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_3CH _2)_2NCO(CH_2)_(10)CH_3) in heptane. The property was also examined for ester C=O of an alkyl ester (CH _3(CH_2)_8COOCH_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 (ν~1(OH_f)) at about 3690 cm ~(-1) and the bonded OH shows a much broader stretching band (ν~1(OH_b)) of a significantly lower frequency. The difference Δν_(fb) (= ν~1(OH_f) - ν~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_2O) ? C=O?H-O-H is larger by about ten times for amide C=O than for ester C=O. Factors Δν_(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.
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