Chemical transfer energetics of the -CH2-group in aqueous glycerol: Solvent effect on hydrophobic hydration and its three-dimensional structure

摘要

Standard transfer Gibbs energies, Delta G(t)(0) and entropies Delta S-t(0) of the -CH2- group have been evaluated in aqueous glycerol (GL) by determining the solubilities of a series of homologous alkylcarboxylates (RCOO-), mono-alkylammonium ions (RNH3+) and alpha-amino acids at five equidistant temperatures ranging from 15 degrees C to 35 degrees C by spectrophotometry and 'formol' titrimetry. The chemical contributions Delta G(t),(0)(ch)(i) as obtained by subtracting the cavity effect, Born type and ion-dipole type electrostatic effects in case of RCOO- and RNH3+ ions and cavity and dipole-dipole interaction effects for a-amino acids, are indicative of the superimposed effect of increased acidity, increased dispersion and decreased hydrophobic hydration (HbH) effect of GL-water mixtures as compared to that in water, while T Delta S-t,ch(0)(i) are guided by structural effects as well. These values also helped to compute chemical transfer energetics of the -CH2- group, Delta P-t,(0)(ch)(-CH2-) [P=G or S] by subtracting the values of lower homologues from those of the immediately higher homologues and also those of the HbH effect Delta P-t,HbH(0)(-CH2-) by subtracting the dispersion interaction effect of the (-CH2-) group from the corresponding Delta P-t,(0)(ch)(-CH2-). These values are found to be more or less equal for all probes indicating their additivity. While Delta G(t,HbH)(0)(-CH2-) are found to be guided by decreased HbH of GL-water mixtures, Delta S-t,HbH(0)(-CH2-) are indicative of three-dimensional (3D) structuredness of the solvents. (c) 2005 Elsevier B.V.. All rights reserved.