Dynamics of the primary processes of protein folding: Helix nucleation

We report on the folding and unfolding dynamics of a.-helix nucleation in two model helical peptides. One of the peptides studied unfolds at lower temperatures (cold denatures) in solutions of 9% hexafluoroisopropanol. Laser-induced temperature jumps were used to rapidly perturb the helix/coil equilibrium in this peptide from a predominantly unfolded to a more folded ensemble. The peptide conformation was monitored through time-resolved absorption of the amide I' band. These experiments directly probe alpha-helix nucleation, as a majority of alpha-helices formed must start from a completely random coil conformation. In another alpha-helical peptide, the unfolding and folding kinetics of specific residues were monitored through the use of isotopically (C-13=O) labeled amino acids. By selectively measuring the unfolding kinetics of the middle of the helix, one can minimize the contribution of end-fraying effects and explicitly probe the crossing of a nucleation free energy barrier in the helix to coil direction. The results reveal that alpha-helix nucleation occurs on a sub-microsecond time-scale with a substantial enthalpic barrier.