Self-aggregation of Synthetic Protobacteriochlorophyll-d Derivatives¶

Regioisomerically Controlled Self-Aggregation of Zinc 3-Hydroxymethyl-13-formyl-chlorin/Porphyrin and Their 3,13-Inverted Pigments

Zinc 3-hydroxymethyl-13-formyl-chlorin, 1, and its 3,13-inverted (3-formyl-13-hydroxymethyl) regioisomer, 2, and their corresponding 17,18-dehydrogenated porphyrins, 3 and 4, were synthesized for models of natural bacteriochlorophylls-c/d/e possessing 3(1)-OH and 13-C=O groups which self-aggregate in main light-harvesting antenna systems of green photosynthetic bacteria. Zinc chlorins 1 and 2 were monomers in neat THF and gave an obvious difference in their visible absorption spectra, indicating that sole inversion of the 3- and 13-substituents in a chlorin chromophore controlled their optical properties. In an aqueous Triton X-100 solution (a nonionic surfactant), zinc 3(1)-OH-13-CHO-chlorin 1 and porphyrin 3 self-aggregated as do natural bacteriochlorophylls, while zinc 3-CHO-13(1)-OH chlorin 2 and porphyrin 4 (the 3,13-inverted regioisomers of 1 and 3) hardly formed such large oligomers, showing that the inversion of the peripheral 3,13-substituents made their oligomerization unfavorable. FT-IR spectra of aggregated 1-4 in the solid film and their molecular modeling calculations suggested that the 17(2)-C=O moiety in inverted 2/4 interacted with its own 13(1)-OH group to disturb further aggregation.

Synthesis and Optical Properties of Bacteriochlorophyll-a Derivatives Having Various C3 Substituents on the Bacteriochlorin π-System

Methyl bacteriopyropheophorbide-a derivatives having a series of substituents at the C3 position were prepared and their optical properties were compared with the corresponding chlorin analogues. Two kinds of oxidation reaction (C3-vinyl --> formyl --> carboxy group) were found to be applicable with a little alteration of the free-base bacteriochlorin macrocycles. The Qx and Qy electronic absorption peak positions of synthetic bacteriochlorins in CH2Cl2 were affected by the C3 substituents and found to be more sensitive than those of the chlorins. The observed Qx/Qy peaks in their monomeric states were shifted to a longer wavelength in the order of 1-hydroxyethyl < hydroxymethyl < acetoxymethyl < vinyl < acetyl < carboxy < formyl < 2,2-dicyanoethynyl group. Zinc complex with the C3-hydroxymethyl group formed self-aggregates in a nonpolar organic solvent, which showed the largest red-shift of the Qy band (2380 cm(-1), 726 nm in THF to 878 nm in 1% THF-cyclohexane) among those of the synthetic self-aggregative (bacterio)chlorins examined.

Effects of peripheral substituents on diastereoselectivity of the fifth ligand-binding to chlorophylls, and nomenclature of the asymmetric axial coordination sites

A preference for one of the two axial ligand-binding sites on the central metal atom of chlorophylls (Chls) and bacteriochlorophylls (BChls) was confirmed. In recently reported crystallographic data on PS2 and LHC2 complexes, there are 42 Chl molecules whose fifth ligands were identified; 33 of 42 molecules bound the fifth ligand at the axial position where the C13(2)-methoxycarbonyl group protrudes (denoting as the 'back'-type isomer). Among 151 (B)Chl a/b molecules found in eight types of (B)Chl proteins including PS2 and LHC2, 124 molecules (82%) are the 'back'-type isomers. Such a statistical selection was observed not only for Chl a but also for Chl b and BChls a/b, indicating that the C3-, C7-, and C8-substituents as well as the macrocyclic pi-conjugates would have little influence on the ligand-binding site. Computational examinations revealed that the energetic gap between the 'back' and its opposite 'face' complexes was inherent to (B)Chls and that the C13(2)-methoxycarbonyl moiety contributed relatively greatly to the diastereomeric preference in the ligand binding. Nomenclature of the two distinguishable sides on chlorophyllous macrocycles, as well as the two asymmetric ligand-binding sites, is also discussed.