Impact of the lipid environment on the protonation dynamics of bacteriorhodopsin studied with time-resolved step-scan FTIR spectroscopy

The membrane plays an important role in the structure and function of membrane proteins. We studied the influence of the lipid environment on the photocycle of the membrane protein bacteriorhodopsin (BR) with time-resolved step-scan FTIR spectroscopy. Proton transfer dynamics was monitored with microsecond time resolution for BR embedded in the native purple membrane as well as reconstituted into DOPC liposomes. We observed altered protonation dynamics of the Schiff base and the primary proton acceptor Asp85, revealing a faster rise as well as decay of the M state for BR surrounded by DOPC lipids. The purple membrane consists of a lipid composition that adapts better to the protein shape resulting in a stronger protein-membrane interaction as compared to the uniform DOPC lipid environment. Conformational dynamics and the correlated protonation dynamics are affected by the altered protein-membrane interaction explaining the faster photoreaction of BR in DOPC liposomes. Here we demonstrate the high sensitivity of the proton transfer dynamics to the lipid environment of BR.