Theory of hole-burning spectra of pigment-protein complexes

Julian Adolphs, Thomas Renger
Johannes Kepler University Linz, Institute for Theoretical Physics, Austria

We have developed a theory for the calculation of resonant and non-resonant hole-burning (HB) spectra of pigment-protein complexes and applied it to the water-soluble chlorophyll- binding protein (WSCP) from cauliflower [1]. The theory is based on a non-Markovian line-shape theory [2] and includes exciton delocalization, vibrational sidebands and lifetime broadening. Whereas standard approaches were based on a two-level system theory [3], a recent approach by Reppert [4] combined multilevel line-shape theory [2] with a Monte Carlo scheme to describe hole-burning spectra of a model dimer. We found that this approach can explain non-resonant hole-burning spectra, where the burn laser deposits excess energy in the protein. This excess energy is used to drive conformational transitions that let the pigments ''forget'' their original transition energies. We extended this approach in several aspects, the most important is to take into account the fact that for resonant burning, that is, excitation of the zero phonon line of the lowest exciton state, the protein is only able to access conformational sub-states in the neighborhood of the pre-burn state. In this way, the pigments ''remember'' their original site energies. Application of the new theory to HB experiments on WSCP from cauliflower, gives excellent agreement with experimental data [5] for both, resonant and non-resonant burning. Additionally, it allows to obtain an upper bound of the lifetime of the upper exciton state directly from the HB experiments in agreement with lifetimes measured recently in time domain 2D experiments [6].

References

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[6] J. Alster et al., J. Phys. Chem. B 2014, 118, 3524−3531