Chlorosomal self-aggregation of synthetic chlorophyll-c derivatives

Shogo Matsubaraa, Hitoshi Tamiakia
aGraduate School of Life Sciences, Ritsumeikan University Kusatsu, Shiga 525-8577, Japan

Green photosynthetic bacteria possess special light-harvesting antennae, so-called chlorosomes, whose core part is constructed by supramolecular self-aggregation of specific chlorophyll (Chl) molecules without protein support. The supramolecules are formed by π-π interaction of the composite chlorin moieties as well as their intermolecular hydrogen bonding of their 31-hydroxy with 13-keto-carbonyl groups and coordination of the hydroxy group with the central magnesium. Here we report on synthesis of Chl derivatives extending π-conjugation and their self-aggregation as chlorosomal models.

We synthesized chlorosomal zinc 31-hydroxy-131-oxo-porphyrin (Figure 1B) from Chl-c possessing an acrylic acid residue at the 17-position and its 171,172-dihydrogenated derivative from Chl-a bearing a propionate acid residue. Their self-aggregation in an aqueous THF solution was analyzed by optical spectroscopy and their deaggregation was examined by addition of pyridine. Chlorosomal J-aggregation of the Chl-a derivative was performed by slipped face-to-face stacking of the tetrapyrrole π-skeletons along the y-axis, giving red shifts of Soret and Qy bands [1,2]. In contrast, the Chl-c derivative showed unique self-aggregation behaviors (Figure 1A), whose visible spectra were time- and thermal-dependent.

References

[1] H. Tamiaki, M. Amakawa, Y. Shimono, S. Tanikaga, A. R. Holzwarth, K. Schaffner, Photochem. Photobiol., 63, 92-98 (1996).
[2] H. Tamiaki, H. Kitamoto, T. Watanabe, R. Shibata, Photochem. Photobiol., 81, 170-176 (2005).