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

The J-K transition of the bacteriorhodopsin photocycle was monitored b y sub-picosecond time-resolved infrared spectroscopy. IR difference sp ectra 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. Spect ral shifts of the bands at 1607 and 1661 cm(-1) reflect the chromophor ic conformational changes during the J to K transition. Kinetics, take n at 1640 and 1607 cm(-1), show rise times determined by the dephasing times of the vibrational modes. The partial decrease of the bleach si gnal at 1640 cm-l is interpreted as a recovery of the vibrationally co oled BR(570) electronic ground state and provides a new method to meas ure the photocycle quantum yield. The development of the bleach at 166 1 cm(-1) occurred faster than 500 fs, suggesting an almost instantaneo us protein response to the electronic excitation.