Many aspects of energy and charge transfer in oxygenic photosynthesis are still poorly understood at both the level of single pigments and multi-pigment complexes. At the single pigment level, chlorophylls and chlorophyll-like pigments (including bacteriochlorophyll, pheophytin, etc.) exhibit similar structure in the Q-band absorption region that has long been accepted to have correspondingly similar electronic structure. However, evidence has been mounting that suggests that despite the qualitative similarities between chlorophyll-like pigments they are in fact unique in their electronic structure. On larger length scales, key processes in photosynthesis such as stable charge separation, are accomplished via multiple pigments embedded within a protein matrix acting in concert. Understanding all the intermediate states that occur during charge separation in oxygenic photosynthesis, particularly in the photosystem II complex, is challenging due to the large degree of spectral overlap between chlorophyll and chlorophyll-like pigments. This has made it nearly impossible to disentangle individual pigment contributions from spectroscopic signatures and understand the structure-function relationship in this important system.
This thesis presents my work that capitalizes on the advantages of two-dimensional electronic spectroscopy (2DES), particularly its ability to maintain simultaneous high temporal and spectral resolution, and incorporates further modifications to the technique that allow for studying oxygenic photosynthesis at the single pigment and multi-pigment regimes. By integrating polarization control among the multi-laser pulse experiment I was able to compare the underlying electronic and vibrational energy level structure of bacteriochlorophyll a and chlorophyll a to show how the structure of chlorophyll a deviates from the simple Gouterman model framework and lends support to the argument for vibronic models. Results of this work were supported by theoretical calculations performed by our collaborators from the group of Eitan Geva in the Chemistry Department at the University of Michigan and the group of Barry Dunietz at Kent State University. Futhermore, I used a multi-spectral 2DES technique, exciting across the Qy band and probing the higher energy Qx and carotenoid transitions in the photosystem II reaction center. The 2DES spectra reveal cross peaks between the highly congested Qy band and Qx and carotenoid transitions, providing insight into the contributions of the individual pigments to the absorption in the Qy region. Analysis of the kinetics of the 2DES data allows us to test the proposed two-pathway model of charge separation in the photosystem II reaction center.
The results of these studies emphasize the importance of the feed-back between experiment and theory in building and refining an overall understanding of oxygenic photosynthesis. We anticipate that the information obtained in these studies will contribute to building the new models of the underlying energy structure of single pigments and excitonic interactions that lead to energy and charge transfer in the reaction center of oxygenic photosynthetic complexes.PHDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/151557/1/emaret_1.pd
