Bidirectional Photochemistry of Antarctic Microbial Rhodopsin: Emerging Trend of Ballistic Photoisomerization from the 13-cis Resting State

[Image: see text] The decades-long ultrafast examination of nearly a dozen microbial retinal proteins, ion pumps, and sensory photoreceptors has not identified structure–function indicators which predict photoisomerization dynamics, whether it will be sub-picosecond and ballistic or drawn out with complex curve-crossing kinetics. Herein, we report the emergence of such an indicator. Using pH control over retinal isomer ratios, photoinduced transient absorption is recorded in an inward proton pumping Antarctic microbial rhodopsin (AntR) for 13-cis and all-trans retinal resting states. The all-trans fluorescent state decays with 1 ps exponential kinetics. In contrast, in 13-cis it decays within ∼300 fs accompanied by continuous spectral evolution, indicating ballistic internal conversion. The coherent wave packet nature of 13-cis isomerization in AntR matches published results for bacteriorhodopsin (BR) and Anabaena sensory rhodopsin (ASR), which also accommodate both all-trans and 13-cis retinal resting states, marking the emergence of a first structure–photodynamics indicator which holds for all three tested pigments.