Pl. Cheng et al., PROPERTIES OF ZINC AND MAGNESIUM METHYL BACTERIOPHEOPHORBIDE-D AND THEIR AGGREGATES, Photochemistry and photobiology, 58(2), 1993, pp. 290-295
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.