Abstract
Recent investigations of protein dynamics performed by the authors using laser spectroscopic techniques are reviewed. The site-selective fluorescence spectroscopy in zinc-substituted myoglobin has revealed that its absorption spectrum is inhomogeneously broadened at liquid helium temperature. The obtained data have enabled the extraction of the density of low-energy vibrational modes weighted by the electron-vibration coupling strengths. The absorption spectra below and above 180K have been found to be reproduced by simulations under the assumptions of frozen and thermalized distributions among the conformational substates, respectively, in the ground state. This result supports the view that myoglobin molecules show a liquid-glass transition around 180K. From the time-resolved measurements of resonance fluorescence spectra and hole spectra, it has also been found that a protein molecule exhibits a peculiar structural relaxation dynamics, which is quite different from those in dye solutions. Information on the distribution of barrier heights separating the conformational substates and local structural change within a protein molecule has been obtained from the studies of the effect of temperature-cycling on the hole spectra burned at low temperatures. Recent trends of experimental studies on the conformational changes related to the liquid-glass transition of proteins are also discussed.
