UV Resonance Raman Investigation
of the Aqueous Solvation
Dependence of Primary Amide Vibrations
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Abstract
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 <i>N</i>-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 <i>ground
state</i> dipolar <sup>–</sup>OCNH<sub>2</sub><sup>+</sup> resonance structure over the neutral OCNH<sub>2</sub> resonance structure. Thus, vibrations with large CN
stretching show increased UVRR cross sections because the CN
displacement between the electronic ground and excited state increases
along the CN bond. In contrast, vibrations dominated by CO
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)