Self-Assembly of Methyl Zinc (3R)- and (31S)-Bacteriopheophorbides d

Abstract

The methyl zinc (31R)- and (31S)-[8Et,12Me]bacteriopheophorbides danalogs of the light-harvesting bacteriochlorophylls in photosynthetic bacteriaself-assemble in nonpolar solvents. While in dilute dichloromethane solution both epimers prevail in their monomeric form, complex equilibria of aggregates with gradually red-shifting Qy absorptions are formed at either higher concentration or lower temperature or upon dilution with hexane. Dynamic 1H-NMR and FT-IR spectroscopies show that the 31-hydroxy group participates directly in the self-assembly through oxygen ligation to zinc and through hydrogen bridging to the 131-keto group of the ligated hydroxyls in intermediate unsymmetrical dimers. The basic unit, combining three molecules through >CO···H−O···Zn bonding, and the existence of equilibria between monomers, dimers, and oligomers of varying size depending on the conditions parallel the scheme proposed previously for bacteriochlorophylls c and d (Chiefari, J.; et al. J. Phys. Chem. 1995, 99, 1357−1365. Holzwarth, A. R.; Schaffner, K. Photosynth. Res. 1994, 41, 225−233). In some respects the CD, 1H-NMR, and IR spectral behavior of the two epimers indicates diastereoselective conformational and kinetic differences at the levels of dimers and larger oligomers. Inter alia, the self-assembly of a 1:1 mixture of the (31R) and (31S) epimers proceeds more rapidly than that of the separate epimers. This is possibly of relevance with regard to the rodlike aggregates in the interior of the chlorosomes of certain bacterial species which are composed of both 31-epimeric bacteriochlorophylls.