Low temperature FTIR spectroscopy provides new insights in the pH-dependent proton pathway of proteorhodopsin

► Photocycle study of proteorhodopsin (PR) using low temperature FTIR spectroscopy. ► At pH 5.1 proper isomerization, large amide changes and an altered proton acceptor. ► At pH 5.5 changes concerning the backbone movements and the protonation steps. ► Proposal of a three-state model that explains the complex pH dependence of PR.