Fast folding dynamics of an α-helical peptide with bulky side chains

Abstract

The fast folding/unfolding dynamics of the α-helical polypeptide poly-N⁵-(3-hydroxypropyl)-L-glutamine, PHPG, was observed in D₂O after a nanosecond laser-induced temperature jump of 1 °C. The sudden rise in temperature disturbs the helix–coil equilibrium of the peptide and the ensuing structural relaxation was monitored by time-resolved IR-spectroscopy of the amide I′ band with a time resolution of 12 ns. After an initial relaxation on the time scale of a few nanoseconds, probably arising from a local rearrangement of random coil or side chain structures, the helix–coil relaxation proceeds on the time scale of a few hundred nanoseconds. The observed time constant of 227 ns is very similar to the helix–coil relaxation time constants observed for short alanine-based peptides, in spite of the greater length and more complex and bulky side chain of PHPG.