Hydrophobicity scale of amino acids as determined by absorption millimeter spectroscopy: Correlation with heat capacities of aqueous solutions

Abstract

Hydration indexes of protein α-amino acids were measured by the new method of absorption millimeter spectroscopy (AMS) at 10 mm wavelength (1.05 cm-1; 31.42 GHz). This region of electromagnetic waves, located between the region of high-frequency dielectric spectroscopy and that of far-infrared spectroscopy, allows to measure hydration on the basis of different rotational and reorientational mobilities of water in hydration shell. Contribution to hydration of the methylene group is shown to be significantly greater than that of polar OH, -CONH2 and -COOH groups, while the contribution of the charged -CH(N+H3)COO-group is even negative. The high sensitivity of AMS method to hydrophobic hydration allows to build up the hydrophobicity scale which gives an acceptable correlation (r=0.95) with the heat capacities of the aqueous solutions of amino acids. Thus, millimeter absorption spectroscopy, the method of hydration determination at a molecular level (determination of V-structure of water with lifetime ~50 ps), allows to quantitatively distinguish hydration of polar and non-polar groups as well as calorimetry does at a macroscopic level.