TEMPERATURE-DEPENDENCE OF FEMTOSECOND EXCITED-STATE DYNAMICS OF BACTERIORHODOPSIN ANALYZED BY THE FOURIER-TRANSFORM OF OPTICAL-ABSORPTION SPECTRA

Mechanisms of the excited state dynamics for the cis-trans photoisomerization of bacteriorhodopsin are investigated by analyzing the temperature dependence of the time-correlation function (tcf) of the modified vibrational wave packet which is obtained by the Fourier transform of the experimentally observed optical absorption spectra at 273, 233, 193, 133, and 78 K. Remarkable temperature dependences of tcf are obtained, especially at the very initial time region up to about 60 fs: The peak at 27 fs, which is clearly observed at 78 K, becomes weak gradually as temperature increases, and instead a deep minimum is created around 30 fs at higher temperatures. We also observed a progression of some maxima with periods of 27-30 fs at lower temperatures (78 and 133 K) and a progression of global maxima with a period of about 60 fs at higher temperatures (233 and 273 K). The implication of these remarkable temperature dependent properties of tcf is discussed in relation to the possible role the protein environment played in the molecular mechanism of the specific, ultrafast photoisomerization reaction of bacteriorhodopsin.