Femtosecond time resolved infrared spectroscopy of the ethylenic stretch vibration during the all-trans to 13-cis isomerization of bacteriorhodopsin

We report on room temperature infrared transient absorption experiments on Bacteriorhodopsin concerning the early picosecond dynamics of the all-trans to 13-cis isomerization of the retinal chromophore. The absorption band of the all-trans chromophore ethylenic stretch vibration bleaches concomitant ly to photoexcitation whereas new absorption strength is created redshifted within less than 1 ps, the initial bleach recovers partly according to the non unity quantum yield of the photoisomerization within 2 ps. With a time constant of 3 ps the low energy absorption band is downshifted 5 - 10 cm(- 1). Thus, vibrational relaxation and cooling processes appear to have ended within a Few ps and on the time scale of 20 ps a stationary BR-K differenc e spectrum is obtained.