Site-Specific Coupling of Hydration Water and Protein Flexibility Studied in Solution with Ultrafast 2D-IR Spectroscopy

There is considerable evidence for the slaving of bimolecular dynamics to the motions of the surrounding solvent environment, but to date there have been few direct experimental measurements capable of site-selectively probing. both :the dynamics of the water and the protein with ultrafast time resolution Here, two dimensional infrared spectroscopy (2D-IR) is used to study the ultrafast hydration and protein dynamics sensed by a metal carbonyl vibrational probe covalently attached to the surface of hen egg white lysozyme dissolved in D2O/glycerol solutions. Surface labeling provides direct.:access to the dynamics at the protein water interface, where both the hydration and the protein dynamics can be observed simultaneously through the vibrational probe's frequency frequency correlation function In pure D2O, the correlation function shows a fast initial 3 ps decay corresponding to fluctuations of the hydration water, followed by a significant static Offset attributed to fluctuations of the protein.. that are not sampled within the <20 Ps experimental window. Adding glycerol increases the bulk solvent viscosity while leaving, the protein structurally intact and hydrated The hydration dynamics exhibit a greater than 3,fold slowdown between 0 and 86% glycerol (v/v), and the contribution from the protein's dynamics is found to slow in a nearly identical fashion. In addition, the, magnitude of the dynamic slowdown associated with hydrophobic hydration is directly measured and shows quantitative agreement with predictions from molecular dynamics simulations.