Self‐Aggregation of Synthetic Zinc Chlorins Possessing a 13‐Ester‐Carbonyl Group as Chlorosomal Chlorophyll Models

Abstract

We have synthesized zinc 31‐hydroxychlorins 1–4 possessing a methoxycarbonyl group at the 13‐position by modifying naturally occurring chlorophyll‐a. Synthetic zinc chlorins 1–4 have three specific substituents, namely 31‐OH, central Zn and 13‐C=O moieties, along the Qy axis, and 1–3 self‐aggregate in an aqueous medium containing Triton X‐100, as do natural bacteriochlorophylls in the main light‐harvesting antennas (chlorosome) of green photosynthetic bacteria. The 13‐methoxycarbonyl group in 1–3 is a prerequisite for such self‐aggregation, while the other methoxycarbonyl groups at the 151‐ and/or 172‐positions are not. This indicates that such a linear situation of these substituents is significant for chlorosomal self‐aggregation of chlorophyllous pigments and also that the 13‐methoxycarbonyl group is an alternative to the 13‐keto‐carbonyl group in natural pigments. Furthermore, the resulting oligomers of 1–3 have fluorescence emission peaks at less than 700 nm, which are considerably more blue‐shifted than previously prepared self‐aggregates of chlorins possessing the 13‐keto‐carbonyl group fixed onto an exo five‐membered ring, and are a new class of artificial light‐harvesting systems. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Self-aggregation behavior of synthetic zinc 3-hydroxymethyl-13/15-carbonyl-chlorins as models of main light-harvesting components in photosynthetic green bacteria

“…The weak hydrogen bonding in (1a) n decreased the p-p interaction among the composite chlorophyllous pigments, leading to the lower red-shifted value in the Q y absorption peak. The similar suppression was reported in self-aggregation of zinc 3 1 -hydroxy-chlorin possessing the 13-methoxycarbonyl group (Kunieda and Tamiaki 2006).…”

“…4 ? 1 (as shown in Scheme 1 below) was performed by the same procedures as in the previous report (Kunieda and Tamiaki 2006): 3a and 3b were not purified and the crude products were used for further reduction. Purpurin-18 stearyl ester (2c) was prepared by similar procedures [1-ethyl-3-[3-(N,N-dimethylamino)propyl]carbodiimide (EDC) and 4-(N,N-dimethylamino)pyridine (DMAP) in Scheme 1] as described in the literature (Shibata and Tamiaki 2006).…”

“…The 3-vinyl group of separated 2a and 2b was oxidized with osmium tetraoxide and sodium periodate and the resulting 3-formyl group of 3a and 3b was reduced with tert-butylamine borane complex (Kunieda and Tamiaki 2006) to give 3-hydroxymethyl-chlorins 4a and 4b in a moderate yield (about 50%) for the two steps. The free bases 4a and 4b were metallated with zinc acetate (Kunieda and Tamiaki 2006) to afford desired zinc complexes 1a and 1b nearly quantitatively after purification of RP-HPLC.…”

“…The films gave almost the same visible absorption spectra as the corresponding aggregated solutions. For monomerization of the chlorophyllous molecules in the film, pyridine vapor was treated as reported previously (Kunieda and Tamiaki 2006). The observed data are summarized in Table 2.…”

The Length of Esterifying Alcohol Affects the Aggregation Properties of Chlorosomal Bacteriochlorophylls

“…Photochemistry and Photobiology, 2008, 84: 1187-1194 nonpolar solvents or in aqueous buffers with the addition of lipids, detergents or carotenoids (20)(21)(22)(23). Self-aggregation was also reported for synthetic chlorins with the hydroxy and keto groups located along the Q y axis of the chlorin derivative and with coordinated central metal ions (24,25); however, it is also possible to use other recognition groups for the formation of synthetic BChl aggregate mimics (26). Extensive research into BChl aggregates and artificial self-assembling porphyrins is reviewed elsewhere (1,2,4,(27)(28)(29).…”