Carotenoids are essential for light harvesting in certain microbial rhodopsins, with xanthorhodopsins that bind salinaxanthin being the most well-known example. But how do carotenoids that lack the typical 4-keto group still support fast and efficient energy transfer?In our latest study, we investigate the proton-pumping rhodopsin Kin4B8, focusing on the xanthophylls zeaxanthin and lutein. Using a combination of long-timescale molecular dynamics, polarizable QM/MM simulations, and excitonic modeling, we show that protein-guided chromophore alignment, not specific carotenoid functional groups, enables ultrafast (<100 fs) and high-efficiency (>70%) energy transfer to retinal.
Our simulations of absorption and circular dichroism (CD) spectra not only reproduce experimental data, but also reveal the structural origins of the characteristic biphasic CD band shapes, including the notable blue-shifted minimum in the retinal region. These insights clarify recent mutagenesis and carotenoid screening experiments, and point to general design principles for tuning light-harvesting efficiency in both natural and engineered systems.
Salvadori, G., Saraceno, P., Santomieri, A., John, C., & Pedraza-González, L.
Structural and Spectroscopic Basis of Excitation Energy Transfer in Microbial Rhodopsins Binding Xanthophylls
Chemical Science, Accepted manuscript, 2025
Read the full paper here: https://doi.org/10.1039/D5SC04961J
