Charge Regulation of Natural Amino Acids in Aqueous Solutions

Abstract

The aim of this work is to develop a predictive model to describe the electrostatic behavior of 20 natural amino acids under diverse solution conditions. A coarse-grained model is proposed to account for the key ingredients of thermodynamic nonideality arising from the interaction of amino acids with various solvated ions in an aqueous solution including the molecular volume excluded effects, solvent-mediated electrostatic interactions, van der Waals attraction, and hydrophobic and hydration forces. With a small set of parameters characterizing the intermolecular interactions and dissociation equilibrium, the thermodynamic model is able to correlate extensive experimental data for the activity coefficients and solubility of amino acids in pure water and in aqueous sodium chloride solutions. Moreover, it predicts apparent equilibrium constants for the charge regulation of all amino acids in excellent agreement with experimental data. Importantly, the thermodynamic model allows for the extrapolation of the intrinsic equilibrium constants for amino-acids conversion among different charge states (viz., negative, neutral, and positive charges) thereby enabling the prediction of the charge behavior for all natural amino acids under arbitrary solution conditions.

Molecular thermodynamics for amino‐acid adsorption at inorganic surfaces

“…Toward that end, we employ an augmented primitive model (APM) that explicitly accounts for the different charged states of natural amino acids. 21 Due to the variation of hydration structure in response to the deprotonation or protonation of ionizable sites, each amino-acid molecule has a unique hard-sphere diameter σ i and valence Z i in different charged states.…”

“…We demonstrated in our previous work that the augmented model is able to reproduce both the titration behavior and the thermodynamic properties of amino-acid solutions in good agreement with experimental observations. 21 The coarse-grained model can be similarly applied to the adsorption of amino acids at an inorganic surface. In this case, the surface is represented by a hard wall with the charge density determined from the dissociation equilibrium of ionizable sites and ionic distributions self-consistently.…”

“…30 These values were obtained in our previous work based off correlations with experimental data for all natural amino acids in bulk NaCl solutions. 21 The activity coefficients account for the effect of solvent-mediated interactions among all chemical species in the solution.…”

“…In our previous work, we developed a coarse-grained model for aqueous solutions of amino acids that appropriately takes into account bulk interactions. 21 The thermodynamic model provides a faithful description of the charge behavior of amino acids at different solution conditions. In addition, we proposed a coarse-grained model to describe the charge regulation of various ionizable surfaces by an explicit consideration of the inhomogeneous ion distributions and surface reactions.…”

A Molecular Theory of Polypeptide Adsorption at Inorganic Surfaces

“…These oligopeptides are composed of some or all of the following amino-acid residues: alanine, lysine and aspartic acid. The equilibrium constants for two ionizable residues are taken from our previous work: p K Lys = 10.95 and p K Asp = 3.97. The experimental data used for the validation of our theoretical results are from Imamura et al Unfortunately, no titration data were reported for the titanium dioxide surface used in the experimental work for oligopeptide adsorption.…”