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 demon strate an extraordinary sensitivity of the amide III (Am III) vibration and the CalphaH bending vibration to the amide backbone conformation. We demon strate that this sensitivity results from a Ramachandran dihedral V angle d ependent coupling of the amide N-H motion to (C)CalphaH motion, which resul ts in a psi dependent mixing of the Am III and the (C)CalphaH bending motio ns. The vibrations are intimately mixed at psi similar to 120 degrees, whic h is associated with both the beta -sheet conformation and random coil conf ormations. In contrast, these motions are essentially unmixed for the alpha -helix conformation where psi similar to -60 degrees. Theoretical calculat ions demonstrate a sinusoidal dependence of this mixing on the psi angle an d a linear dependence on the distance separating the N-H and (C)CalphaH hyd rogens. Our results explain the Am III frequency dependence on conformation as well as the resonance Raman enhancement mechanism for the (C)CalphaH be nding UV Raman band. These results may in the future help us extract amide psi angles from measured UV resonance Raman spectra.