UV resonance Raman study of TrpZip2 and related peptides: π-π interactions of tryptophan

Aromatic interactions are important stabilizing forces in proteins but are difficult to detect in the absence of high-resolution structures. Ultraviolet resonance Raman spectroscopy is used to probe the vibrational signatures of aromatic interactions in TrpZip2, a synthetic β-hairpin peptide that is stabilized by edge-to-face and face-to-face tryptophan π-π interactions. The vibrational markers of isolated edge-to-face π-π interactions are investigated in the related β-hairpin peptide W2W11. The bands that comprise the Fermi doublet exhibit systematic shifts in position and intensity for TrpZip2 and W2W11 relative to the model peptide, W2W9, which does not form aromatic interactions. Additionally, hypochromism of the B_b absorption band of tryptophan in TrpZip2 leads to a decrease in the relative Raman cross-sections of B_b-coupled Raman bands. These results reveal spectral markers for stabilizing tryptophan π-π interactions and indicate that ultraviolet resonance Raman may be an important tool for the characterization of these biological forces.