Investigation of the chromophore binding cavity in the 11-cis acceptable microbial rhodopsin MR

► A unique photoactive protein MR can also bind 11-cis retinal besides all-trans and 13-cis retinal. ► MR is less restricted to the retinal chromophore. ► The presence of the 11-cis conformer is regulated by the residues located around the retinal. ► Some long-range effect(s) (>5 Å) is also important for the maintenance of the chemical properties of MR.