Ultrafast Energy Transfer in LHC-II Revealed by Three-Pulse Photon Echo Peak Shift Measurements

Abstract

We report the results of three-pulse photon echo peak shift (3PEPS) measurements on the light-harvesting complex II (LHC-II) of the green algae Chlamydomonas reinhardtii. Experiments were performed at two different excitation wavelengths, 670 and 650 nm, corresponding to Chl-a and Chl-b excitation, respectively. The 3PEPS data are analyzed using a new theory that incorporates the effect of energy transfer on third-order response functions. Our theoretical model separates energy transfer dynamics from the solvation dynamics arising from coupling of the electronic transitions to the protein environment. We suggest that the protein fluctuations can be described by an ultrafast solvation on a sub-100 fs time scale and a long time correlation (static disorder). Decay of the 670 nm peak shift reveals spectral equilibration time scales for Chl-a molecules that range from 300 fs to 6 ps and agree well with other experiments. 3PEPS data at 650 nm (Chl-b excitation) reveal rapid Chl-b to Chl-b energy transfer (<1 p...