Dynamics of the folded and unfolded villin headpiece (HP35) measured with ultrafast 2D IR vibrational echo spectroscopy.

A series of two-dimensional infrared vibrational echo experiments performed on nitrile-labeled villin headpiece [HP35-(CN)(2)] is described. HP35 is a small peptide composed of three alpha helices in the folded configuration. The dynamics of the folded HP35-(CN)(2) are compared to that of the guanidine-induced unfolded peptide, as well as the nitrile-functionalized phenylalanine (PheCN), which is used to differentiate the peptide dynamic contributions to the observables from those of the water solvent. Because the viscosity of solvent has a significant effect on fast dynamics, the viscosity of the solvent is held constant by adding glycerol. For the folded peptide, the addition of glycerol to the water solvent causes observable slowing of the peptide's dynamics. Holding the viscosity constant as GuHCl is added, the dynamics of unfolded peptide are much faster than those of the folded peptide, and they are very similar to that of PheCN. These observations indicate that the local environment of the nitrile in the unfolded peptide resembles that of PheCN, and the dynamics probed by the CN are dominated by the fluctuations of the solvent molecules, in contrast to the observations on the folded peptide.