Two-Dimensional Electronic Spectroscopy of the Photosystem II D1D2-cyt.b559 Reaction Center Complex: Experiment and Simulation.

Abstract

Oxygenic photosynthesis is key to life on this planet, and photosystem II is key to oxygenic photosynthesis. The only natural molecule capable of splitting water, it has been studied extensively with a wide range of linear and nonlinear spectroscopic methods. Still, the energy and charge transfer pathways remain poorly understood. Two dimensional electronic spectroscopy (2DES) extends previous non-linear spectroscopies into an additional frequency axis, uncovering information about electronic coupling and energy transfer that is difficult to discern in other methods.

This thesis presents technical advances to 2DES with a pulse shaper in the pump-probe geometry, particularly phase-cycling for isolating signals of interest and for reducing scatter signals. This method is applied to the first 2DES measurements of the Qy band of the D1D2-cyt.b559 reaction center of photosystem II (PSII RC). A new method for extracting kinetic information from such a rich data set is presented: two dimensional decay associated spectra. The 2DES data directly reveal excitonic coupling between “blue” and “red” states within the band. The rapid growth of a cross-peak below the diagonal provides unambiguous evidence for energy equilibration within the reaction center on the order of 100 fs. Spectrally dependent lifetimes of 2-3 ps are observed, in agreement with a recent model in which charge separation occurs along two distinct pathways. Slower time constants of ~7 ps and ~50 ps are consistent with slow energy transfer from peripheral chlorophylls and secondary charge transfer, respectively.

The first simulations of the PSII RC are presented and compared to experiment. The simulations examine a well-tested model for the excitonic structure of the PSII RC, which provides a good description for linear absorption, linear dichroism, circular dichroism, steady-state fluorescence, triplet-minus-singlet as well as Stark spectra. The resulting simulations match neither the experimental lineshapes nor the observed kinetics, revealing the power of 2DES for constraining theoretical models. An improved version of this model is proposed that gives qualitatively better lineshapes, although still fails to predict the observed kinetics. The thesis concludes with a brief discussion of future experimental and simulation work that is needed that builds on the work presented here.