Probing protein hydration by the difference O-H (O-D) vibrational spectroscopy: Interfacial percolation network involving highly polarizable water-water hydrogen bonds

The difference DO-H (HO-H, DO-H) IR and NIR vibrational absorption spectroscopy has been applied to aqueous solutions of native and thermally unfolded Bovine Serum Albumin (BSA). Subsequent global Gaussian analysis of the obtained and matching published data for the BSA "dry films" " (J. Grdadolnik and Y. Marechal, Biopolymers, 62 (2001) 40-53) disclosed two kinds of intensive and very broad O-H (O-D) sub-bands, centered at 3260 and 2840 cm(-1) (for O-D, at 2350 and 2050 cm(-1), accordingly). These bands are assigned to the stretching vibrations of water molecules involved in strong protein-water and water-water hydrogen-bonding, respectively, latter probably resulting in two dimensional (2D) percolating networks with migrating protons, as suggested recently by G. Careri (Progr. Biophys. Mol. Biol. 70 (1998) 223-249). For the BSA solution sample, upon the protein denaturation at ca. 65-70 degreesC, a partial liberation of unexchangeable N-H protons has been observed accompanied also by the partial release of bound interfacial water. (C) 2003 Elsevier Science B.V. All rights reserved.