ULTRAFAST PUMP-PROBE SPECTROSCOPY OF THE PHOTORECEPTOR STENTORINS FROM THE CILIATE STENTOR-COERULEUS

Polarized absorption difference profiles have been obtained with 3-6-ps resolution between 565 and 630 nm for the chromoproteins stentorin I and stentorin II, isolated from the protozoan ciliate Stentor coeruleus. Control experiments were performed on hypericin and on the stentorin chromophore in methanol. The isotropic decays of stentorin I and the stentorin chromophore are dominated by components with lifetimes conservatively longer than 1 ns. Stentorin I, the stentorin chromophore, and hypericin all develop, within <5 ps, an intense excited-state absorption band that is blue-shifted from and polarized essentially perpendicular to the main long-wavelength absorption band. The presence of this excited-state absorption band is signaled by the appearance of strongly wavelength-dependent initial anisotropies r(0), which assume large positive as well as negative values (>0.4 and <-0.2). To our knowledge, such anisotropies have not been reported in polarized pump-probe studies of rotational diffusion in solution or of energy-transfer kinetics in photosynthetic antennae. Hypericin itself exhibits 0.4-6-ps excited-state absorption decay kinetics, which are likely associated with intramolecular excited-state proton transfer. The corresponding proton transfers may be faster in stentorin I and in the stentorin chromophore. Stentorin II rapidly develops a unique absorption difference spectrum that lacks the excited-state absorption feature displayed by the other three systems. The ultrafast process in stentorin II (thought to be the active chromoprotein in ciliary motion) may be intermolecular proton transfer to an appropriately situated protein residue. The present experiments redefine the upper limit for the dominant lifetime component in this photoprocess to <3 ps.