Energy relaxation dynamics in the optical excited state of myoglobin

In order to obtain information on the fast dynamics of the conformational fluctuation of a protein, absorption, steady-state, and time-resolved fluorescence spectra have been measured in the spectral region of the lowest optical absorption band of Zn-substituted myoglobin between 170 and 290 K. The obtained spectra have been analyzed using the single-site spectra calculated from the experimentally determined effective density of states of vibrational modes of Zn-substituted myoglobin. A configuration-coordinate model has been adopted to take into account the inhomogeneous broadening of the spectra. The fluorescence spectrum has also been compared with that calculated from the absorption spectrum on the basis of the Einstein relation. The results of the analyses show that the conformational relaxation occurs in the excited state, but is not completed within the lifetime of the excited state even at room temperature. The dynamic Stokes shift, which is expected in such a case, has not been observed in the whole temperature range examined. This indicates that the excited-state dynamics in myoglobin is different from that due to the thermal crossing over static potential barriers. The experimental results are explained well by a model based on hierarchically constrained dynamics. © 1996 The American Physical Society.