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 t he 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 h ydrogen bonding interactions is emphasized. Experimental evidence is p rovided for cooperativity in hydrogen bonding of the bound molecules o f water at the amide oxygen due to increased dielectric constant of th e medium and further solvation with molecules of water. It is demonstr ated that O-17 shielding constants can contribute valuable information on specific hydration of the peptide oxygen. Very good linear correla tion between delta(O-17) and v(CO) (the amide I stretching vibrational frequency) was found for different solvents which have varying dielec tric constants and solvation abilities. For solvents in which two IR b ands were observed the weighted mean of these bands was used. The rela tive advantages and disadvantages of both methods in studying and quan titating specific hydration at the amide and peptide oxygen are critic ally evaluated.