Crystal and Molecular Structure of Methyl Bacteriopheophorbide a. A Model for a Primary Electron Acceptor in Bacterial Photosynthesis

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Bacteriopheophytin is a key component in the mechanism by which photosynthetic bacteria transduce solar energy into chemical form. The structure of methyl bacteriopheophorbide a, a bacteriopheophytin derivative, has been determined by three-dimensional X-ray diffraction. The compound crystallizes in space group PI in a unit cell containing a molecule of methyl bacteriopheophorbide and a benzene molecule of crystallization, with lattice constants a = 7.2502 (8) Å, b = 8.1070 (9) Å, c = 17.224 (5) Å, α = 90.80 (2)°, β = 94.17 (2)°, and 7 = 110.38 (1)°. The structure has been refined with isotropic thermal parameters on F by full-matrix least squares to RF = 0.110, based on 860 reflections. The molecules associate in chains via π-π interactions with ring I of one molecule overlapping ring III of its neighbor with a vertical separation of 3.6 Å. The one-dimensional chains form two-dimensional layers without the intervention of the hydrogen bonding by water molecules previously reported for chlorophyllides. This work represents the first crystallographic determination of a bacteriochlorin and confirms the canonical structures and relative orientations of the five chiral centers of bacteriochlorophyll a derivatives. © 1981, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society

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