Mastigocladopsis repens Rhodopsin (MastR), the newest member of a family of retinal-binding anionic transmembrane pumps, can trap light energy to allow inward chloride transport across a cellular membrane. The pumping action is driven by retinal isomerization, which is thermodynamically unfavorable in the ground state but occurs at a higher rate under photo-excitation in the host protein environment. Particularly interesting is MastR’s conversion to an outward proton pump (MastR-T74D) due to single-point mutation of an amino acid located close to the retinal chromophore. Although flash photolysis studies have characterized the MastR/MastR-T74D photocycle on a microsecond timescale, its ultrafast dynamics involving retinal isomerization have not yet been investigated. We performed ultrafast transient absorption studies on MastR and MastR-T74D to look at the femtosecond to picosecond dynamics that lead to retinal isomerization and compared them to the well-studied ultrafast dynamics of bacteriorhodopsin.
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