No abstract
An unprecedented, high degree of helicity as judged by CD spectroscopy is observed in N-templated model peptides of the type AcHel-(Ala Lys) Ala -NH (AcHel-Ala peptide pictured; AcHel is an N-terminal helix-inducing template for polypeptides). These results raise concern over the current methods for determining 100 % helicity.
A two-state helix-coil model underlies all calculations of fractional helicities FH from CD spectra of helical polypeptides. The presence of an isodichroic point near 203 nm is widely assumed to validate this model, but is shown here to provide inadequate validation for alanine-rich peptides. A parametric correlation with constant slope B between CD ellipticities at a pair of wavelengths is introduced as a more rigorous two-state test. Correlations of temperature-dependent [theta](222) vs [theta](208) values are reported for a variety of peptides. Constant slopes B are observed for literature CD data obtained from fragments of helical proteins and dimeric helical coiled coils, but parametric correlations of CD data for alanine-rich peptides consistently exhibit anomalous concave upward curvature, characterized by local slopes that are linearly temperature dependent. Low-temperature CD studies together with parametric correlations at a series of wavelengths demonstrate that the curvature anomaly is maximal at 222 nm and localized in the 215-230 nm wavelength region. Precedented structural variation of the phi, psi dihedral angles of the alpha-helix is suggested as a possible explanation. For the important case of alanine-rich peptides, experiments are proposed that may yield temperature corrections for [theta](222) and permit reliable calculations of FH from [theta](222) values.
The H,O-soluble dendritic cyclophanes (dendrophanes) 5 5 of first to third generation with molecular weights up to nearly 20 kD were synthesized, purified, and characterized. Cyclophane 2, which served as the initiator core (generation zero), was prepared from tetrabromocyclophane 10 in a four-step sequence which involved as the first transformation a high-yielding, four-fold Pd(0)-catalyzed Suzuki cross-coupling reaction with 4-(benzyloxy)-phenyl-boronic acid to give 18. The X-ray crystal-structure analysis of tetrabromocyclophane 10 displayed an open, nearly rectangular box with opposite aromatic walls being 8.3 and 11.4 A apart and of suitable size for the incorporation of steroidal substrates. 'H-NMR Binding titrations in borate-buffered D20/CD30D 1 : 1 showed that cyclophane-tetracarboxylate 2 forms 1 : 1 inclusion complexes with steroids ( Table 2). Complexation was found to be enthalpically driven with higher binding affinities measured for the more apolar substrates. 'H-NMR Titrations in the same solvent also provided clear evidence for core-selective complexation of testosterone (21) by the dendrophanes 3 (Ist), 4 (2nd), and 5 (3rd generation) carrying up to 108 carboxylate surface groups. The stability of the corresponding 1 : 1 complexes was hardly affected by the size of the dendritic shell, although some generation-dependent conformational changes in the receptor cavity seemed to take place. Remarkably, host-guest exchange kinetics in all recognition processes were fast on the 'H-NMR time scale.Together with the naturally abundant cyclodextrins [I], cyclophanes do form the major part of synthetic receptors for inclusion complexation of apolar substrates [2]. Only a limited number of synthetic hosts are capable of steroid recognition [3] [4], a process of fundamental importance in biology [5]. Recent X-ray structural data for steroid-binding proteins, enzymes, and antibodies [6] revealed that natural receptors, similar to cyclophane hosts, prefer complexing the voluminous steroidal substrates in binding sites largely shaped by aromatic amino-acid side chains, thus taking advantage of favorable desolvation processes and apolar dispersion as well as polar CH. . -z interactions.
Water-soluble dendritic cyclophanes (dendrophanes) of first (1, 4), second (2, 5), and third generation (3, 6) with poly(ether amide) branching and 12, 36, and 108 terminal carboxylate groups, respectively, were prepared by divergent synthesis, and their molecular recognition properties in aqueous solutions were investigated. Dendrophanes 1-3 incorporate as the initiator core a tetraoxa[6.1.6.l]paracyclophane 7 with a suitably sized cavity for inclusion compiexation of benzene or naphthalene derivatives. The initiator core in 4-6 is the [6.1.6.l]cyclophane 8 shaped by two naphthyl(pheny1)methane units with a cavity suitable for steroid incorporation. The syntheses of 1-6 involved sequential peptide coupling to monomer 9, followed by ester hydrolysis (Schemes 1 and 4).Purification by gel-permeation chromatography (GPC; Fig. 3) and full spectral characterization were accomplished at the stage of the intermediate poly(methy1 carboxylates) 10-12 and 23-25, respectively. The third-generation 108-ester 25 was also independently prepared by a semi-convergent synthetic strategy, starting from 4 (Scheme 5). All dendrophanes with terminal ester groups were obtained in pure form according to the I3C-NMR spectral criterion (Figs. 1 and 5). The MALDI-TOF mass spectra of the third-generation derivative 25 (mol. 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 fragmentation 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-NMR and fluorescence binding titrations in basic aqueous buffer solutions showed that dendrophanes 1-3 complexed benzene and naphthalene derivatives, whereas 4-6 bound the steroid testosterone. Complexation occurred exclusively at the cavity-binding site of the central cyclophane core rather than in fluctuating voids in the dendritic branches, and the association strength was similar to that of the complexes formed by the initiator cores 7 and 8, respectively (Tables f and 3). Fluorescence titrations with 6-(p-tolnidino)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 evaluated by 'H-NMR titrations. The rapid complexation-decomplexation kinetics 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.
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