Observation of Coherence in the Photosystem II Reaction Center and Chlorophyll a.

Abstract

Oxygenic photosynthesis powers life on Earth by converting solar energy into chemical energy. The primary conversion steps occur in the photosystem II reaction center (PSII RC). The design principles of the PSII RC are not yet fully understood. In particular open questions remain about its electronic structure and charge separation mechanism.

Two dimensional electronic spectroscopy (2DES) is a powerful method for investigating the electronic structure of photosynthetic systems. 2DES has improved our ability to excite and observe coherent dynamics in a wide range of systems. Observation of coherent dynamics in photosynthetic systems have generated considerable debate about their physical origin and possible functional relevance. Coherent dynamics in the PSII RC have been observed to persist on a similar timescale to charge separation. The coherences are believed to reflect electronic/vibrational resonances that may be important for charge separation. This thesis aims to understand the physical origin of the coherent dynamics in the PSII RC by comparison studies with the chlorophyll a monomer. In both the PSII RC and chlorophyll a we use 2DES to determine the spectroscopic signatures of the coherent dynamics of several modes. We compare these with the expected signatures within several simple models. We find that the PSII RC can be well described by a vibronic model and that chlorophyll a shows deviations from the simple displaced harmonic oscillator.

We have developed two-color rapid acquisition coherence spectroscopy (TRACS), a more sensitive approach to study coherent dynamics. We have applied this method to chlorophyll a and showed dramatic increase in signal to noise and decrease in total experiment time compared to typical 2DES measurements. We demonstrate that TRACS can distinguish between ground and excited state coherent dynamics, and provide high frequency resolution of the coherent dynamics.

To address the role of coherent dynamics observed in the PSII RC, we have utilized TRACS to investigate the 790-820 nm region which has anion band markers for charge separation. Here, we report observation of coherent dynamics in the 790-820 nm spectral region, but require further studies to resolve the anion band contribution and to probe the functional significance of these coherent processes.