Structural and Theoretical Models of Photosynthetic Chromophores. Implications for Redox, Light Absorption Properties and Vectorial Electron Flow.

Single-crystal x-ray data for a homologous series of methyl bacteriopheophorbides d, derived from the antenna chlorophylls of the green photosynthetic bacterium Chorobium vibrioforme are presented. Crystallographically significant conformational variations are shown to be possible for the same chlorophyll skeleton. The possibility was considered that such conformational variations can affect the highest occupied and lowest unoccupied molecular orbital levels of the chlorophylls and thereby modulate their redox potentials and light adsorbing properties. This concept was tested by demonstrating that a sterically distorted porphyrin of known structure exhibited experimentally optical and redox properties in solution consonant with theory. 20 refs., 3 figs.