Transfer energetics of a series of homologous α-amino acids and hence of CH2 group—a possible probe for the solvent effect on hydrophobic hydration and the three-dimensional-structuredness of aqueous cosolvents

Abstract Standard transfer Gibbs energies, Δ G t 0 and entropies, Δ S t 0 of a series of homologous α-amino acids (i) like glycine (gly), alanine (ala), α-amino butyric acid (aba) and nor-valine (n-val) from water to aqueous mixtures of three-dimensional-structure making (SM) 2-propanol (2-PrOH) and three-dimensional-structure breaking (SB) acetonitrile (ACN) have been determined from solubility measurements at five equidistant temperatures ranging from 15 to 35 °C by ‘formol titrimetry’. The chemical contributions Δ P t, ch 0 i as obtained by subtracting the cavity effect. Δ P t, cav 0 i and dipole–dipole interaction effect Δ P t, dd 0 i ( P = G or S ), reflect that while Δ G t, ch 0 i are dictated by decreased hydrophobic hydration (H b H) and the superimposed increased basicity and dispersion effects. T Δ S t, ch 0 i are guided by superimposed structural interaction effect as well. These values also helped to compute the transfer energetics of the smallest hydrophobic moiety (CH 2 ), Δ P t ,ch 0 (CH 2 ), by subtracting the values of lower homologues from those of immediately higher homologues and also those of Δ P t ,H b H 0 (CH 2 ) by subtracting the dispersion interaction effect of the (CH 2 ) group, Δ P t ,disp 0 (CH 2 ), from the corresponding Δ P t ,ch 0 (CH 2 ). These values are found to be additive in nature and are nearly equal to those obtained from the homologous series of RCOO − and RNH 3 + salts studied earlier. While Δ G t ,H b H 0 (CH 2 ) are found to increase with cosolvent composition indicating the effect of decreased H b H, Δ S t ,H b H 0 (CH 2 ) are found to pass through a characteristic maximum in the case of SM 2-PrOH, but a broad minimum in the case of SB ACN. The latter are indicative of the effect of three-dimensional-structuredness of solvents. This leads us to conclude that transfer energetics of the CH 2  group are not only an unambiguous reflector of the solvent effect of H b H, but also the best probe for the three-dimensional-structuredness of aquo-ionic and aquo-non-ionic cosolvents without being obscured by the effect of ‘buffer bonds’, as in the cases of HBz, p NA and R 4 N + ions studied earlier.