Self-assembled zinc pheoporphyrin dimers. Models for the supramolecular antenna complexes of green photosynthetic bacteria?

The molecular structure of the zinc porphyrin analog of methyl-8,12-diethylbacteriopheophorbide d has been determined by X-ray diffraction. The crystallographic study was undertaken to provide structural foundations for the several models proposed for the supramolecular organization of bacteriochlorophyll (BChl) c, d and e antenna chromophores in vivo and in vitro. Surprisingly, the Zn porphyrins form hydrogen-bonded dimers with extended pi-pi interactions without the expected ligation of the Zn by the 3-(1-hydroxyethyl) group that distinguishes the BChls c, d and e from all other photosynthetic (bacterio) chlorophylls. The crystallographic results provide the first structural benchmarks for this class of metalloporphyrin derivatives and set structural boundaries for other modes of aggregation considered for the chromophores in vitro and in vivo. (C) 1997 Elsevier Science B.V.