O-17 NMR CHEMICAL-SHIFTS AS A TOOL TO STUDY SPECIFIC HYDRATION SITES OF AMIDES AND PEPTIDES - CORRELATION WITH THE IR AMIDE-I STRETCHING VIBRATION

A model of specific hydration sites of amides and peptides based on O-17 NMR chemical shifts is presented. Solvation of the amide hydrogen (NH) is shown to induce a very small modification of the shielding of the amide oxygen and thus can be neglected. On the contrary, long-range dipole-dipole interactions and specific hydration at the amide oxygen are demonstrated to induce large and specific modifications of the O-17 shielding constants. The significance of wave function overlap in hydrogen bonding interactions is emphasized. Experimental evidence is provided for cooperativity in hydrogen bonding of the bound molecules of water at the amide oxygen due to increased dielectric constant of the medium and further solvation with molecules of water. It is demonstrated that O-17 shielding constants can contribute valuable information on specific hydration of the peptide oxygen. Very good linear correlation between delta(O-17) and v(CO) (the amide I stretching vibrational frequency) was found for different solvents which have varying dielectric constants and solvation abilities. For solvents in which two IR bands were observed the weighted mean of these bands was used. The relative advantages and disadvantages of both methods in studying and quantitating specific hydration at the amide and peptide oxygen are critically evaluated.