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 BR570 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−1 is interpreted as a recovery of the vibrationally cooled BR570 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.