Ultrafast Chemical Dynamics of Hydrogen Bonding Environments: From Alcohols to Crowded Proteins.

Abstract

Ultrafast two-dimensional infrared (2DIR) spectroscopy is used to study and characterize the hydrogen bonding dynamics of several systems including: linear alcohols, fragile glasses near the glass transition temperature, and hydration environments around small hydrophobes and proteins. The hydrogen bonding dynamics of linear alcohols (methanol to 1-hexanol) are characterized using 2DIR spectroscopy. The spectral diffusion, a common 2DIR observable, of a metal carbonyl vibrational probe is measured for the series of alcohols and demonstrates a monotonic slowing of the hydrogen bond dynamics as the chain length of the alcohol increases. In addition, the influence of hydrogen bonding between the probe molecule and the solvent on intramolecular vibrational redistribution (IVR) is measured by monitoring the cross peak amplitude between two vibrational modes of the vibrational probe. The experimentally measured IVR time constants, coupled with molecular dynamics simulations, allowed a slower IVR time to be related to an increased number of hydrogen bonds, demonstrating solvent-hindered IVR. Hydrogen bonding dynamics of a fragile glass former are studied as the system is cooled to a glass transition temperature. The spectral diffusion of dirhenium decacarbonyl (DRDC, Re2(CO)10) in 1,2-hexanediol is measured as the system is cooled within a few degrees of Tg. Near the glass transition temperature the frequency-frequency correlation function shows non-exponential relaxation, illustrating the presence of ultrafast dynamic heterogeneity of fragile glasses near the glass transition. Additionally, a non-Arrhenius temperature dependence of the spectral diffusion is observed, suggesting that alpha-like relaxation (slow, cooperative motions found in fragile glasses near the glass transition temperature) are manifested on the ultrafast timescale. The dynamics of water, in particular water near hydrophobic molecules and surfaces, is extensively studied using 2DIR. The dynamics associated with hydrophobic hydration are studied for small hydrophobic molecules as well as extended protein surfaces. In addition, the coupling between the protein dynamics and the hydration dynamics are observed using co-solvent additives. It is found that there is a measurable slowdown (factor of 2) of water around isolated protein surfaces that originates from an excluded volume effect, where limiting the number of possible hydrogen bond acceptors constrains the hydrogen bond rearrangements near hydrophobic surfaces.