DENDROPHANES - WATER-SOLUBLE DENDRITIC RECEPTORS AS MODELS FOR BURIEDRECOGNITION SITES IN GLOBULAR-PROTEINS

Water-soluble dendritic cyclophanes (dendrophanes) of first (1, 4), se cond (2, 5), and third generation (3, 6) with poly(ether amide) branch ing and 12, 36, and 108 terminal carboxylate groups, respectively, wer e prepared by divergent synthesis, and their molecular recognition pro perties in aqueous solutions were investigated. Dendrophanes 1-3 incor porate as the initiator core a tetraoxa[6.1.6.1]paracyclophane 7 with a suitably sized cavity for inclusion complexation of benzene or napht halene derivatives. The initiator core in 4-6 is the [6.1.6.1]cyclopha ne 8 shaped by two naphthyl(phenyl)methane units with a cavity suitabl e for steroid incorporation. The syntheses of 1-6 involved sequential peptide coupling to monomer 9, followed by ester hydrolysis (Schemes I and 4). Purification by gel-permeation chromatography (GPC; Fig. 3) a nd full spectral characterization were accomplished at the stage of th e intermediate poly(methyl carboxylates) 10-12 and 23-25, respectively . The third-generation 10g-ester 25 was also independently prepared by a semi-convergent synthetic strategy, starting from 4 (Scheme 5). All dendrophanes with terminal eater groups were obtained in pure form ac cording to the C-13-NMR spectral criterion (Figs. 1 and 5). The MALDI- TOF mass spectra of the third-generation derivative 25 (mel. wt. 19328 D) displayed the molecular ion as base peak, accompanied by a series of ions [M - n(1041 +/- 7)](+),tentatively assigned as characteristic fragment ions of the poly(ether amide) cascade. A similar fragmentatio n pattern was also observed in the spectra of other higher-generation poly(ether amide) dendrimers. Attempts to prepare monodisperse fourth- generation dendrophanes by divergent synthesis failed. H-1-NMR and flu orescence binding titrations in basic aqueous buffer solutions showed that dendrophanes 1-3 complexed benzene and naphthalene derivatives, w hereas 4-6 bound the steroid testosterone. Complexation occurred exclu sively at the cavity-binding site of the central cyclophane core rathe r than in fluctuating voids in the dendritic branches, and the associa tion strength was similar to that of the complexes formed by the initi ator cores 7 and 8, respectively (Tables I and 3). Fluorescence titrat ions with 6-(p-toluidino)naphthalene-2-sulfonate as fluorescent probe in aqueous buffer showed that the micropolarity at the cyclophane core in dendrophanes 1-3 becomes increasingly reduced with increasing size and density of the dendritic superstructure; the polarity at the core of the third-generation compound 3 is similar to that of EtOH (Table 2). Host-guest exchange kinetics were remarkably fast and, except for receptor 3, the stabilities of all dendrophane complexes could be eval uated by H-1-NMR titrations. The rapid complexation-decomplexation kin etics are explained by the specific attachment of the dendritic wedges to large, nanometer-sized cyclophane initiator cores, which generates apertures in the surrounding dendritic superstructure.