UV resonance Raman investigation of the aqueous solvation dependence of primary amide vibrations

We investigated the normal mode composition and the aqueous solvation dependence of the primary amide vibrations of propanamide. Infrared, normal Raman, and UV resonance Raman (UVRR) spectroscopy were applied in conjunction with density functional theory (DFT) to assign the vibrations of crystalline propanamide. We examined the aqueous solvation dependence of the primary amide UVRR bands by measuring spectra in different acetonitrile/water mixtures. as previously observed in the UVRR spectra of N-methylacetamide, all of the resonance enhanced primary amide bands, except for the amide I (amI), show increased UVRR cross sections as the solvent becomes water-rich. These spectral trends are rationalized by a model wherein the hydrogen bonding and the high dielectric constant of water stabilizes the ground state dipolar ^-O-C=NH₂⁺ resonance structure over the neutral O=C-NH₂ resonance structure. Thus, vibrations with large C-N stretching show increased UVRR cross sections because the C-N displacement between the electronic ground and excited state increases along the C-N bond. In contrast, vibrations dominated by C=O stretching, such as the amI, show a decreased displacement between the electronic ground and excited state, which result in a decreased UVRR cross section upon aqueous solvation. The UVRR primary amide vibrations can be used as sensitive spectroscopic markers to study the local dielectric constant and hydrogen bonding environments of the primary amide side chains of glutamine (Gln) and asparagine (asn).