Infrared and near-infrared study of the interaction of amide C=O with water in ideally inert medium.

Hydration property of amide C=O was investigated from the OH stretching fundamentals and the first combinations of the water in the hydrate, formed in hydrophobic solution of an N,N-dialkyl amide ((CH(3)CH(2))(2)NCO(CH(2))(10)CH(3)) in heptane. The property was also examined for ester C=O of an alkyl ester (CH(3)(CH(2))(8)COOCH(3)) for comparison. The hydrates of C=O...H-O-H (I) and C=O...H-O-H...O=C (II) occur, the latter emerging above 5% concentration only and increasing in relative population with increasing concentration. The free OH of the water in I shows a sharp stretching band (nu(1)(OH(f))) at about 3690 cm(-1) and the bonded OH shows a much broader stretching band (nu(1)(OH(b))) of a significantly lower frequency. The difference Deltanu(fb) (= nu(1)(OH(f))-nu(1)(OH(b))), which is used as an indicator of hydration ability, is much larger for amide C=O (205 cm(-1)) than for ester C=O (134 cm(-1)). Equilibrium constant k(H) for the hydration process [C=O + (H(2)O) <--> C=O...H-O-H] is larger by about ten times for amide C=O than for ester C=O. Factors Deltanu(fb) and k(H) demonstrate that amide C=O has a strong hydration ability, implying that amide C=O of peptide linkage has an important contribution to high hydration ability of proteins.