PICOSECOND OPTICAL STUDIES IN PROTEINS

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The specific goal of the proposed research is to determine how vibrational energy propagates in biopolymer systems. The mechanism of energy propagation and dispersion in proteins is central to a microscopic understanding of large amplitude correlated nuclear motion. Structural changes to a specific stimulus are known to be important factors in regulation of activity. Allosteric control of O2 affinity in hemoglobin, muscle contraction and the unwinding of DNA are just a few examples. Biomechanics has long been recognized as an essential component in the regulation of activity. Given the importance of the problem, several theoretical models have been developed to come to grips with the enormous number of nuclei coupled in the motion and the apparent contradiction in time scales for the motions. Theories ranging from nondispersive solitons (vibrational wave packets) to full scale molecular dynamics calculations have been employed. The proposed research constitutes the first direct experimental approach to this problem. Optical excitation in the Soret region of heme proteins provides a selective means to deposit energy into a well defined spatial position in the protein. By use of real-time holography with the novel feature of optical heterodyne detection, the randomization of the optical energy into vibrational modes and the propagation of that energy from the heme center to the protein exterior water layer can be followed with 100 fsec resolution. Optical heterodyne detection acts to amplify the signal and enables the detection of less that 10-80C temperature changes in internal energy. By studying hot band transitions originating spatially from the heme center, tryosine residue in the protein backbone and water exterior, a detailed mapping of the spatial profile and dynamics of the energy propagation can be made. These results can be compared to recent molecular dynamics simulations. Slow structural relaxation processes triggered by CO and O2 photodissociation will also be studied by following changes in the aqueous lattice through density contributions to the real-time holographic image. Emphasis will be place on correlating the first steps in energy transduction during ligand binding or photodissociation to the large amplitude structural responses to the stimulus. These studies will allow for the very first time examination of protein dynamics on all timescales, from the 100 fsec to the msec regime.