Transient UV Raman Spectroscopy Finds No Crossing Barrier between the Peptide α-Helix and Fully Random Coil Conformation

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Abstract
Transient UV resonance Raman measurements excited within the amide π → π* transitions of a 21 unit α-helical peptide has for the first time determined a lower bound for the unfolding rate of the last α-helical turn to form a fully random coil peptide. A 3 ns T-jump is generated with 1.9 μm laser pulses, which are absorbed by water. Subsequent 3 ns 204 nm UV pulses excite the amide Raman spectra at delay times between 3 ns and 1 ms, to monitor the peptide conformational evolution. We find ∼180 ns relaxation times which result in a rate constant of >5 × 106 s-1 for unfolding of the last α-helical turn. Our data are inconsistent with slow α-helix nuclei melting.