Two-dimensional electronic spectroscopy of the Q_x to Q_y relaxation of chlorophylls a in photosystem II core complex

Using two-dimensional electronic spectroscopy, we measured the Q_x to Q_y transfer dynamics of the chlorophyll a (Chl a) manifold in the photosystem II (PSII) monomeric core complex from Arabidopsis thaliana. A PSII monomeric core consists of 35 Chls a and no Chl b, thus allowing for a clear window to study Chl a Q_x dynamics in a large pigment-protein complex. Initial excitation in the Q_x band results in a transfer to the Q_y band in less than 60 fs. Upon the ultrafast transfer, regardless of the excitation frequency within the Q_x band, the quasi-transient absorption spectra are very similar. This observation indicates that Chl a’s Q_x to Q_y transfer is not frequency selective. Using a simple model, we determined that this is not due to the lifetime broadening of the ultrafast transfer but predominantly due to a lack of correlation between the PSII core complex’s Chl a Q_x and Q_y bands. We suggest the origin to be the intrinsic loss of correlation during the Q_x to Q_y internal conversion as observed in previous studies of molecular Chl a dissolved in solvents.