PROPERTIES OF ZINC AND MAGNESIUM METHYL BACTERIOPHEOPHORBIDE-D AND THEIR AGGREGATES

Light-harvesting bacteriochlorophylls are believed to be aggregated in oligomeric forms in chlorosomes of green photosynthetic bacteria. Zn and Mg methyl bacteriopheophorbides d (MBPd) were synthesized from chl orophyll a and studied as model compounds for bacteriochlorophyll d. M onomeric Zn and Mg MBPd in methanol have Q(y) absorption maxima at 650 nm and 657 nm and fluorescence decay lifetimes of 5.1 ns and 5.4 ns, respectively, compared to 5.6 ns for bacteriochlorophyll d. Zn and Mg MBPd both form oligomers in nonpolar solvents and exhibit Q(y) absorpt ion maxima at 728 nm and 731 nm and fluorescence decay lifetimes of 14 ps and 19 ps, respectively, compared to 730 nm and 9 ps for similar b acteriochlorophyll d aggregates. One of the diastereomers at the 3(1) position, R-Mg MBPd, forms intermediate-sized aggregated species that are equivalent to the dimer and a highly fluorescent species formed by bacteriochlorophylls c and d. The similarities of quantitative proper ties between the model compounds, and the antenna pigments bacteriochl orophyll c and d indicate that Mg and Zn MBPd are good models for stud ying pigment interactions in chlorosomes and that the long hydrocarbon tail in the natural pigment is not required for oligomer formation. T he dimer and the highly fluorescent species do not appear to be the bu ilding blocks of the oligomer.