Time-Resolved Spectroscopy of the Early Photolysis Intermediates of Rhodopsin Schiff Base Counterion Mutants

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Abstract
Time-resolved absorption difference spectra of COS-cell expressed rhodopsin and rhodopsin mutants (E113D, E113A/A117E, and G90D), solubilized in detergent, were collected from 20 ns to 510 ms after laser photolysis with 7 ns pulses (λmax = 477 nm). The data were analyzed using a global exponential fitting procedure following singular value decomposition (SVD). Over the entire time range excellent agreement was achieved between results for COS-cell and rod outer segment rhodopsin both in kinetics and in the λmax values of the intermediates. The Schiff base counterion mutant E113D showed strong similarities to rhodopsin up to lumi, following the established scheme: batho ⇌ bsi → lumi. Including late delay times (past 1 μs), the mutant E113D lumi decayed to metarhodopsin II (MII), showing that the detergent strongly favors MII over metarhodopsin I (MI). However, a back-reaction from MII to lumi was observed that was not seen for rhodopsin. The kinetic schemes for the mutants E113A/A117E and G90D were significantly different from that of rhodopsin. In both mutants batho decay into an equilibrium with bsi was too fast to resolve (12−C13 of the retinal chromophore. The λmax values of the bsi and lumi intermediates in the mutant pigments are interpreted in terms of movement of the Schiff base relative to its counterion.