Femtosecond spectroscopy of the first events of the photochemical cycle in bacteriorhodopsin

The first steps in the photochemistry of bacteriorhodopsin (BR) are
investigated with light pulses of 160 fs duration. Four samples are
studied: (i) the purple membrane, (ii) deuterated purple membrane,
(iii) BR trimers and (iv) BR monomers. In all samples the first
intermediate J is formed within 430±50 fs. No isotope effect is
observed in the formation of J upon deuteration, in contrast to
previous reports with much higher excitation energies. Thus proton
movement to or from the retinal Schiff's base is not relevant
during the first step. Comparing the data for trimeric and
monomeric BR suggests an upper limit of 50 fs for the transfer of
excitation energy from the excitonically coupled trimer to a single
retinal chromophore.