1.H.2 Energy relaxation of biological proteins in picosecond time scale: Myoglobin

　　There are many biological proteins that use light energy to create chemical energy or for a signal transducer function. Such proteins generally have chromophores inside the proteins to absorb the light from the sun. The energy that is not stored as chemical energy or not used for the biological function should be eventually damped into the surrounding medium. How is the energy dissipated into the solvent; through the protein indirectly or directly into the solvent? How can we observe the route? Energy transfer from a photoexcited chromophore embedded in a protein matrix is important to understand the molecular mechanism of the damping of the light energy that is used for triggering a protein dynamics and has been attracting many scientists. We investigated the energy dissipation route of deoxyMyoglobin in buffer from photo-excited chromophore to the solvent by the time-resolved transient grating (TG) method. The observed peak delay of the deoxymyoglobin was found to be -15 ps; i.e., the acoustic peak delay of the signal was anormalously negative after the photoexcitation of deoxyMb. This negative value is explained by the two acoustic waves created from the medium as well as from the protein thermal expansion. We believe that this is the first direct experimental evidence for the heating of the protein matrix after the heme excitation and the thermal expansion of the protein structure by the excess energy.


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