Rings Formed by Hydrogen Bonding

The following set of oxides provides a useful series of six-rings for comparison of substituent effects. Where R = R and is bulky, six-membered rings are planar (6a) as in (f-Bu2GeO)3 formed above65 and the Sn analogues66 with R = f-Bu or r-amyl. The stannylene Ar(f)2Sn, discussed later, readily forms a similar cyclic oxide67 and use of a 

A mixed-member six-ring, —GeMe2SeCH2CMe2CH2Se—, has a symmetrical twisted boat configuration76 with GeSe = 235 pm, GeMe = 196 pm, CGeC = 111.5° and SeGeSe = 103.2°. 

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Further evidence for an acyclic structure for glucosazone is provided by the formation of a formazan after treatment with benzenediazonium chloride 238). The osazone may have a six-membered chelate ring formed by hydrogen bonding of the two hydrazine radicals. (See under Hy-drazones.)... 

The H atoms were also located and are included in the refinement. The centrosymmetric dimer (CgH CO NHSO g mols. are formed by hydrogen bonds between the N of imide and 0 of the carbonyl atoms, (NH.0C), of the five membered ring system of saccharin. The six-sided ring formed by the hydrogen bonds around the center of symmetry is completely planar. The location of the H atom eliminates the possibility of the lactim structure for... 

The limited flexibility of their molecules is due to the presence of glucose rings and additiortal cyclic structures formed by hydrogen bonds of side groups (Table 1). 

To provide sufficient rigidity to the core at least two ring systems are usually required. Rare exceptions are the 2,4-alkyldienoic carboxylic acids in which dimers are formed by hydrogen bonding of the carboxylic acid groups (which becomes a pseudo-ring system) indeed, most carboxylic acid systems behave in the same manner, e.g., 4-alkoxybenzoic acids (6). 

Fig. 3. (a) Chemical stmcture of a synthetic cycHc peptide composed of an alternating sequence of D- and L-amino acids. The side chains of the amino acids have been chosen such that the peripheral functional groups of the dat rings are hydrophobic and allow insertion into Hpid bilayers, (b) Proposed stmcture of a self-assembled transmembrane pore comprised of hydrogen bonded cycHc peptides. The channel is stabilized by hydrogen bonds between the peptide backbones of the individual molecules. These synthetic pores have been demonstrated to form ion channels in Hpid bilayers (71). 

There are several other far less common types of helices found in proteins. The most common of these is the Sjq helix, which contains 3.0 residues per turn (with 10 atoms in the ring formed by making the hydrogen bond three residues up the chain). It normally extends over shorter stretches of sequence than the a-helix. Other helical structures include the 27 ribbon and the 77-helix, which has 4.4 residues and 16 atoms per turn and is thus called the 4.4ig helix. 

All of these functions are made possible by the characteristic chemical features of carbohydrates (1) the existence of at least one and often two or more asymmetric centers, (2) the ability to exist either in linear or ring structures, (3) the capacity to form polymeric structures via glyeosidie bonds, and (4) the potential to form multiple hydrogen bonds with water or other molecules in their environment. 

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