Energy conversion in Purple Bacteria Photosynthesis

The study of how photosynthetic organisms convert light offers insight not only into nature’s evolutionary process, but may also give clues as to how best to design and manipulate artificial photosynthetic systems – and also how far we can drive natural photosynthetic systems beyond normal operating conditions, so that they can harvest energy for us under otherwise extreme conditions. In addition to its interest from a basic scientific perspective, therefore, the goal to develop a deep quantitative understanding of photosynthesis offers the potential payoff of enhancing our current arsenal of alternative energy sources for the future. In the following Chapter, we consider the excitation dynamics of photosynthetic membranes in Rps. Photometricum purple bacteria. The first studies on purple bacteria photosynthetic membranes were concerned with the complex underlying detailed structure (Jamieson & al, 2002; McDermott et al., 1995; Roszack & et al, 2003; Waltz et al., 1998). The interested reader might find helpful the first section of this chapter where we present a summary of structures and processes that allow photosynthesis in purple bacteria. As improved resolution became available for light-harvesting structures, so too the community’s interest increased in understanding the details of the rapid femtoto picosecond timescales for the excitation transfer process within a given harvesting complex – even to the regime where quantum effects are expected. Indeed such quantum effects have recently been confirmed in, for instance, the Fenna-Matthews-Olson complex of green sulfur bacteria (Engel et al., 2007). However, the processes occurring at this level of detail in terms of both structure and properties of the excitation harvesting, have not yet been shown as being crucial to the performance of the full harvesting membrane, nor with the primary goal of any given photosynthetic organism: to fuel its metabolism. We focus on the transfer among different complexes, in particular the inter-complex excitation transfer. We consider first a system of few complexes in order to understand the consequences on the dynamics of the complexes’ connectedness. We also consider the relative amounts of harvesting complexes (i.e. stoichiometry) on small sized networks in order to help establish our understanding of the behavior of complete chromatophore vesicles. 3