactions are subject to significant modulation by the solvent’s dielectric response. However, at the molecular scale, the effective dielectric constant is diminished, thereby enhancing localized electrostatic coupling. Furthermore, the interference between the electrostricted hydration shell of the ionic headgroups and the water surrounding hydrophobic residues leads to a nonadditive contribution to the molar Gibbs energy (Vafaei et al., Reference Vafaei2014).

Temperature also plays a key role in influencing the chemical 3behavior of amino acids in solution. As temperature increases, the kinetic energy of molecules rises, which can disrupt hydrogen bonding and solvation shells around amino acids. This may enhance solubility for some amino acids or promote aggregation for others, depending on their hydrophobicity and structural features. Moreover, temperature affects the rates of acid–base reactions and conformational flexibility, which are especially relevant in enzymatic and protein folding processes. It is evident that the temperature exerts a substantial influence on the solubility of glycine, a compound that is frequently employed in the domain of pharmaceutical crystallization and injectable preparations. Conversely, the temperature-induced aggregation of hydrophobic amino acids, such as leucine, has the potential to compromise the stability of therapeutic proteins. These effects are also critical in processes such as heat-induced denaturation in vaccine storage and enzyme inactivation in industrial biocatalysis (Record et al., Reference Record1978; Matthew, Reference Matthew1985; Nagai et al., Reference Nagai2008; Chua et al., Reference Chua2018).

Another important factor is ionic strength, which refers to the concentration of ions in solution. In order to address the requirement for a quantitative description of the effects of ionic strength, the Davies equation (for I ≤ 0.5 M) is utilized:

$$ \mathrm{p}{K}^0=\mathrm{p}{K}_c+A\Delta {Z}^2\left(\frac{\sqrt{I}}{1+\sqrt{I}}-0.3\cdot I\right) $$

where K ⁰ is the thermodynamic dissociation constant (pK ⁰ = − log K ⁰), K _c is the apparent dissociation constant (pK _c = − log K _c), A is the constant dependent on temperature and the dielectric permeability of the solvent, ΔZ ² is the difference of charge squares of the reaction products and the starting reagents, and I is the ionic strength of the solution.

The high ionic strength of the solution has been demonstrated to induce shielding of electrostatic interactions between charged amino acid groups, thereby reducing both repulsion and attraction between molecules (Asp pK _a ~ 3.70 and Arg pK _a > 12.00). This can lead to changes in aggregation behavior, solubility, and buffering capacity. In biological systems, physiological ionic strength is tightly regulated to maintain protein stability and function. In experimental contexts, salts such as NaCl are often used to manipulate ionic strength and study its effect on amino acid and protein behavior. The alterations in ionic strength exert a substantial influence on the solubility of arginine. An increase in ionic strength within the range of approximately 0.5–1.0 M has the effect of increasing arginine solubility. This phenomenon can be attributed to the stabilization of its strong molecular dipole, which is characterized by high charge density (the guanidine group). This is of crucial importance in the formulation of liquid drug forms. In a similar manner, the stability of histidine-tagged proteins in purification processes that employ immobilized metal affinity chromatography (IMAC) has been demonstrated to be modulated by ionic strength (Arakawa and Tsumoto, Reference Arakawa and Tsumoto2003; Willey, Reference Willey2004; Chan and Warwicker, Reference Chan and Warwicker2009).

The presence of co-solutes – such as ethanol, urea, glycerol, or polyethylene glycol – can significantly modulate amino acid behavior by altering solvent polarity, disrupting hydrogen bonds, or stabilizing specific conformations. For instance, urea is recognized for its ability to disrupt hydrogen bonding networks. This disruption can occur through its interference with the hydration layers of amino acids, thereby affecting their structure and function. Conversely, glycerol is frequently utilized as a stabilizing agent and to counteract aggregation (Ball, Reference Ball2017). Macromolecular crowding, a phenomenon in which large biomolecules occupy a substantial proportion of the available cellular volume, has been shown to restrict the available space and alter the thermodynamic properties of solutes. This can enhance hydrophobic interactions and modify pK _a values (Zhou et al., Reference Zhou2008). Co-solutes, such as urea derivatives or trimethylamine N-oxide (TMAO), have been demonstrated to perturb or stabilize protein structures by altering solvation dynamics and hydration layers around amino acids (Yancey et al., Reference Yancey1982). The capacity of salts to stabilize or destabilize amino acid structures is contingent upon their position in the Hofmeister series. Kosmotropic salts (e.g., sulfate, phosphate) facilitate the structuring of water and the stabilization of amino acids. In contrast, chaotropic salts (e.g., perchlorate, thiocyanate) have been demonstrated to disrupt water structure and the destabilization of amino acids (Zhang and Cremer, Reference Zhang and Cremer2006).

Pressure is an environmental variable that has received comparatively less attention in research, yet it is of particular relevance in the context of high-pressure biochemical applications. Increased pressure can affect the volume and compressibility of amino acid solutions, shift equilibria between conformational states, and influence the strength of noncovalent interactions. Certain amino acid side chains respond differently to pressure depending on their hydrophobicity and ability to pack efficiently in different molecular environments. It has been demonstrated that hydrophobic amino acids, such as leucine and isoleucine, play a substantial role in pressure sensitivity within protein cores. High-pressure treatment is also employed in the food and pharmaceutical industries for the inactivation of enzymes and pathogens while preserving the structural integrity of proteins. The amino acid composition of proteins is a determining factor in their structural resilience. In addition, pressure-stable amino acid analogues are being investigated for their potential in drug stabilization (Silva and Weber, Reference Silva and Weber1993; Heremans and Smeller, Reference Heremans and Smeller1998).

The influence of L-α-amino acids chemistry on biological processes

The behavior of amino acids in aqueous solution has profound biological consequences, as water is the primary medium in which virtually all biochemical processes occur. The physicochemical properties of amino acid residues in proteins, including their solubility, ionization, and capacity for hydrogen bonding, are pivotal to their biological functions. The following functions must be considered: firstly, enzymatic activity; secondly, protein structure and dynamics; thirdly, metabolic regulation; and finally, cellular homeostasis (Sorokina et al., Reference Sorokina2022).

It is imperative to acknowledge the significance of amino acids in the context of protein folding, a consequence that is intrinsically linked to their behavior in water. In aqueous environments, amino acids arrange themselves according to their polarity: hydrophilic residues tend to orient toward the solvent, while hydrophobic side chains are buried within the protein core. The distribution of the substance under investigation is driven by the hydrophobic effect and modulated by electrostatic interactions and hydrogen bonding. The correct folding of a protein is imperative for its function; misfolded proteins frequently lose activity and may aggregate, potentially leading to diseases such as Alzheimer’s or Parkinson’s (Dobson, Reference Dobson2002).

The ionization states of amino acids, which vary with pH, directly influence protein charge, solubility, and interactions. For instance, in the context of enzymes, the accurate dissociation constants of catalytic residues are paramount for optimal activity. The histidine is a key component of catalytic triads in many hydrolase enzymes due to its ability to act as both a proton donor and an acceptor. Glutamic acid fulfils a comparable function in aspartic proteases, facilitating the hydrolysis of peptide bonds in pharmaceutical enzymes, including pepsin and HIV protease inhibitors. In contrast, tryptophan is a biochemical precursor to serotonin, a neurotransmitter that plays a pivotal role in mood regulation, thereby underscoring the pharmacological significance of amino acid derivatives. The serine, cysteine, and glutamate residues frequently function as nucleophiles, while lysine and arginine have been observed to stabilize transition states or negatively charged intermediates (Polgár, Reference Polgár2005). The functionality of these roles is contingent upon the ionization state of the residues, which is, in turn, influenced by the local environment and the overall solution pH (Warshel et al., Reference Warshel2006).

In the context of biological membranes and transport processes, free amino acid solubility and charge have been demonstrated to influence the movement of ions across lipid bilayers, as well as their recognition by transport proteins. The requirement for facilitated diffusion or active transport via specialized proteins is a feature of charged or polar amino acids. The aqueous compatibility of amino acids and the stability of their ionization states under cellular conditions are prerequisites for this process (Chan et al., Reference Chan2006; Shin et al., Reference Shin2009). In the domain of clinical pharmacology, transport systems that recognize neutral amino acids, such as the LAT1 transporter, are utilized to facilitate the delivery of drugs across the blood–brain barrier. For instance, L-DOPA (a product of tyrosine metabolism) is a precursor of dopamine and is actively transported via this system for the treatment of Parkinson’s disease (Pardridge, Reference Pardridge2007; Nagatsu and Sawada, Reference Nagatsu and Sawada2009).

In addition, amino acid residues have been demonstrated to play a pivotal role in the regulation of cellular pH. It is evident that, due to their buffering capacity, they play a pivotal role in maintaining optimal pH levels within and outside of cells by effectively accepting or donating protons. The zwitterionic nature of amino acids is such that they are able to resist drastic changes in pH, a property that is critical for the stability of many metabolic pathways. This buffering capacity is of particular significance in tissues exhibiting fluctuating metabolic activity, such as muscle or liver. Carnosine, a dipeptide composed of β-alanine and histidine, has been demonstrated to possess significant buffering capacity within skeletal muscle tissue. The present study (Boldyrev et al., Reference Boldyrev2010) investigates the potential of the substance under consideration as a supplement to delay the onset of muscle fatigue and acidosis in athletes, as well as to mitigate age-related muscle decline.

Within the domain of metabolism, the behavior of amino acids in water has been demonstrated to exert a significant influence on their conversion to other biomolecules. For instance, the occurrence of transamination and deamination reactions is contingent upon the aqueous solubility and protonation of amino groups. L-glutamine is extensively utilized in cell culture media due to its function as a nitrogen donor in the biosynthesis of nucleotide and amino sugars. In a similar manner, L-cysteine functions as a precursor for glutathione, a vital antioxidant that plays a crucial role in maintaining redox homeostasis within cells (Lu, Reference Lu2009). Furthermore, the process of urea formation, a significant nitrogenous waste product in humans, necessitates the presence of soluble intermediates derived from amino acids, underscoring the significance of water-compatible chemical behavior in detoxification pathways (Casey et al., Reference Casey2010). Nitric oxide, synthesized from arginine in an enzymatic reaction, and serotonin, which is derived from tryptophan, have been identified as key players in both cell signaling and immune responses. The solubility, reactivity, and transport of these precursors are governed by their behavior in the aqueous phase, which in turn influences the kinetics and specificity of signaling pathways (Wu and Morris, Reference Wu and Morris1998).

Branched-chain amino acids, including leucine, isoleucine, and valine, have found extensive application in the domain of clinical nutrition, particularly among patients afflicted with liver disease or experiencing muscle wasting. The metabolic roles of these substances, as well as their sensitivity to pH and ionic strength, have a significant impact on their therapeutic administration and formulation. Another derivative, S-adenosylmethionine (SAM), synthesized from methionine, serves as a major methyl group donor in numerous methylation reactions, including those involved in the regulation of deoxyribonucleic acid (DNA) and neurotransmitters. The utilization of SAM in the therapeutic management of depression and the treatment of liver disease has been well-documented (Lu and Mato, Reference Lu and Mato2012).

Finally, in biotechnological and therapeutic applications, it is imperative to understand amino acid behavior in water for the formulation of drugs, the engineering of proteins, and the design of peptide drugs. It is imperative to meticulously consider solubility, aggregation tendencies, and pH sensitivity to ensure stability and efficacy. For instance, monoclonal antibody formulations frequently necessitate the use of buffers and excipients that facilitate amino acid–based interactions, thereby preventing precipitation or degradation (Manning et al., Reference Manning1989).

Experimental and computational methods to determine the acid–base properties of L-α-amino acids

Potentiometric titration is widely regarded as the gold standard for determining the protonation constants (pK _a) of free amino acids and their coordination complexes. However, contemporary advancements in this domain extend beyond the realm of pH measurement precision, primarily attributable to the progression of data processing algorithms. Conventional identification of inflection points on titration curves has been superseded by nonlinear least-squares refinement, implemented in specialized software such as HYPERQUAD, SUPERQUAD, or BEST. The programs under discussion facilitate the simultaneous optimization of stability and protonation constants. Furthermore, the ionic strength corrections are incorporated through the utilization of the Davies equation (Jozanović et al., Reference Jozanović2021).

Nuclear magnetic resonance (NMR) spectroscopy provides atomic-level insights that complement potentiometric data. Chemical shifts (¹H and ¹³C) exhibit a high degree of sensitivity to the protonation state of amino acid residues within proteins. The implementation of sophisticated two-dimensional techniques (2D NMR), including HSQC and NOESY, facilitates the delineation of intra- and intermolecular interactions. This encompasses the overlap of hydration shells in polarized media (Wüthrich, Reference Wüthrich1986; Kovermann et al., Reference Kovermann2016).

In the domains of UV–Vis and FTIR spectroscopy, advancement is evident in the integration of chemometric methods, such as principal component analysis (PCA) and band deconvolution. These tools allow for the precise monitoring of conformational changes and zwitterionic stability under varying pH and temperature conditions (Barth, Reference Barth2007).

Molecular dynamics (MD) simulations are a computational method that models the time-dependent interactions between amino acids and solvent molecules at the atomic level. These simulations provide valuable insights into solvation shells, conformational flexibility, hydrogen bonding, and electrostatic interactions. It has been demonstrated that MD is a particularly useful technique for predicting physicochemical properties, including diffusion coefficients, hydration energies, and folding kinetics. The correlation of vibrational data with theoretical solvation models should be facilitated, with the effects of kosmotropic and chaotropic agents being accounted for in accordance with the Hofmeister series (Karplus and McCammon, Reference Karplus and McCammon2002).

Quantum chemical methods, including Density Functional Theory (DFT), facilitate the precise calculation of molecular geometries, ionization energies, and electronic distributions. These methods are critical for accurate pK _a predictions and for understanding mechanisms of proton transfer and catalysis. Hybrid quantum mechanics/molecular mechanics (QM/MM) models have been demonstrated to be particularly efficacious in simulating enzymatic reactions and transition states involving amino acid residues (Cremer, Reference Cremer2001).

The applications and implications of free and bound of L-α-amino acids

In order to facilitate the rational design of proteins and therapeutic molecules, it is imperative to comprehend the chemical properties of amino acids in aqueous environments. Protein engineering is a process that utilizes site-directed mutagenesis and computational modeling to modify amino acid sequences. The objective of this process is to enhance the stability, activity, or specificity of enzymes and receptors. Precise knowledge of pK _a values, hydrophobicity, and steric effects of side chains guides these modifications to optimize protein folding and function. Specific mutations introducing proline residues have been shown to improve protein thermostability due to proline’s conformational rigidity. Within the domain of pharmaceutical protein engineering, such stabilizing mutations have the potential to prolong the shelf-life of drugs and enhance their efficacy (Eijsink et al., Reference Eijsink2004).

Within the domain of drug design, the pH-dependent behavior and interaction profiles of amino acids are of paramount importance in predicting binding affinities between drugs and protein targets. It has been demonstrated that minor alterations in the dissociation constants of key residues can significantly modify the charge distribution within active sites, thereby impacting both ligand binding and catalytic turnover. It is evident that histidine is known to participate in acid–base catalysis in enzyme active sites due to its imidazole side chain, which possesses a pK _a close to the physiological pH. This property can be replicated in the structures of drug molecules to regulate binding or inhibition (Garcia-Moreno, Reference Garcia-Moreno2009; Rapin et al., Reference Rapin2010). The role of glutamate in receptor-ligand binding is of particular significance in the design of glutamatergic drugs for neurological disorders such as Alzheimer’s disease and epilepsy. The development of drugs that mimic the binding behavior of glutamate has enabled the modulation of synaptic transmission (Meldrum, Reference Meldrum2000).

Furthermore, peptides and peptidomimetics designed with noncanonical amino acids are being explored as therapeutics due to their tunable solubility, stability, and target selectivity. The incorporation of nonnatural amino acids, such as α-methylated leucine or fluorinated phenylalanine, has been demonstrated to enhance metabolic stability and receptor selectivity in peptide therapeutics. These modifications have already been utilized in pharmaceuticals targeting cancer and autoimmune diseases (Gentilucci et al., Reference Gentilucci2010; Vlieghe et al., Reference Vlieghe2010).

Within industrial contexts, amino acid chemistry plays a pivotal role in optimizing biocatalysis, fermentation processes, and the formulation of protein-based products. The performance of enzymes in bioreactors is highly sensitive to environmental conditions, including pH, ionic strength, and temperature. These conditions directly influence the ionization states of amino acid residues involved in catalysis and substrate recognition (Zaks, Reference Zaks2001; Schoemaker et al., Reference Schoemaker2003).

The buffering capacity of amino acids is of considerable importance in ensuring the stability of operating conditions during large-scale bioprocesses. For instance, glutamic acid and histidine are frequently utilized in buffer systems due to their advantageous pK _a values and minimal toxicity. Furthermore, it is imperative to possess a comprehensive understanding of amino acid solubility and aggregation tendencies to avert precipitation during the critical protein purification and formulation stages (Wang, Reference Wang2005; Wang and Roberts, Reference Wang and Roberts2018). Arginine is extensively utilized in formulation buffers for monoclonal antibodies, a practice that is primarily driven by its capacity to mitigate aggregation and enhance solubility, particularly in contexts involving freeze–thaw cycles and analogous stress conditions (Ren, Reference Ren2023).

Advances in metabolic engineering exploit amino acid biosynthetic pathways to achieve increased production of valuable compounds, including lysine, glutamine, and branched-chain amino acids. In this instance, an advanced comprehension of feedback inhibition and cofactor requirements at the molecular level has resulted in enhanced yields and robustness in microbial production strains. One industrial example is Corynebacterium glutamicum, a microbial workhorse engineered to overproduce L-glutamate, which is used in the food industry as a flavor enhancer (monosodium glutamate, MSG) (Park and Lee, Reference Park and Lee2008; Becker and Wittmann, Reference Becker and Wittmann2012).

As scientific instruments become more precise, future research will increasingly focus on the dynamic interplay between amino acid chemistry and biological complexity. A promising avenue for further research is the utilization of advanced quantum chemical and molecular dynamics simulations to predict pK _a shifts and reaction mechanisms in highly heterogeneous environments, such as crowded cells or membrane interfaces (Warshel et al., Reference Warshel2006).

Another expanding area of research is the study of posttranslational modifications (PTMs) and their chemical implications. Posttranslational modifications (PTMs), which include phosphorylation, methylation, and glycosylation, alter the chemical nature of amino acids, introducing new functional groups that affect protein interactions, stability, and signaling capacity. The quantification of the pH sensitivity and solvation behavior of these modified residues remains a significant challenge (Walsh et al., Reference Walsh2005).

Furthermore, synthetic biology seeks to extend the genetic code to encompass novel amino acids with engineered side chains, thereby facilitating the production of proteins with nonnatural properties. These innovations demand a comprehensive understanding of the impact of chemical modifications on solvation, ionization, and folding behaviors (Davis and Chin, Reference Davis and Chin2012).

The coordination of selected L-α-amino acids with metal ions under variable pH conditions

The stability of complexes is described by two sets of constants: the overall stability constants (log β) and the stepwise dissociation constants (log K). A particular role is played by the chelation effect, which is defined as the increase in complex stability resulting from the presence of multidentate ligands such as amino acids. The coordination behavior of amino acids in an aqueous solution is strongly influenced by pH, which affects the dissociation state of functional groups involved in metal ions binding. Aspartic acid, serine, and histidine represent acidic, neutral, and basic amino acids, respectively, with each amino acid offering distinct donor atoms for coordination.

In aspartic acid, at low pH, the α- and β-carboxyl groups are largely protonated, which limits their ability to coordinate metal ions. However, under certain conditions (temperature, ionic strength, solvent), weak complexes can be formed through interactions of the side chains of aspartic acid-containing peptides with metal ions such as Co(II) or Zn(II). At a neutral pH level, both carboxyl groups are deprotonated, thereby enabling bidentate complexes with Ca(II), Mg(II), and Zn(II) ions with several Asp residues. The complexes in question have been demonstrated to function in the activation of enzymes, the transportation of ions, and the folding of proteins. They facilitate the maintenance of structural integrity in metalloproteins, thereby supporting catalytic activity in processes such as the citric acid cycle and neurotransmission. These complexes become more stable at elevated pH levels due to deprotonation of the amino group. However, this process can also result in metal hydroxide precipitation or the formation of polynuclear species, such as Fe^III(Asp) or Al^III(Asp), which are of relevance in toxicological contexts. Across the pH range, aspartic acid and its metal complexes fulfil essential roles in metal buffering, metabolic regulation, and coordination chemistry of biological systems. The log β for the [Cu(Asp)] complex is approximately 9, whereas the zinc(II) analogue log β is approximately 6. Potentiometric studies emphasize the pivotal role of protonated species, such as [M(HAsp)]⁺, where the protonation status of the beta-carboxyl group exerts a profound influence on the overall stability (Smith and Martell, Reference Smith and Martell1989; Pettit and Powell, Reference Pettit and Powell2006). Conversely, when incorporated into peptides or proteins, the alpha-termini are sequestered within peptide bonds, thereby shifting the coordination burden to the distal beta-carboxylate side chain. While the intrinsic affinity of a single side-chain carboxylate is relatively modest (log K approximate 2–4), the strategic spatial orientation of multiple aspartate residues within binding pockets enables high-affinity metal sequestration through cooperative chelation effects. Binary complexes of aspartic acid (L) with divalent metal ions were investigated in anionic (sodium lauryl sulfate), cationic (cetyltrimethylammonium bromide), and nonionic (Triton X-100) micellar media. The speciation models revealed the formation of MLH, ML₂, and ML₂H species for Co(II), whereas for Ni(II) and Cu(II), the ML, MLH, and ML₂ complexes were identified. In the Zn(II) systems, the presence of ML₂, ML₂H, and ML₂H₂ was confirmed. The range of the log β values was 7–9 for ML, 11–15 for MLH, 11–15 for ML₂, 15–19 for ML₂H, and 24–25 for ML₂H₂. The magnitude of these constants exhibited a significant dependence on the concentration and micellar nature of the respective surfactants (Schmidbaur et al., Reference Schmidbaur1990; Dudev and Lim, Reference Dudev and Lim2001; Samba Siva Rao et al., Reference Samba Siva Rao2009).

Serine plays a pivotal role in a number of significant biological processes, including metabolism, neurotransmission, and biosynthesis of proteins and nucleotides. This phenomenon can be attributed, at least in part, to the presence of a polar side chain, which has the capacity to engage in hydrogen bonding and metal coordination. At low pH, protonation of the majority of donor groups, including the amino and hydroxyl groups, limits metal binding. Nevertheless, weak interactions with soft metal ions, such as Zn(II) or Cu(II), may still occur via the partially deprotonated carboxylate group. At a neutral pH level, serine is known to predominate in its zwitterionic form, characterized by a deprotonated carboxylate group and a protonated amino group. The carboxylate group thus becomes the primary site for monodentate or bidentate coordination to Cu(II), Zn(II), and Co(II) ions, which are involved in metalloenzyme activity, redox regulation, and protein-metal interactions. The hydroxyl group on the side chain has been demonstrated to enhance the stability of such complexes through additional coordination or hydrogen bonding. At elevated pH levels, the process of deprotonation of the hydroxyl group results in an enhancement of the ligand’s capacity for coordination. This process gives rise to the formation of more stable or polynuclear Ni(III) and Fe(III) complexes, which are of relevance in both biological and environmental systems. Across the pH range, serine’s capacity to coordinate metal ions – though weaker compared to acidic amino acids – contributes to metal buffering, transport, and enzyme modulation, particularly in systems where mild coordination is biologically advantageous (Dudev and Lim, Reference Dudev and Lim2008; Mandal et al., Reference Mandal2015). For Cu(II) complexes, the log β ₁₁₀ approximates 8 and log β ₁₂₀ approximates 14, whereas Ni(II) and Zn(II) log β ₁₁₀ approximate 5.5 and 5.0, respectively (Smith and Martell, Reference Smith and Martell1989; Pettit and Powell, Reference Pettit and Powell2006). The Ser side chain is of critical importance as an auxiliary donor for Ca(II) and Mg(II) ions in metalloenzymes and transport proteins, such as albumin. In such cases, it has been demonstrated that the compound contributes to the precise structural orientation of the metal center through cooperative effects. The Fe(III) and Ser form the Fe(HSer)³⁺, Fe(Ser)²⁺, Fe(OH)Ser⁺, Fe(OH)²⁻ Ser, Fe(OH)Ser₂, and Fe₂(OH)₂(Ser)₂²⁺ complexes, over a broad concentration range of serine to iron ([Ser]/[Fe] = 5:1 to 500:1), from pH 1.5 to 4.0 (Djurdjevic and Jelic, Reference Djurdjevic and Jelic1997).

In histidine, its biological importance and metal-binding properties undergoing substantial variation across the pH spectrum can be attributed to the presence of the imidazole side chain, which functions as a potent and multifaceted ligand. At low pH, the imidazole nitrogen N1 is predominantly protonated, thereby reducing its metal-binding capacity. However, it has been observed that metal ions, including Zn(II), Cu(II), and Co(II), tend to form weak or only partially protonated complexes. This phenomenon has been observed in species such as Cu(HisH), where HisH represents a protonated histidine. It is evident that at a neutral pH level, the imidazole group undergoes deprotonation, thus becoming a potent ligand for transition metal ions. This process facilitates the formation of stable Zn(II) and Ni(II) complexes, which are commonly observed in metalloenzymes, transport proteins, and the catalytic centers of biological molecules. These complexes contribute to a variety of processes, including oxidative phosphorylation, antioxidant defense, and metal ion homeostasis. At elevated pH levels, there is a possibility for additional deprotonation of the amino group, or indeed, the imidazole ring. This mechanism has the capacity to augment coordination flexibility and gives rise to the formation of polynuclear Co(III) and Fe(III) complexes. The functionality of these complexes has been demonstrated in the regulation of redox reactions and in the design of artificial metalloenzymes. Histidine’s capacity to bind both soft and hard metal ions over a broad pH range renders it one of the most biologically significant amino acids in the domain of metal coordination chemistry. In the presence of water, the resulting chelates demonstrate elevated stability, exhibiting log β ₁₁₀ values of approximately 10 for Cu(II), 8 for Ni(II), and 7 for Zn(II). At lower pH levels, the formation of protonated species [M(HHis)]²⁺ is favored, with a β ₁₁₁ value of approximately 14 for Cu(II) (Smith and Martell, Reference Smith and Martell1989; Pettit and Powell, Reference Pettit and Powell2006). In proteomic contexts, the imidazole side chain remains the preferred anchor for transition metals, forming the catalytic core of enzymes such as carbonic anhydrase or the structural scaffold of zinc fingers (Jakab et al., Reference Jakab2007; Jancsó et al., Reference Jancsó2011). Furthermore, complexes have been observed between His and heterocyclic ligands (uracil, nicotinic acid) in pure water (pH 5.4) and in aqueous buffer solutions (pH 7.4), exhibiting stoichiometry of 1:1 and 1:2, respectively. It was determined that the presence of a buffer solution fosters more favorable conditions for the occurrence of complex formation, as evidenced by the higher values of log K at pH 7.4 in comparison to those at pH 5.4 (Tyunina et al., Reference Tyunina2019).