Transient UV Raman spectroscopy finds no crossing barrier between the peptide alpha-helix and fully random coil conformation

Transient UV resonance Raman measurements excited within the amide pi --> p i* transitions of a 21 unit alpha -helical peptide has for the first time d etermined a lower bound for the unfolding rate of the last alpha -helical t urn to form a fully random coil peptide. A 3 ns T-jump is generated with 1. 9 mum 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 similar to 180 ns relaxation times which result in a rate constant of >5 x 10(6) s(-1) for u nfolding of the last alpha -helical turn. Our data are inconsistent with sl ow alpha -helix nuclei melting.