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New Features in the Ultrafast Ligand Dynamics and Energy Dissipation in Myoglobin

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Abstract

Numerous transformations in heme proteins can be triggered upon photoexcitation, providing perfect model systems for the study of protein dynamics. In particular, the recombination of CO after photodissociation has been investigated in great detail. It was found to occur on the ns time scale. In contrast, the geminate recombination of NO proceeds much faster, within several 100 ps, i.e. on the same time scale as the relaxation of the protein to its unligated structure [1]. Furthermore, the recombination of NO was found to be nonexponential. Several mechanisms have been suggested to explain this behavior: (i) distribution of the barrier to rebinding due to different protein substates (inhomogeneous model), (ii) time dependence of the barrier due to protein/heme relaxation to the unligated structure or (iii) dissociation of the ligand to different intermediate sites in the protein. We are investigating these possibilities by time resolved IR measurements of the NO recombination. Furthermore, the comparison of native heme proteins with specially designed mutants will discriminate between the different mechanisms. We have also investigated the effect of the amount of excess energy in the precursor state on the resulting dynamics. The question is whether a higher excitation energy leads to the release of a ligand with higher kinetic energy, and thereby results in different intermediate protein sites, or whether the excess energy simply leads to hotter heme product which then undergoes vibrational cooling.

© 1996 Optical Society of America

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