Host-guest experiments

In order to show that HIT-1 was a-helix compatible and induces a-helical conformations in short peptides from the N- and C-terminus as well as from internal positions, template-linked peptides have been compared to nonconstrained reference peptides. In the N- and C-cap projects a hydrophobic 12-mer peptide was used as a standard reference. The 12-mer reference peptide 104 was originally developed for host guest experiments to evaluate the a-helix propensities of unnatural amino acids (see Section II.A) and had later been used to confirm N-cap induced helix formation.141,202,214 In the C-cap series and for position-independent templates the same reference peptide could be used after minor modification of the terminal protection groups. The suitably protected reference peptide and the corresponding template-linked model peptides used for the evaluation of N-terminal, C-terminal, and internal helix induction are depicted in Figure 39. 

PPII in Globular Proteins and Host-Guest Experiments.392... 

Giesa, S. Gross, J.H. Kiatschmer, W. Gleiter, R. Experiments Towards an Analytical Application of Host-Guest Complexes of [60] FuUerene and Its Deriva-... 

The metathesis reaction of the imine bond was also able to dynamically select for oligomers that form the most stable host-guest complex [88]. It has been shown that rodlike and dumbbell-shaped guests have a higher affinity and specificity for oligomers of 20-22 repeat units [58,61 ]. Imine starter sequences 60,61a, and 61b, which can potentially form 16- (63a), 22- (63b), and 28-mers (63c) as well as smaller molecular weight (MW) materials (62a and 62b), were chosen as starter sequences for an imine ligation experiment (Fig. 37). When equiUbrated in chloroform the product distribution of 63a-c was close to 1 1 1,... 

Dynamic combinatorial chemistry (DCC) is marvelously effective at discovering receptors for a broad array of analytes. The nature of the internal competition experiment ensures (normally) that the most effective binder for the analyte of interest is amplified for subsequent identification and characterization. In the context of a host-guest assembly, the issue of stereochemistry can be manifested in a number of scenarios. These include various permutations of chiral or achiral guests, along with achiral, enan-tiopure, or racemic dynamic library components. 

Receptor models 71a and 71b for dinucleotides (dinucleoside monophosphates) have been developed out of 69 by replacing the naphthoyl residue, which is not involved in any host-guest interactions in the complexes, by a second carbazolyl cleft. The adenine selectivity of the two pincers was reflected in single extraction experiments dissolved in CH2CI2,71a removes one full equivalent of both adenylyl(3 -+ 5 )adenosine (ApA) and 2 -deoxyadenylyl(3 ->5 )-2 -deoxyadenosine [d(ApA)] 72 out of aqueous solutions of guest (8-fold excess) but only half the amount of d(ApG) [103]. 

An interesting finding in the CB7-MV2+ system is that, in clear contrast to host-guest systems involving CD hosts, the voltammetric data do not contain any indication that complex dissociation must precede any of the electron transfer processes. Furthermore, the electrochemistry of the inclusion complex is as fast—in the timescale accessible in these cyclic voltammetric experiments—as that of the free guest. This is clearly illustrated by the voltammograms depicted in Fig. 3.2, which show the comparative results of a scan-rate study on the MV2+/MV+ and CB7 MV2 + / 7 MV + redox couples. In both cases, the observed anodic and cathodic peak potentials are basically invariant as the scan rate is increased up to... 

In an attempt to better understand the mechanism for encapsulation of cobaltocenium inside 46 we performed careful titrations of this redox-active cation with variable concentrations of host 4. To our surprise, we quickly realized that only 2-3 equivalents were necessary to fully shut down the electrochemical response of cobaltocenium.51 This result is in strong contrast to the NMR data, which clearly indicate a 6 1 [host/guest] stoichiometry for full encapsulation. There are two important differences between these two types of experiments. In the NMR experiments, solutions were prepared in pure, deuterated CD2CI2 and the only solutes present are cobaltocenium hexafluorophosphate (ca. 1 mM) and host 4 (0-8 mM). In the electrochemical experiments, solutions were prepared in isotopically unenriched CH2CI2 also containing 0.1 M tetradodecylammonium bromide as supporting electrolyte. The concentrations of cobaltocenium hexafluorophosphate and host 4 were similar to those used in the NMR experiments. It clearly became evident that the nature of the supporting electrolyte, especially the nature of its anion, was crucial to... 

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