Time-resolved FTIR spectroscopy of the photointermediates involved in fast transient H ⁺ release by proteorhodopsin

Proteorhodopsin (pR) is a homologue of bacteriorhodopsin (bR) that has been recently discovered in oceanic bacterioplankton. Like bR, pR functions as a light-driven proton pump. As previously characterized by laser flash induced absorption spectroscopy (Krebs, R. A.; Alexiev, U.; Partha, R.; DeVita, A. M.; Braiman, M. S. BMC Physiol 2002, 2, 5), the pR photocycle shows evidence of light-induced H ⁺ release on the 10-50 μs time scale, and of substantial accumulation of the M intermediate, only at pH values above 9 and after reconstitution into phospholipid followed by extensive washing to remove detergent. We have therefore measured the time-resolved FTIR difference spectra of pR intermediates reconstituted into DMPC vesicles at pH 9.5. A mixture of K- and L-like intermediates, characterized by a 1516 cm ^-1 positive band and a 1742 cm ^-1 negative band respectively, appears within 20 μs after photolysis. This mixture decays to an M-like state, with a clear band at 1756 cm ^-1 due to protonation of Asp-97. The 50-70 μs rise of M at pH 9.5 is similar to (but a little slower than) the rise times for M formation and H ⁺ release that were reported earlier based on flash photolysis measurements of pR reconstituted into phospholipids with shorter acyl chains. We conclude that, at pH 9.5, H ⁺ release occurs while Asp-97 is still protonated; i.e., this aspartic acid cannot be the H ⁺ release group observed by flash photolysis under similar conditions.