title: Statistical considerations on the formation of circular photosynthetic light-harvesting complexes from Rhodopseudomonas palustris.
abstract: Depending on growth conditions, some species of purple photosynthetic bacteria contain peripheral light-harvesting (LH2) complexes that are heterogeneous owing to the presence of different protomers (containing different αβ-apoproteins). Recent spectroscopic studies of Rhodopseudomonas palustris grown under low-light conditions suggest the presence of a C 3-symmetric LH2 nonamer comprised of two distinct protomers. The software program Cyclaplex, which enables generation and data-mining of virtual libraries of molecular rings formed upon combinatorial reactions, has been used to delineate the possible number and type of distinct nonamers as a function of numbers of distinct protomers. The yield of the C 3-symmetric nonamer from two protomers (A and B in varying ratios) has been studied under the following conditions: (1) statistical, (2) enriched (preclusion of the B-B sequence), and (3) seeded (pre-formation of an A-B-A block). The yield of C 3-symmetric nonamer is at most 0.98 % under statistical conditions versus 5.6 % under enriched conditions, and can be dominant under conditions of pre-seeding with an A-B-A block. In summary, the formation of any one specific nonamer even from only two protomers is unlikely on statistical grounds but must stem from enhanced free energy of formation or a directed assembly process by as-yet unknown factors.
authors: Department of Chemistry, North Carolina State University, Raleigh, NC, 27695-8204, USA, mtanigu@ncsu.edu.
authors: Masahiko
authors: M
authors: Taniguchi
authors: Sarah
authors: S
authors: Henry
authors: Richard J
authors: RJ
authors: Cogdell
authors: Jonathan S
authors: JS
authors: Lindsey
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism*; D010788:Photosynthesis / Q000502:physiology; D012241:Rhodopseudomonas / Q000737:chemistry / Q000378:metabolism*
chemical_list: D001429:Bacteriochlorophylls; D045342:Light-Harvesting Protein Complexes