Self-aggregation behavior of synthetic zinc 3-hydroxymethyl-13/15-carbonyl-chlorins as models of main light-harvesting components in photosynthetic green bacteria.
Zinc complexes of 3-hydroxymethyl-13/15-carbonyl-chlorins having a six-membered lactone as the E-ring were prepared by modifying purpurin-18 as models of bacteriochlorophyll-d, one of the chlorophyllous pigments in the main light-harvesting antenna systems (chlorosomes) of green photosynthetic bacteria. The synthetic 13-carbonylated compound self-aggregated in 1%(v/v) tetrahydrofuran and hexane to give large oligomers possessing red-shifted and broadened electronic absorption bands and intense circular dichroism bands at the shifted Q ( y ) region, indicating that the supramolecular structure of the resulting self-aggregate was similar to those of natural and artificial chlorosomal aggregates. The red-shift value observed here was smaller than the reported values in chlorosomal pigments having a five-membered keto-ring, which was ascribable to a weaker intermolecular hydrogen-bonding of 13-C=O with 3(1)-OH in a supramolecule of the former self-aggregate and suppression of the pi-pi interaction among the composite chlorins. On the other hand, the isomeric 15-carbonylated molecule was monomeric even in the nonpolar organic solvent, confirming the reported proposal that the linear orientation of three interactive moieties, OH, C=O and Zn, in a molecule is requisite for its chlorosomal self-aggregation.
Tamiaki
Hitoshi
H
Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, 525-8577, Japan. tamiaki@se.ritsumei.ac.jp
Yoshimura
Hideaki
H
Shimamura
Yasuhide
Y
Kunieda
Michio
M
D001419:Bacteria / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D015394:Molecular Structure; D019906:Nuclear Magnetic Resonance, Biomolecular; D010788:Photosynthesis; D011166:Porphyrins / Q000737:chemistry / Q000378:metabolism*
D011166:Porphyrins; C006969:chlorin