Unusual Photoisomerization Pathway in a Near-Infrared Light Absorbing Enzymerhodopsin.

Microbial and animal rhodopsins possess retinal chromophores which capture light and normally photoisomerize from all- to 13- and from 11 to all--retinal, respectively. Here, we show that a near-infrared light-absorbing enzymerhodopsin from (OmNeoR) contains the all- form in the dark but isomerizes into the 7- form upon illumination. The photoproduct (λ = 372 nm; P) possesses a deprotonated Schiff base, and the system exhibits a bistable nature. The photochemistry of OmNeoR was arrested at <270 K, indicating the presence of a potential barrier in the excited state. Formation of P is accompanied by protonation changes of protonated carboxylic acids and peptide backbone changes of an α-helix. Photoisomerization from the all- to 7- retinal conformation rarely occurs in any solvent and protein environments; thus, the present study reports on a novel photochemistry mediated by a microbial rhodopsin, leading from the all- to 7- form selectively.