Understanding the role of vibrational energy in activating biomolecules to perform their various functions is a major goal that will extend existing knowledge of biomolecular reaction dynamics. Temperature dependent ultrafast kinetics, resonance Raman scattering, and femtosecond coherence spectroscopy, are used in this project to probe the structure/function/dynamics relationships of biomolecules such as heme proteins. Coherence techniques are a unique probe of the vibrational spectrum in the region below room temperature (kBT ~ 200cm-1), which contains the thermally accessible modes most likely to be utilized as reaction coordinates. Low-frequency heme distortions (e.g., ruffling and doming) are involved in the reorganization of energy associated with such fundamental reactions such as electron transport and ligand binding. The coherence spectroscopy studies use heme model complexes; DFT calculations, x-ray structures, and normal coordinate structural decomposition to quantify the protein-induced symmetry lowering distortions of the heme that activate these (anharmonic) low frequency modes. Phase and amplitude excitation profiles of these modes increase understanding of the non-radiative electron-nuclear coupling mechanisms that trigger the coherent oscillations of reaction coordinates. Other techniques, such as nuclear resonance vibrational spectroscopy and magnetic field perturbations are applied to aid in the assignment and characterization of the low frequency (anharmonic) motions. Novel temperature dependent ultrafast kinetic studies reveal the protein-specific entropic and enthalpic contributions to ligand binding transition state barriers. New experiments suggest a dynamic ligand discrimination mechanism in heme protein systems that depend upon the lifetime of the dissociated state relative to its entropy production time. Femtosecond stimulated Raman scattering instrumentation is also being developed and applied to the study of metal-ligand charge transfer bands. This will be used as an important independent probe of the structure, function, dynamics, and anharmonicity of the heme chromophore that can be applied to other biological systems as well.
This project investigates fundamental aspects of protein dynamics and focuses on the electron-nuclear coupling at the active site of heme proteins. Studies of such interactions are crucial to the basic understanding of living systems. Students (both graduate and undergraduate) will be trained in cutting edge optical and biological techniques that benefit society through the enhancement of the human resource infrastructure of the academic, biotechnology, and photonics industries. The knowledge obtained will be disseminated through lectures, seminars, and published papers. Outreach to underrepresented groups and the general public will also take place as a result of this project. This project is jointly supported by the Molecular Biophysics Program in the Division of Molecular and Cellular Biosciences in the Directorate for Biological Sciences and the Division of Physics in the Mathematical and Physical Sciences Directorate.
