ON THE NATURE OF THE PRIMARY LIGHT-INDUCED EVENTS IN BACTERIORHODOPSIN : ULTRAFAST SPECTROSCOPY OF NATIVE AND C13=C14 LOCKED PIGMENTS

The primary light-induced events in the photosynthetic retinal protein bacteriorhodopsin (bR) are investigated by ultrafast optical spectroscopy over the 440−1000 nm spectral range. The study compares the early dynamics of the native all-trans pigment bR570 with those of two synthetic analogues, bR5.12 and bR5.13, in which isomerization around the critical C13C14 bond is blocked by a five-membered ring into all-trans and 13-cis configurations, respectively. Nearly identical spectral evolution is observed in both native and artificial systems over the first 100−200 fs of probe delay. During this period stimulated near-IR (∼900 nm) emission, and intense ∼460 nm absorption bands, due to analogous fluorescent I states (denoted as I460, I5.12 and I5.13, respectively), appear concurrently within 30 fs. In all systems continuous spectral shifting over tens of femtoseconds is observed in the 500−700 nm range. Native bR goes on to produce the J625 absorption band within ∼1 ps, which is accompanied by disappearance...