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

Samuel M. Blau, Doran I.G. Bennett, Christoph Kreisbeck, Gregory D. Scholes, Alán Aspuru-Guzik

Research output: Contribution to journalArticlepeer-review

58 Scopus citations


The mechanisms controlling excitation energy transport (EET) in light-harvesting complexes remain controversial. Following the observation of long-lived beats in 2D electronic spectroscopy of PC645, vibronic coherence, the delocalization of excited states between pigments supported by a resonant vibration, has been proposed to enable direct excitation transport from the highest-energy to the lowest-energy pigments, bypassing a collection of intermediate states. 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 quantum mechanics/molecular mechanics (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 construct a general model for vibronic dimers and establish the parameter regimes of coherent and incoherent vibronic transport. We demonstrate that direct down-conversion in PC645 proceeds incoherently, enhanced by large reorganization energies and a broad collection of high-frequency vibrations. We suggest that a similar incoherent mechanism is appropriate across phycobiliproteins and represents a potential design principle for nanoscale control of EET.

Original languageEnglish (US)
Pages (from-to)E3342-E3350
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number15
StatePublished - 2018

All Science Journal Classification (ASJC) codes

  • General


  • Excitation energy transfer
  • Light harvesting
  • Molecular dynamics
  • Photosynthesis
  • Quantum coherence


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