NANOSECOND LASER PHOTOLYSIS OF PHOBORHODOPSIN: FROM Natronobacterium pharaonis APPEARANCE OF KL AND L INTERMEDIATES IN THE PHOTOCYCLE AT ROOM TEMPERATURE

Abstract— Photochemical and subsequent thermal reactions of pharaonis phoborhodopsin (ppR; absorption maximum, 498 nm) from Natronobacrerium pharaonis were investigated by nanosecond laser photolysis at 20°C. The experimental results clearly showed the presence of two intermediates in the photocycle of ppR besides the K, M and O intermediates detected previously. One was formed immediately after the excitation of ppR with a blue pulse (pulse width, 17 ns; wavelength, 460 nm), and the other was formed by the thermal reaction of this species. The new intermediates' absorption maxima were 512 and 488 nm, their extinction coefficients were 0.85- and 0.68-times smaller than that of ppR, and their lifetimes were 990 ns and 32 μs, respectively. The absorption and kinetic characteristics of these intermediates relative to ppR were similar to those of the KL and L intermediates of bacteriorhodopsin (bR). The formation of KL intermediates from both ppR and bR were observed only at room temperatures. On the other hand, the formation of L intermediate of bR was observed at both of room and low temperature, whereas that from ppR only at room temperature. The unique formation of L intermediate of ppR at room temperature is discussed in relation to high thermal stability of K intermediate of ppR.