Synthesis of chlorophyll-a derivatives methylated in the 3-vinyl group and their intrinsic site energy.

Wittig reaction of methyl pyropheophorbide-d possessing the 3-formyl group gave readily methyl pyropheophorbides-a bearing a variety of 3-alkenyl groups as semi-synthetic models of chlorophyll-a. The 3-substituents rotated around the C3-C3(1) bond from the coplanar conformation with the chlorin π-system, moving the redmost visible absorption maxima to a shorter wavelength. The model experiments showed that natural chlorophyll-a carrying the 3-vinyl group would take a similar rotamer to control its intrinsic site energy.

Self-aggregation of synthetic zinc chlorophyll derivatives possessing 3(1)-hydroxy or methoxy group and 13(1)-mono- or dicyanomethylene moiety in nonpolar organic solvents as models of chlorosomal bacteriochlorophyll-d aggregates.

Methyl 13(1)-(di)cyanomethylene-pyropheophorbides were synthesized by Knoevenagel reactions of the corresponding 13(1)-oxo-chlorins prepared from modifying chlorophyll-a with malononitrile or cyanoacetic acid. Alternatively, methyl 13(1)-cyanomethylene-pyropheophorbides were produced by Wittig reactions of 13(1)-oxo-chlorins with Ph3P=CHCN. Self-aggregation of zinc complexes of the semi-synthetic chlorophyll derivatives possessing a hydroxy or methoxy group at the 3(1)-position was examined in 1%(v/v) tetrahydrofuran or dichloromethane and hexane by electronic absorption and circular dichroism spectroscopy. Although intermolecular hydrogen-bonding between the 3(1)-hydroxy and 13(1)-oxo groups of bacteriochlorophylls-c/d/e/f was essential for their self-aggregation in natural light-harvesting antenna systems (=chlorosomes), zinc 3(1)-hydroxy-13(1)-di/monocyanomethylene-chlorins self-aggregated in the less/lesser polar organic solvents to form chlorosome-like large oligomers in spite of lacking the 13(1)-oxo moiety as the hydrogen-bonding acceptor. Zinc 3(1)-methoxy-13(1)-dicyanomethylene-chlorin gave similar self-aggregates regardless of lack of both the 3(1)-hydroxy and 13(1)-oxo groups. The present self-aggregation was ascribable to stronger coordination of the 3(1)-oxygen atom to the central zinc than the conventional systems, where the electron-withdrawing cyano group(s) increased the coordinative ability of the central zinc through the chlorin π-system.