The Amide I Mode of Peptides in Aqueous Solution Involves Vibrational Coupling between the Peptide Group and Water Molecules of the Hydration Shell

We have measured polarized Raman spectra of the model peptides glycylglycine and N -acetylglycine in aqueous solution with different excitation 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 their residuals and reduced χ 2 numbers. This strategy enables us to obtain reliable spectral parameters of even strongly overlapping bands. Our experimental results show that the amide I bands of glycylglycine and N -acetylglycine in H 2 O are composed of two subbands, whereas the corresponding amide I‘ band of glycylglycine in D 2 O can be fitted by one single band. Moreover, we found that the amide I band region in the Raman spectra of glycylglycine in different H 2 O/D 2 O mixtures (i.e., 25%/75%, 50%/50%, and 75%/25%) significantly deviate from the weighted superposition of the corresponding spectra of glycylglycine in pure H 2 O and D 2 O. These results are rationalized by invoking vibrational coupling between the amide I mode and the bending modes of the surrounding water molecules which provide a continuum of vibrational states. This coupling is absent in D 2 O because deuteration causes a downshift of the water's bending mode. In H 2 O/D 2 O mixtures the undeuterated species exhibits a reduced splitting of its amide I band due to the lower density of H 2 O molecules. Hence our results show that peptides and their aqueous environment form a dynamic entity. For glycylglycine the analysis of amide II also reveals a splitting into two subbands which most likely results from two different conformers with respect to the orientation of the carboxyl group.