title: Mimics of the self-assembling chlorosomal bacteriochlorophylls: regio- and stereoselective synthesis and stereoanalysis of acyl(1-hydroxyalkyl)porphyrins.
abstract: Diacylation of copper 10,20-bis(3,5-di-tert-butylphenylporphyrin) using Friedel-Crafts conditions at short reaction times, high concentrations of catalyst, and 0-4 degrees C affords only the 3,17-diacyl-substituted porphyrins, out of the 12 possible regioisomers. At longer reaction times and higher temperatures, the 3,13-diacyl compounds are also formed, and the two isomers can be conveniently separated by normal chromatographic techniques. Monoreduction of these diketones affords in good yields the corresponding acyl(1-hydroxyalkyl)porphyrins, which after zinc metalation are mimics of the natural chlorosomal bacteriochlorophyll (BChl) d. Racemate resolution by HPLC on a variety of chiral columns was achieved and further optimized, thus permitting easy access to enantiopure porphyrins. Enantioselective reductions proved to be less effective in this respect, giving moderate yields and only 79% ee in the best case. The absolute configuration of the 3(1)-stereocenter was assigned by independent chemical and spectroscopic methods. Self-assembly of a variety of these zinc BChl d mimics proves that a collinear arrangement of the hydroxyalkyl substituent with the zinc atom and the carbonyl substituent is not a stringent requirement, since both the 3,13 and the 3,17 regioisomers self-assemble readily as the racemates. Interestingly, the separated enantiomers self-assemble less readily, as judged by absorption, fluorescence, and transmission electron microscopy studies. Circular dichroism spectra of the self-assemblies show intense Cotton effects, which are mirror-images for the two 3(1)-enantiomers, proving that the supramolecular chirality is dependent on the configuration at the 3(1)-stereocenter. Upon disruption of these self-assemblies with methanol, which competes with zinc ligation, only very weak monomeric Cotton effects are present. The favored heterochiral self-assembly process may also be encountered for the natural BChls. This touches upon the long-standing problem of why both 3(1)-epimers are encountered in BChls in ratios that vary with the illumination and culturing conditions.
authors.affiliation: Institute for Nanotechnology, Karlsruhe Institute of Technology, Postfach 3640, D-76021 Karlsruhe, Germany. silviu.balaban@int.fzk.de
authors.forename: Teodor Silviu
authors.initials: TS
authors.lastname: Balaban
authors.forename: Anil Dnyanoba
authors.initials: AD
authors.lastname: Bhise
authors.forename: Gerhard
authors.initials: G
authors.lastname: Bringmann
authors.forename: Jochen
authors.initials: J
authors.lastname: Bürck
authors.forename: Cyril
authors.initials: C
authors.lastname: Chappaz-Gillot
authors.forename: Andreas
authors.initials: A
authors.lastname: Eichhöfer
authors.forename: Dieter
authors.initials: D
authors.lastname: Fenske
authors.forename: Daniel C G
authors.initials: DC
authors.lastname: Götz
authors.forename: Michael
authors.initials: M
authors.lastname: Knauer
authors.forename: Tadashi
authors.initials: T
authors.lastname: Mizoguchi
authors.forename: Dennis
authors.initials: D
authors.lastname: Mössinger
authors.forename: Harald
authors.initials: H
authors.lastname: Rösner
authors.forename: Christian
authors.initials: C
authors.lastname: Roussel
authors.forename: Michaela
authors.initials: M
authors.lastname: Schraut
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Tamiaki
authors.forename: Nicolas
authors.initials: N
authors.lastname: Vanthuyne
mesh_terms: D000107:Acetylation; D001429:Bacteriochlorophylls / Q000737:chemistry*; D040761:Biomimetic Materials / Q000138:chemical synthesis* / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid; D003300:Copper / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008665:Metalloporphyrins / Q000138:chemical synthesis / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D013237:Stereoisomerism
chemical_list: D001429:Bacteriochlorophylls; D008665:Metalloporphyrins; D003300:Copper