SELF-ASSEMBLY OF METHYL ZINC (3(1)R)-BACTERIOPHEOPHORBIDE-D AND (3(1)S)-BACTERIOPHEOPHORBIDE-D

The methyl zinc (3(1)R)- and (3(1)S)-[8Et, 12Me]bacteriopheophorbides d-analogs of the light-harvesting bacteriochlorophylls in photosynthet ic bacteria-self-assemble in nonpolar solvents. While in dilute dichlo romethane solution both epimers prevail in their monomeric form, compl ex equilibria of aggregates with gradually red-shifting Q(y) absorptio ns are formed at either higher concentration or lower temperature or u pon dilution with hexane. Dynamic H-1-NMR and FT-IR spectroscopies sho w that the 3(1)-hydroxy group participates directly in the self-assemb ly through oxygen ligation to zinc and through hydrogen bridging to th e 13(1)-keto group of the ligated hydroxyls in intermediate unsymmetri cal dimers. The basic unit, combining three molecules through >C=O ... H-O ... Zn bonding, and the existence of equilibria between monomers, dimers, and oligomers of varying size depending on the conditions par allel the scheme proposed previously for bacteriochlorophylls c and d (Chiefari, J.; et al. J. Phys. Chern. 1995, 99, 1357-1365. Holzwarth, A, R.; Schaffner, K, Photosynth. Res. 1994, 41, 225-233). In some resp ects the CD,H-1-NMR, and IR spectral behavior of the two epimers indic ates diastereoselective conformational and kinetic differences at the levels of dimers and larger oligomers, Inter alia, the self-assembly o f a 1:1 mixture of the (3(1)R) and (3(1)S) epimers proceeds more rapid ly than that of the separate epimers. This is possibly of relevance wi th regard to the rodlike aggregates in the interior of the chlorosomes of certain bacterial species which are composed of both 3(1)-epimeric bacteriochlorophylls.