Local protein solvation drives direct down-conversion in phycobiliprotein PC645 via incoherent vibronic transport

Mechanisms controlling excitation energy transport (EET) in light-harvesting complexes remain controversial. Following the observation of long-lived beats in two-dimensional electronic spectroscopy of PC645, vibronic coherence, the delocalization of excited states between pigments supported by a resonant vibration, has been proposed to enable direct down-conversion from the highest-energy states to the lowest-energy pigments. Here, we instead show that for phycobiliprotein PC645 an incoherent vibronic transport mechanism is at play. We quantify the solvation dynamics of individual pigments using ab initio QM/MM nuclear dynamics. Our atomistic spectral densities reproduce experimental observations ranging from absorption and fluorescence spectra to the timescales and selectivity of down-conversion observed in transient absorption measurements. We demonstrate that bilin solvation controls EET pathways and that direct down-conversion proceeds incoherently, enhanced by large reorganization energies and a broad collection of high-frequency vibrations. We thus suggest that engineering local solvation dynamics represents a potential design principle for nanoscale control of EET.