Pigment-protein architecture in the light-harvesting antenna complexes of purple bacteria: does the crystal structure reflect the native pigment-protein arrangement?

Structural analysis of crystallized peripheral (LH2) and core antenna compl exes (LH1) of purple bacteria has revealed circular aggregates of high rota tional symmetry (C-8, C-9 and C-16, respectively). Quantum-chemical calcula tions indicate that in particular the waterwheel-like arrangements of pigme nts should show characteristic structure-sensitive spectroscopic behavior i n the near infrared absorption region. Laser-spectroscopic data obtained wi th non-crystallized, isolated LH2 of Rhodospirillum molischianum are in lin e with a highly symmetric (C8) circular aggregate, but deviations have been found for LH2 of Rhodobacter sphaeroides and Rhodopseudomonas acidophila. For both the latter, C-shaped incomplete circular aggregates (as seen only recently in electron micrographs of crystallized LH1-reaction center comple xes) may be a suitable preliminary model. (C) 2000 Federation of European B iochemical Societies. Published by Elsevier Science B,V, All rights reserve d.