Lys216 is buried in the middle of the 7-TMS structure of bR, and retinal lies mostly parallel to the membrane and between the helices Light absorption converts retinal from all-trans to 13-cis configuration, triggering conformation changes that induce pKa changesThis facilitates proton transfers from Asp96 to the Lys Schiff base to Asp85 and net proton transport across the membrane