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.

Probing volumetric properties of biomolecular systems by pressure perturbation calorimetry (PPC) – The effects of hydration, cosolvents and crowding

“…Electrostatic contraction between charged groups and adjacent water molecules' electrical dipoles cause a partial loss in the mobility of hydrating waters and result in a decrease in the partial molar volume. For charged groups, water molecules in hydration shells are more densely packed, thereby occupying less volume compared to the bulk state [17][18][19]. Although the effect of polar groups on the adjacent water molecules is qualitatively similar to that of charged groups, they have often different origin.…”

“…Interestingly, above $50°C, where the hydration contribution becomes insignificant, expansivities of polar and nonpolar residues converge to a similar and positive value of 1 ± 0.3 Â 10 À3 K À1 . The reason for the shift to more positive a-values can be attributed to the contribution to a that arises from the mutual thermal motions of the solute and solvent molecules at the solute-solvent interface, an effect denoted as thermal volume by Chalikian et al [16][17][18]. Interestingly, the magnitude of the negative expansivities for the nonpolar amino acid side chains is not proportional to their hydrophobicity, but correlates with the size of side chain [1].…”

Thermodynamics of Fractions and Its Application to the Hydration Study of the Swelling Process in Functionalized Polymer Particles

“…The contribution V I is the specific interaction volume, which represents the change in solvent volume due to the interactions of each atomic group on the particle surface with surrounding water molecules (electrostriction, hydrophobic interactions, hydrogen bonding) and makes a negative contribution to the total partial specific volume. By differentiating eq 91 with respect to pressure at constant entropy (S), the specific partial adiabatic compressibility of the particle at the limit of zero concentration can be obtained as 3,18,19,25,[27][28][29]37 where the effect of the pressure on V c was neglected and k cav and ∆k h are the cavity and hydration contributions, respectively, to k f;1 0 . As in the case of V f;1 0 , the contribution k M is positive and the contribution ∆k h is negative.…”

“…Now, differentiating eq 93 with respect to temperature gives 19 where the contributions to ∆e h are obtained assuming n h to be constant and defining e sh as the specific partial expansibility of the water in the hydration shell and e S0 as the specific partial expansibility of water in bulk. In this case, e cav is positive, but ∆e h can be positive or negative depending on the difference between e sh and e S0 .…”

“…lim (T,P,t 2 ,t 3 ,...,t h ,t h+1 ,...,t n )f(T,P,t′ 2 ,t′ 3 ,...,t′ h ,0,...,0) j (n) (T,P,t 2 ,...,t h ,t h+1 ,...,t n ) ) j (h) (T,P,t′ 2 ,t′ 3 ,...,t′ h ) (18) lim (T,P,t 2 ,t 3 ,...,t n )f(T,P,0,...,t′ i )1,0,...,0) j i;1,2,...,i-1,i+1,...,n (T,P,t 2 ,t 3 ,...,t i ,...,t n ) ) j i / (T,P) (19) lim (T,P,t 2 ,t 3 .....,t n )f(T,P,0,...,0,t′ i )1,0,...,0) j (n) (T,P,t 2 ,t 3 ,...,t i ,...,t n ) ) j i / (T, P) (20)…”