FEMTOSECOND TIME-RESOLVED INFRARED-LASER STUDY OF THE J-K TRANSITION OF BACTERIORHODOPSIN

The J-K transition of the bacteriorhodopsin photocycle was monitored by sub-picosecond time-resolved infrared spectroscopy. IR difference spectra in the region between 1670 and 1600 cm(-1) were taken at 1.5 and 9 ps, respectively, after photoexcitation of BR(570) at 540 nm. Spectral shifts of the bands at 1607 and 1661 cm(-1) reflect the chromophoric conformational changes during the J to K transition. Kinetics, taken at 1640 and 1607 cm(-1), show rise times determined by the dephasing times of the vibrational modes. The partial decrease of the bleach signal at 1640 cm-l is interpreted as a recovery of the vibrationally cooled BR(570) electronic ground state and provides a new method to measure the photocycle quantum yield. The development of the bleach at 1661 cm(-1) occurred faster than 500 fs, suggesting an almost instantaneous protein response to the electronic excitation.