Self-Aggregation of Synthetic Zinc Oxo-Bacteriochlorins Bearing Substituents Characteristic of Chlorosomal Chlorophylls

Abstract

We prepared novel zinc 8-ethyl-8-methyl-7-oxo- and 7-ethyl-7-methyl-8-oxo-bacteriochlorins 1 and 2 possessing substituents characteristic of chlorosomal chlorophylls, exclusively observed in extramembraneous light-harvesting antennas of photosynthetic green bacteria. The electronic absorption spectra of monomeric 1 and 2 in THF were obviously different: the Q(y) maximum of the former was 724 and that of the latter was 683 nm. This observed spectral difference was clearly explained by theoretical ZINDO/S calculation of their energetically minimized molecules. The optical properties of monomeric 1/2 were controlled by the electronic effect of the 7/8-oxo groups. Specific spectral changes in the electronic, CD, and FT-IR absorption spectra by dilution of the monomeric THF solutions of 1/2 with a 100/200-fold volume of cyclohexane showed the formation of chlorosomal self-aggregation species constructed by 13-C=O...H-O(3(1))...Zn and pi-pi stacking. Especially, the red-shift values in the Q(y) band of 1/2 by self-aggregation were 2450/1970 cm(-1), indicating that exciton interaction among the composite molecules in the self-aggregation of 1 was stronger than in those of 2. Molecular model calculations for dodecamers of 1/2 based on a parallel chain arrangement gave partially different supramolecular structures; the specific hydrogen-bonding distances in 2-dodecamer were larger than those of 1-dodecamer, while both coordinations gave the same Zn-O distance. These modeling results showed that 1 was more tightly packed in the self-aggregates to give a larger red-shift value in the Q(y) band by self-aggregation than 2. The difference in the supramolecular structures is mainly ascribable to the steric effect of 8/7-dialkyl groups in self-aggregates of 1/2.

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

“…Moreover, chlorosome-like self-aggregation of synthetic porphyrins possessing three chlorosomal moieties has also been reported, [36,45] where the 13-formyl-(or keto)-carbonyl group in synthetic zinc 3 1 -hydroxyporphyrin acts as a chlorosomal substituent without the E-ring, thus suggesting that conformationally free 13-COR (R = H, CH 3 ) is also acceptable for the formation of such selfaggregates. Furthermore, our FT-IR experiments have indicated that the zinc chlorins 1-3 self-aggregate using only the 13-methoxycarbonyl group, in a similar manner to natural BChl-e (7-CHO) [18] and previously reported (bacterio)chlorins, [42,46,47] where an additional C=O moiety at the 7-, 8-, or 13 2 -position is excluded from the hydrogen-bonding network. Thus, the 13-COOCH 3 group in the zinc chlorins 1-3 is sufficient to induce chlorosomal self-aggregation.…”

“…Since most of the synthetic models prepared mimic the molecular structure of natural BChls by possessing the E-ring, their absorption and emission properties are similar to natural BChls. The self-aggregated porphyrin [23,48] and bacteriochlorin molecules [23,42,49] reported previously have their oligomeric Qy maxima at shorter (640-650 nm) and longer wavelength regions (780-880 nm), respectively, compared with the typical chlorin self-aggregates. These variations are accomplished by modifying the π-conjugated systems of the composite molecules.…”

“…Furthermore, the dihedral angles C12-C13-C13 1 -O13 2 and C14-C13-C13 1 -O13 2 in 1-4 and 5/6 are different from each other. The energy-minimized molecular structures of 1/3/5 obtained by MM+/PM3 calculations [42] show that the 13-methoxycarbonyl group in 1/3 deviates slightly from the chlorin π-plane (1.20/6.79°for C12-C13-C13 1 -O13 2 and -178.94/-175.56°for C14-C13-C13 1 -O13…”

“…Furthermore, the dihedral angles C12-C13-C13 1 -O13 2 and C14-C13-C13 1 -O13 2 in 1-4 and 5/6 are different from each other. The energy-minimized molecular structures of 1/3/5 obtained by MM+/PM3 calculations [42] show that the 13-methoxycarbonyl group in 1/3 deviates slightly from the chlorin π-plane (1.20/6.79°for C12-C13-C13 1 -O13 2 and -178.94/-175.56°for C14-C13-C13 1 -O13 2 ), while the 13-keto-carbonyl group in 5 is almost planar with it (-0.09°for C12-C13-C13 1 -O13 2 and 179.30°f or C14-C13-C13 1 -O13 2 ) due to fixation of the E-ring. The conformational variation and the slightly deviated situation of the 13-methoxycarbonyl group of 1-4 should alter their π-conjugated systems and shift their absorption bands to lower wavelength.…”

One‐Dimensional, Cofacial Porphyrin Polymers Formed by Self‐Assembly of meso‐Tetrakis(ERE Donor) Zinc(II) Porphyrins

“…The way in which bacteriochlorophyll (BChl) molecules self-assemble to form the light-harvesting complexes found in the chlorosomes of green sulfur bacteria, however, remains a subject of debate in the absence of Xray crystallographic evidence. In particular, Tamiaki and coworkers [8,[84][85][86][87] and Balaban et al [7,[88][89][90] have put much effort in devising (semi-)synthetic mimics of self-assembling BChl units to investigate the self-assembling structures. Balaban recently made a very convincing case for a model, in which BChl monomers are assumed to selfassemble through intermolecular interactions between the central magnesium atom of one BChl molecule, the ketone oxygen atom of one of its BChl neighbors, and the hydroxy oxygen atom of the other neighbor (Figure 14, right).…”