Ultrafast multidimensional vibrational spectroscopy: Experimental and theoretical studies on proteins and model compounds

Ultrafast multidimensional vibrational echo techniques have been developed both experimentally and theoretically and used to probe dynamics in condensed phased systems. In particular, experiments have focused on the ultrafast dynamics of myoglobin-CO (MbCO), and on a model inorganic dicarbonyl, acetylacetonatodicarbonyl rhodium I (Rh(CO)₂acac) dissolved in organic glasses. Experiments have shown that the standard description of beating phenomena seen in nonlinear experiments is inadequate, and has led to a new unified theoretical description of beating which explains all oscillatory behavior seen in third-order nonlinear spectroscopies.; Experiments on Rh(CO)₂acac have probed the nature of the solute-solvent interaction, and the mechanisms for inhomogeneous broadening. The CO stretching modes in Rh(CO)₂acac are coupled. Spectrally resolved vibrational echo decays on this system with short pulses exhibit quantum beats with a quantum beat frequency that depends on the detection frequency for each transition, which is indicative of the mechanism of inhomogeneous broadening in the system. The experiments were interpreted in the context of a model for coupled harmonic oscillators, and indicate that solute/solvent interactions lead to local CO stretch frequency perturbations that are correlated.; Multidimensional vibrational echo experiments on MbCO have led to the structural assignments of the conformational substrates seen in MbCO. The IR spectrum of the CO stretch of MbCO shows multiple bands, which correspond to slightly different conformations of the protein. Using multidimensional vibrational echo experiments in combination with molecular dynamics (MD) simulations, the structural origins for the IR absorption bands seen in MbCO have been assigned. Different conformational substates of the protein give rise to different dynamical line shapes for CO. The dynamical line shape for each band in the CO IR spectrum in MbCO acts as a signature for a particular protein structure. By comparing the dynamical line shapes measured with vibrational echo experiments and the dynamical line shapes calculated from molecular dynamics simulations for different possible conformations of the protein, it is possible to distinguish between possible “candidate” structures for the protein. This approach has determined the structural origins of the different bands seen in the infrared spectrum, and quantified the sources of dephasing for CO with atomic level detail.