Differential Hydration of α,ω-Aminocarboxylic Acids in D2O and H2O

Abstract

We report the relative molar sound velocity increments, [U], partial molar volumes, V°, expansibilities, E°, and adiabatic compressibilities, K°S, for a homologous series of eight α,ω-aminocarboxylic acids in D2O solution within the temperature range of 18−55 °C. We use the resulting data to estimate the volume, expansibility, and adiabatic compressibility contributions of the component aliphatic (methylene groups) and charged (oppositely charged amino and carboxyl termini) chemical groups. We compare these group contributions with similar group contributions for the same set of α,ω-aminocarboxylic acids in H2O (Chalikian, T. V.; Sarvazyan, A. P.; Breslauer, K. J. J. Phys. Chem. 1993, 97, 13017−13026). We use these data to characterize quantitatively the differential hydration properties of charged and hydrophobic groups in D2O and H2O. Taken together, our results suggest that the hydration properties of hydrophobic and charged groups in D2O, as reflected in their volume, expansibility, and compressibility contributions, are measurably distinct from those in H2O. Significantly, these volumetric characteristics of the solute hydration differ not only in their absolute values but also in their temperature dependences. Such characteristics should prove useful in developing a better understanding of the role of differential D2O/H2O hydration in modulating thermal and thermodynamic stability of proteins. In addition, these results represent a further step in building up an empirical database of differential volumetric parameters of protein functional groups in D2O and H2O. Such a database is required for developing a methodology in which differential volumetric measurements in D2O and H2O can be employed to gain insight into the amount and chemical nature of solvent-exposed protein groups in the absence of structural information.