Dihedral psi angle dependence of the amide III vibration: a uniquely sensitive UV resonance Raman secondary structural probe.

UV resonance Raman studies of peptide and protein secondary structure demonstrate an extraordinary sensitivity of the amide III (Am III) vibration and the C(alpha)H bending vibration to the amide backbone conformation. We demonstrate that this sensitivity results from a Ramachandran dihedral psi angle dependent coupling of the amide N-H motion to (C)C(alpha)H motion, which results in a psi dependent mixing of the Am III and the (C)C(alpha)H bending motions. The vibrations are intimately mixed at psi approximately 120 degrees, which is associated with both the beta-sheet conformation and random coil conformations. In contrast, these motions are essentially unmixed for the alpha-helix conformation where psi approximately -60 degrees. Theoretical calculations demonstrate a sinusoidal dependence of this mixing on the psi angle and a linear dependence on the distance separating the N-H and (C)C(alpha)H hydrogens. Our results explain the Am III frequency dependence on conformation as well as the resonance Raman enhancement mechanism for the (C)C(alpha)H bending UV Raman band. These results may in the future help us extract amide psi angles from measured UV resonance Raman spectra.