Preresonance Raman Single-Crystal Measurements of Electronic Transition Moment Orientations in N-Acetylglycinamide

Abstract

We have examined electronic coupling between the two amide electronic transitions in a dipeptide and have found strong excitonic interactions in a case where the amide planes are almost perpendicular. We compared the absorption and resonance Raman spectra of N-methylacetamide (NMA) and acetamide (AM) to that of the dipeptide N-acetylglycinamide (NAGA), which is composed of linked primary and secondary amides. We measured the transition moment magnitudes of each of these species and also determined the orientation of the preresonance Raman tensor of NAGA in a single crystal. From these single-crystal tensor values, we calculated the NAGA diagonal Raman tensor orientations and compared them to those expected for unperturbed primary and secondary amides oriented as in the NAGA crystal. Because the primary and secondary amide III vibrations are vibrationally uncoupled and nonoverlapping, we can use their intensities to determine the contributions to their resonance enhancement from the coupled NAGA electronic...