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From Femtoseconds To Biology: Mechanism Of The Light-Driven Proton Pump In Bacteriorhodopsin

Femtosecond time-resolved absorption and time-resolved resonance Raman spectroscopy are used to examine the molecular mechanism of the light-driven proton pump in bacteriorhodopsin. Transient absorption spectroscopy with 6-fs pulses is used to directly observe the excited-state, double bond torsional isomerization of the retinal chromophore. Time-resolved resonance Raman spectroscopy is used to determine the structure of the retinal prosthetic group in each of the subsequent photolytic intermediates. Based on these results, a new "C-T Model" for the molecular mechanism of the proton pump has been developed. The key feature of this model is an isomerization-driven conformational change of the protein from the T-form to the C-form which acts as a "reprotonation switch" and stores energy to drive later events in the photocycle.