title: 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.
abstract: 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.
authors.affiliation: Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, Japan.
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Tamiaki
authors.forename: Masaki
authors.initials: M
authors.lastname: Kuno
authors.forename: Masaki
authors.initials: M
authors.lastname: Ohata
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry*; D002734:Chlorophyll / Q000737:chemistry*; D013058:Mass Spectrometry; D008956:Models, Chemical; D066244:Proton Magnetic Resonance Spectroscopy; D012997:Solvents / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D015032:Zinc / Q000737:chemistry*
chemical_list: D001429:Bacteriochlorophylls; D012997:Solvents; D002734:Chlorophyll; D015032:Zinc