Channel-type complexes

The cyclodextrins also form channel-type complexes, in which the host... 

The cyclodextrins also form channel-type complexes, in which the host molecules are stacked on top of each other, like coins in a row.110 For example, a-cyclodextrin (cydo-hexaamylose) forms cage complexes with acetic, propionic, and butyric acids, but channel complexes with valeric and higher acids. 

When complexes are prepared in the presence of iodide, four different complex types can be obtained, depending on the conditions. Here we will only discuss the blackish-brown complex (a-cyclodextrin)2-Cdo.5-l5-27H2O. On the Cf, axis of the molecule are found four of the five iodine atoms of Is", the central fifth one being disordered. The disordered iodine atom is located between the two a-cyclodextrin molecules which face each at their wide bases, 11.18. Structures of 11.18 are packed one on top of the other in a way as to create a continuous channel filled with iodine atoms. This is a channel-type complex (Noltemeyer and Saenger 1980). With guest molecules other than iodine, either one or the other of the two types of crystalline structures discussed here, i.e. cage or channel, can be observed. 

In the channel type complexes (Fig. 1c), cyclodextrin molecules are stacked on top of each other like coins in a roll such that the cavities align in the form of endless channels. This packing mode is stabilized by hydrogen bonding between the cyclodextrin molecules and we observe two kinds of arrangements, head-to-head and head-to-tail. 

This latter kind of arrangement appears to be special for y-cyclodextrin as it was not observed until now in a and B-cyclodex-trin crystal structures. For a-cyclodextrin, channel-type complexes in both head-to-head and head-to-tail arrangements have been found whereas for (3-cyclodextrin, only the head-to-head arrangement has been visualized thus far. 

C36H60O30,C3H7O3S-,Na+,9 H20 Cyclomaltohexaose-sodium 1-pro-panesulfonate, nonahydrate (ACDPRS)129 P212121 Z= 2 Dx = 1.42 R = 0.08 for 2,219 intensities. This structure is a channel type, with the molecules of cyclomaltohexaose stacked head-to-tail. It is almost isomorphous with the sodium benzenesulfonate complex.275... 

C36H60O30,C6H5O3S -Na+-10 H20 Cyclomaltohexaose - sodium ben-zenesulfonate, decahydrate (CDXBZS)275 P212121 Z = 2 D = 1.47 R = 0.07 for 2,894 intensities. This is a channel-type structure, with a conformation very similar to that of the potassium acetate280 and Methyl Orange complexes.281 All of the pyranose residues have the 4Cl conformation, and the primary alcohol groups are in the gauche-trans orientation. 

The detailed structures of several clathrates have been characterized, and a certain degree of selectivity on complexation with different isomers has been detected 21). Most of these complexes are of the channel type, but some of them have structures which simultaneously qualify for channel and cage type descriptors representative examples are illustrated in Figs. 19-21. The crystal data of the complexes are summarized in Table 1. 

Wiggins et al. [456] used neutrons from the thermal column of a 10 kW pool-type research reactor and from a 120 pg Cf source to study the prompt photon emission resulting from neutron capture in magnesium nodules (ter-romanganese oxides) from the ocean floor. Spectra were recorded with a Ce(Ii) detector and a 1024-channel analyser. Complex spectra were obtained by irradiation of seawater, but it was possible to detect and estimate manganese in nodules in a simulated marine environment by means of the peaks at 7.00, 6.55, 6.22, and 6.04 pV. 

Ion channels are macromolecular complexes that form aqueous pores in the lipid membrane. We have learned much about ion channel function from voltage clamp and patch clamp studies on channels still imbedded in native cell membranes [1-6, 8]. A diversity of channel types was discovered in the different cells in the body, where the repertoire of functioning channels is adapted to the special roles each cell plays [5]. The principal voltage-gated ones are the Na+, K+ and Ca2+ channels, and most of these are opened by membrane depolarizations. Figure 6-5A summarizes the major functional properties of a voltage-gated... 

Dolmetsch R, Pajvani U, Fife K, Spoils JM, Greenberg ME (2001) Signaling to the nucleus by an L-type caldiun channel-calmodulin complex through the MAP kinase pathway. Science 294 333-339... 

Synthetic drugs of comparable selectivity and affinity to the 1,4-dihydropyridines do not yet exist for the other channel types, T, N, P/Q, and R these remain characterized by complex polypeptide toxins of the aga- and conotoxin classes. Neuronal pharmacology, including that of the central nervous system (CNS), is dominated by the N, P/Q, and R channels. This underscores the normally weak effect of L-channel antagonists on CNS function. Drugs that act at the N, P, and R channels with comparable selectivity and affinity to the 1,4-dihydropyridines may be expected to offer major potential for a variety of CNS disorders, including neuronal damage and death from ischemic insults. 

Figure 7. Schematic representation of the packing arrangement in cyclodextrin complexes (a) channel type, (b) cage or herringbone type, and (c) brick type. (Reproduced with permission from W. Sanger in Inclusion Compounds, Vol. 2, J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Academic Press, New York, 1984, p. 231.)...

By introducing a crown ether unit at the C-terminal region of hydrophobic helical peptides, Otoda et al.19 were able to demonstrate increased stabilization of the peptide aggregate in the membrane by the formation of sandwich-type complexes with large cations. Ion channel activity was also increased due to the ability of the crown peptide to bind ions to the terminal portion of the hydrophobic helix bundle at the water-lipid interface. Ueda et al.20 considered the problem of insolubility of hydrophobic peptides which restricts the distribution of peptides in water to a phospholipid bilayer membrane. In consequence they constructed a hydrophobic helix bundle shielded by hydrophilic peptides that acts rather like an umbrella. 

K. Miki, A. Masui, N. Kasai, M. Miyata, M. Shibakami, K. Takemoto, New Channel-Type Inclusion Compound of Steroidal Bile Acid Structure of a 1 1 Complex between Cholic Acid and Acetophenone , J. Am. Chem. Soc., 110, 6594 (1988)... 

Most of the supramolecular assemblies have been constructed from the solutions of each component. More recently, the authors found that CD formed inclusion complexes with PEG with high selectivity only by mixing powdered crystals of CDs and polymer samples without any solvents under ambient conditions (Scheme 2) [120]. When crystals of a-CD and PEG (Mw = 400) were mixed without solvent (a-CD ethylene glycol unit =1 2), the X-ray powder patterns changed a peak at 26 = 22° characteristic for the cage type decreased, and a peak at 26 = 20° characteristic for the channel type appeared... 

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