POLYPEPTIDE HYDRATION IN MIXED-SOLVENTS AT LOW-TEMPERATURES

The hydration of the nonapeptide oxytocin dissolved in a mixed solvent of 60% H2O and 40% [H-2(6)]acetone was investigated by nuclear magnetic resonance (NMR) spectroscopy. In the temperature range between 0 and -15-degrees-C the nuclear Overhauser effects (NOEs) between the individual proton resonances of the peptide and the water signal were found to change their sign. At the temperature of the sign change the residence time of the hydration water molecules on the peptide surface is about 500 ps. The temperature-dependent transition from positive to negative NOEs provides a sensitive probe for assessing differences in the residence times of the hydration water molecules bound to different atom groups of a polypeptide chain.