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J. Am. Chem. SOC.1990, 112, 9408-9410
Hydrogen-Bonding Self- Assembly of Multichromophore Structures Paolo Tecilla,’ Robert P. Dixon, Gregory Slobodkin, David S. Alavi, David H. Waldeck, and Andrew D. Hamilton’ Department of Chemistry, University of Pittsburgh Pittsburgh, Pennsylvania 15260 Received June 25, 1990 The process of self-assembly involves the noncovalent interaction of two or more molecular subunits to form an aggregate whose novel structure and properties are determined by the nature and positioning of the components. Complex, multicomponent aggregates require intermolecular interactions that are both directional and selective, such as hydrogen bonds. Natural examples of such self-assembly are provided by double-helical* and triple-helica13 DNA or the multichromophore structures of the photosynthetic reaction center4 or light-harvesting antennae a p p a r a t ~ s . ~In these cases, the structure of the aggregate is determined by the chemical information encoded in the oligonucleotide sequence or in the arrangement of binding groups on the protein surfaces. Our goal in this work was to develop simple chemical analogues of self-assembly in solution. We sought to link redox or photoactive chromophores to simple hydrogen-bonding subunits6 Depending on the complementarity of the binding regions, the subunits would form ordered aggregates with electron or energy communication between the chromophores.’ The simplest aggregate would be of type 1 in which two different subunits are linked by a single binding region (Scheme I). Incorporation of two H-bonding sites into the central subunit would lead to symmetrically self-assembled triad 2. This novel noncovalent approach to the construction of photoactive systems is simpler and potentially more general than using covalent linkages between chromophores.8 The self-asscmbling interactions were based on the hexa-hydrogen-bonding complementarity between barbiturate derivatives and two 2,6-diaminopyridine units linked through an isophthalate s p a c ~ r . More ~ basic 4-alko~ypyridines’~ were used in the recognition design in order to maximize the association constants” and so provide significant subunit aggregation, even at low (75%) component is given in the table.2' The biexponential decay may be due to the presence of two conformers, which may differ in the side-chain torsional angles or have different backbone conformations. 'In acetonitrile, the remote heavy atom effect enhances the intersystem-crossing rate constant by an additional (26.6 ns)-l - (54.4 ns)-l = 2.0 x IO7 s-1.
a strong helical bias9 and two guest aromatic residues (see Figure I ) . The guest aromatic residues in both octamers were installed (9) Short oligopeptides rich in a-aminoisobutyric acid (Aib) have been selected for the present work due to their proven abilityI0to form 3i0 helices in solution" and crystallinei2 hases arising from the steric constraints of the gem-dimethyl groups of Aibf3 (10) Venkataram Prasad, B. V.; Balaram. P. Crit. Reu. Biochem. 1984, 16,307. Karle, I . L.; Sukumar, M.; Balaram, P. Proc. Narl. Acad. Sci. U.S.A. 1986, 83, 9284 and references therein. ( I I ) (a) Toniolo, C.; Bonora, G. M.; Barone, V.; Bavoso, A.; Benedetti, E.; Di Blasio, E.; Grimaldi, P.; Lelj, F.; Pavone, V.; Pedone, C. Macromolecules 1985, 18, 895. (b) Nagaraj, R.; Balaram, P. Biochemisrry 1981, 20, 2828. (c) Vijayakumar, E. K.S.;Balaram, P. Biopolymers 1983, 22, 2133. Paterson, Y.; Stimson, E.; Evans, D. J.; Leach, S.J.; Scheraga, H. A. Int. J . Pept. Prorein Res. 1982, 20, 468. (12) (a) Toniolo, C.; Bonora, G. M.; Bavoso, A,; Benedetti, E.; Di Blasio, E.; Pavone, V.; Pedone. C. Macromolecules 1986, f9.472. (b) Bavoso, A.; Benedetti, E.; Di Blasio, E.; Pavone, V.; Pedone, C.; Toniolo, C.; Bonora, G. M. Proc. Nail. Acad. Sci. U.S.A. 1986, 83, 1988. ( I 3) Paterson, Y.; Rumsey, S.M.;Benedetti, E.; Nemethy, G.;Scheraga, H. A . J . Am. Chem. Soc. 1981, 103, 2947.
0 1990 American Chemical Society
