THE AMIDE-I MODE OF PEPTIDES IN AQUEOUS-SOLUTION INVOLVES VIBRATIONALCOUPLING BETWEEN THE PEPTIDE GROUP AND WATER-MOLECULES OF THE HYDRATION SHELL

We have measured polarized Raman spectra of the model peptides glycylg lycine and N-acetylglycine in aqueous solution with different excitati on wavelengths. The spectra in the region between 1500 and 1800 cm(-1) were consistently analyzed by using the same band shapes, halfwidths, and frequencies to fit the profiles of corresponding Raman bands. The quality and the statistical significance of the fits were judged by t heir residuals and reduced chi(2) numbers. This strategy enables us to obtain reliable spectral parameters of even strongly overlapping band s. Our experimental results show that the amide I bands of glycylglyci ne and N-acetylglycine in H2O are composed of two subbands, whereas th e corresponding amide I' band of glycylglycine in D2O can be fitted by one single band. Moreover, we found that the amide I band region in t he Raman spectra of glycylglycine in different H2O/D2O mixtures (i,e., 25%/75%, 50%/50%, and 75%/25%) significantly deviate from the weighte d superposition of the corresponding spectra of glycylglycine in pure H2O and D2O. These results are rationalized by invoking vibrational co upling between the amide I mode and the bending modes of the surroundi ng water molecules which provide a continuum of vibrational states. Th is coupling is absent in D2O because deuteration causes a downshift of the water's bending mode. In H2O/D2O mixtures the undeuterated specie s exhibits a reduced splitting of its amide I band due to the lower de nsity of H2O molecules. Hence our results show that peptides and their aqueous environment form a dynamic entity. For glycylglycine the anal ysis of amide II also reveals a splitting into two subbands which most likely results from two different conformers with respect to the orie ntation of the carboxyl group.