H. Tamiaki et al., SYNTHETIC ZINC AND MAGNESIUM CHLORIN AGGREGATES AS MODELS FOR SUPRAMOLECULAR ANTENNA COMPLEXES IN CHLOROSOMES OF GREEN PHOTOSYNTHETIC BACTERIA, Photochemistry and photobiology, 63(1), 1996, pp. 92-99
A comparison of the spectra of in vitro (3-hydroxymethyl-13(1)-oxometa
llochlorin) and in vivo chlorosomal (bacteriochlorophyll-c) aggregates
suggests a similar supramolecular structure for the artificial oligom
ers and the bacteriochlorophyll-c aggregates in the extramembranous an
tenna complexes (chlorosomes) of green photosynthetic bacteria. Synthe
tic zinc and magnesium chlorins have been found to aggregate in 1% (vo
l/vol) tetrahydrofuran and hexane solutions and in thin films to form
oligomers with the Q(y) absorption bands shifted to longer wavelengths
by about 1900 (Zn chlorins) and 2100 cm(-1) (Mg) relative to the corr
esponding monomer bands. Visible absorption and circular dichroism spe
ctra of various zinc chlorins establish that a central metal, a 3(1)-h
ydroxy and a 13(1)-keto group are functional prerequisites for the agg
regation. Vibrational bands measured by IR spectroscopy of solid films
reveal two characteristic structural features of the oligomers: (1) a
five-coordinated metallochlorin macrocycle with an axial ligand (band
s at 1500-1630 cm(-1)), and (2) a hydrogen bond between the keto oxyge
n of one chlorin and the hydroxy group of a second chlorin, the oxygen
of which is chelated to the metal atom of a third molecule, i.e. C=O
... H-O ... M (=Zn or Mg).