Helicate cyclic

Fig. 7. The hexagonal framework in [Cu(L6XMe2CO)ori BF4 (a) is generated by the helical cyclic array (b), where only copper atoms are shown, (c) Space-filling model of the complex, where BF, are omitted. (From Fig. 4 in Munakata, M. Wu, L. P. Kuroda-Sowa, T. Maekawa, M. Moriwaki, K. Kitagawa, S. Inorg. Chem. 1997, 36, 5416.)...

In cyclic nucleotide-regulated channels, this domain serves as a high-affinity binding site for 3-5 cyclic monophosphates. The CNBD of channels has a significant sequence similarity to the CNBD of most other classes of eukaryotic cyclic nucleotide receptors and to the CNBD of the prokaryotic catabolite activator protein (CAP). The primary sequence of CNBDs consists of approximately 120 amino acid residues forming three a-helices (oA-aC) and eight (3-strands ( 31- 38). 

Fig. 2.3 Model of the 2i- and 3i-helical structures proposed for PHB chains with ideal torsion angle values. The 2i-helix was determined by fiber X-ray diffraction of PHB [49-51] while the 3i-helical fold was constructed by using preferred dihedral angles found along the backbone in crystal structures of cyclic oligomers 9 ( oligolides ) [37, 43, 45]...

The structural constraints imposed by the fused cyclic structure of the catenates and the ability to cleanly separate product mixtures greatly facilitates crystallization, and single crystal X-ray structures have been obtained for a number of ladder polysilanes.3 8 For the all-anti molecules, the non-planarity of the SI rings induces a unidirectional helical twist to the ladder structure, as is evident in Figure 63. 

The non-cyclic ethers E-2 (Figure 10.26), with two pyrenes linked at both ends of the chain, show strong intramolecular excimer formation. Addition of alkaline earth metal ions leads to an increase in monomer emission at the expense of the excimer band. The helical structure of the 1 1 complexes is supported by NMR spectra. Thanks to the pseudocyclic structure, the stability constants of the complexes with Ca2+, Sr2+ and Ba2+ in acetonitrile are quite high (106-107 for n — 5), but the selectivity is poor as a consequence of the flexibility of the oxyethylene chain. 

The influence of secondary structure on reactions of deamidation has been confirmed in a number of studies. Thus, deamidation was inversely proportional to the extent of a-helicity in model peptides [120], Similarly, a-hel-ices and /3-turns were found to stabilize asparagine residues against deamidation, whereas the effect of /3-sheets was unclear [114], The tertiary structure of proteins is also a major determinant of chemical stability, in particular against deamidation [121], on the basis of several factors such as the stabilization of elements of secondary structure and restrictions to local flexibility, as also discussed for the reactivity of aspartic acid residues (Sect. 6.3.3). Furthermore, deamidation is markedly decreased in regions of low polarity in the interior of proteins because the formation of cyclic imides (Fig. 6.29, Pathway e) is favored by deprotonation of the nucleophilic backbone N-atom, which is markedly reduced in solvents of low polarity [100][112],... 

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