Conformation of amides

Many studies of separations, primarily by HPLC, have been reported with a view to establishing the mechanisms by which diastereomers separate (56-58). Although substantive criticism has been offered by way of demonstrated exceptions (45, 59), the picture as qualified by Pirkle (57) holds well for simple (otherwise functionally unsubstituted) amides and carbamates. Briefly, solution conformations of amides and carbamates are as shown in Figure 17 the central functional group acts to create a plane, and the asymmetric centers extend alkyl (aryl) residues to either side of that plane. An explanation based on a combination of steric bulk and hydrophobicity has been advanced to explain HPLC elution orders for carbamates. Elution order for GLC is inverted from that of HPLC in all instances studied, the least soluble (faster eluting) diastereomer by GLC is always the cisoid molecule. The notable exceptions are those compounds bearing a CF3 group at R. ... 

Amides 1-4 all adopt conformations in wiiich a i-butyl group has an axial-like position and, as a result, a non-chair conformation of the six-membered ring. Comment on the origin of this stmctural effect. 

FIGURE 5.2 The peptide bond is shown in its usnal trans conformation of carbonyl O and amide H. The atoms are the oi-carbons of two adjacent amino acids joined in peptide linkage. The dimensions and angles are the average valnes observed by crystallographic analysis of amino acids and small peptides. The peptide bond is the light gray bond between C and N. (Adapted from Ramachandran, G. A., ct ai, 1974. Biochimica Biophysica Acta 359 298-302.)... 

A relationship between the redox state of an iron—sulfur center and the conformation of the host protein was furthermore established in an X-ray crystal study on center P in Azotobacter vinelandii nitroge-nase (270). In this enzyme, the two-electron oxidation of center P was found to be accompanied by a significant displacement of about 1 A of two iron atoms. In both cases, this displacement was associated with an additional ligation provided by a serine residue and the amide nitrogen of a cysteine residue, respectively. Since these two residues are protonable, it has been suggested that this structural change might help to synchronize the transfer of electrons and protons to the Fe-Mo cofactor of the enzyme (270). 

Sea snake short-chain toxins have a molecular weight of only 6,800. The small size with four disulfide bonds makes these toxins very compact and stable molecules. Therefore, when the Pelamis platurus toxin is subjected to heat treatment at 100 C and subsequent cooling, it does not change its conformation substantially. Amide I and ni bands and S-S stretching vibration did not change by heat treatment. 

Maps and Conformational Transitions Between Low-Energy Conformers of N-Acety 1-N -Methyl Glycine Amide An Ab Initio Study at the 4-21G Lever with Gradient Relaxed Geometries. J. Mol. Struct. (Theochem) 124, 143-153. 

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