The Two Dimensional Vibrational Echo of a Nitrile Probe of the Villin HP35 Protein.

2D IR spectroscopy was used to probe the hydrophobic core structure of the 35-residue Villin headpiece subdomain, HP35, by monitoring the C≡N vibrational stretching band of a cyano substituted phenylalanine (Phe). The presence of two humps in the vibrational frequency distribution in the folded equilibrium state is revealed. They represent two states that exchange more slowly than ca. 10 ps. The two CN stretch mode peak frequencies (and their equilibrium populations) are 2228.7 (44%) and 2234.5 cm(-1) (56%). The two CN modes have different frequency-frequency correlation times of 7.4 ps and 1.6 ps respectively. These results suggest that the population with the higher frequency CN group is partly exposed whereas the other CN mode experiences a hydrophobic like environment.