Dihedral ψ Angle Dependence of the Amide III Vibration: A Uniquely Sensitive UV Resonance Raman Secondary Structural Probe

Abstract

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