THERMAL EQUILIBRATION BETWEEN THE M-INTERMEDIATES AND N-INTERMEDIATESIN THE PHOTOCYCLE OF BACTERIORHODOPSIN

The stages in the photocycle of bacteriorhodopsin (BR) involving the M and N intermediates are investigated using a double pulse excitation method. A first (cycling) pulse at 532 nm is followed, with an appropr iate time delay, by a second pulse (337, 406, 446, or 470 nm) which in duces the M --> BR back-photoreaction. After depletion by the second p ulse a repopulation of M in the millisecond range is observed which is interpreted in terms of a thermal N --> M relaxation. It is thus conc luded that a (thermal) M <-> N equilibrium accounts for the biphasic d ecay of M in the BR photocycle. Other models for this stage of the lig ht-driven proton-pump are therefore unnecessary.