Ultraviolet Raman Optical Activity as a Window Into Peptide Backbone Structure

The spectroscopy of ultraviolet Raman optical activity (UV ROA) promises an excellent sensitivity to peptide and protein conformation, decoupling the backbone signal from most side chains. Compared to the more usual ROA with visible light, stronger intensity and additional spatial sensitivity are expected because most of the signal comes from the amide chromophore. UV ROA experiments are scarce, and links between spectral shapes and molecular structure are rather unexplored. However, on a dedicated instrument, we could acquire spectra of three peptides and analyze them on the basis of molecular dynamics and density functional theory simulations. Radical differences were observed between the spectra obtained with the 244 and 532 nm excitations, and they could be rationalized by the simulations. Bands connected to the peptide backbone vibrations are enhanced at the shorter wavelength, due to a pre-resonance with the n-π^* and π-π^* amide transitions. Further computational experiments on the Ala₄ peptide indicate that sensitivity to the secondary structure is enhanced as well, by a combination of geometric and resonance effects. The results thus confirm the potential of UV ROA for analytical chemistry and biochemistry in terms of novel information it brings about molecular geometric and electronic structure.