Aggregation of Modified Zinc Chlorins in Nonpolar Solvents-Bacteriochlorophyll c Mimics with Interchanged Hydroxy and Carbonyl Functions†
Dipl.-Chem. Aldo Jesorka
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]
Search for more papers by this authorDr. Teodor Silviu Balaban
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]
Search for more papers by this authorCorresponding Author
Prof. Dr. Alfred R. Holzwarth
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]Search for more papers by this authorProf. Dr. Kurt Schaffner
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]
Search for more papers by this authorDipl.-Chem. Aldo Jesorka
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]
Search for more papers by this authorDr. Teodor Silviu Balaban
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]
Search for more papers by this authorCorresponding Author
Prof. Dr. Alfred R. Holzwarth
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]Search for more papers by this authorProf. Dr. Kurt Schaffner
Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim an der Ruhr (Germany), Fax: Int. code +(208) 306-3952, e-mail: [email protected]
Search for more papers by this authorWe thank Dr. D. Müller, Ruhr-Universität Bochum, for FAB high-resolution mass spectra and M. Trinoga, J. Bitter and K. Sand for experimental assistance.
Graphical Abstract
Self-assembly of functionally “inverse” zinc chlorins—31-hydroxy and 131keto groups, which are functionally essential in the bacteriochlorophylls c, d, and e, have been interchanged—is possible in nonpolar solvents (see schematic representation below). This result sets structural conditions for the formation of large aggregates similar to those formed by bacteriochlorophylls in vitro and in light-harvesting antenna systems of photosynthetic bacteria.
References
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- 19 Satisfactory analytical data have been obtained for all new compounds.
- 20 After elution with aqueous methanol from the HPLC column and evaporation of the solvents, a sharp blue-shifted peak at 622 nm is visible in the dichloromethane solutions of 6a-Zn diluted with hexane. This peak is removed after washing with brine a dichloromethane extract of 6a-Zn. We tentatively assign this peak to a methanol-bridged dimeric species.
- 21 For 6a-Zn and 6b-Zn in dichloromethane solution the ester CO band is at 1733 cm−1, that of the aldehyde at 1664 cm−1, and a chlorin mode at 1606 cm−1. Upon addition of methanol the ester CO band remains unshifted, whereas the aldehyde CO appears at 1657 2 cm−1, probably due to hydrogen bonding to methanol. When dichloromethane solutions are diluted with hexane, the ester CO band is slightly shifted to larger wavenumbers (1738 cm−1), whereas the aldehyde carbonyl band is strongly shifted to lower frequencies [broad band at 1640 cm−1 for the more soluble (131S) epimer]. This is reminiscent of the strongly down-shifted 131-carbonyl vibration in BChl c aggregates and chlorosomes [9 b]. We correspondingly assign this latter band to the aldehyde carbonyl group strongly hydrogen-bonded to a hydroxy group, which simultaneously coordinates with a zinc atom.
- 22 The proportion of red-shifted oligomers is slightly greater on aggregation in pentane than for that in hexane and heptane. Furthermore, the amount is also determined by the concentration of the CH2Cl2 stock solution: the higher this concentration, the more aggregated species are formed, as is expected from a cooperative process for the self-assembly. This has been shown for BChl c oligomers (predominantly tetramers; cooperativity of 3.6) [15].
- 23 Solutions of 5a-Zn and 5b-Zn in hexane tend to form deposits upon standing, albeit without a red shift in absorption of the supernatant.
- 24 For a first example of a functional unit based on the self-organized assembly of zinc chlorins coupled to a metal-free bacteriochlorin as energy acceptor: H. Tamiaki, T. Miyatake, R. Tanikaga, A. R. Holzwarth, K. Schaffner, Angew. Chem. 1996, 108, 810–812; Angew. Chem. Int. Ed. Engl. 1996, 35, 772–774.
