Amide group conformation

The trans conformation predominates in straight chain amides [46] and in more than nine-membered lactams [47]. Up to the eight-membered lactam the amide group is constrained to adopt the cis conformation [47] which is 1.4 kcal mole less stable than the trans form [48]. This amount of energy is released in the conversion of the c/s-amide group (in lactams up to enantholactam) into the trans form of the polymer 

The planar cis-amide conformation in the seven-membered lactam already can impose some strain on the ring and thus increase its polymerizability. 

The nine-membered lactam representing an intermediate between cis and irons lactams was assumed to be sufficiently flexible to allow the more stable irons amide group to co-exist with the strainless cis form [47]. However, a strainless planar irons form is not possible and the actual conformation is rather a skew structure which drastically reduces the jo—tt overlap and decreases resonance stabilization of the amide group. Therefore, an increased amount of delocalization energy is released in the 

In the ten-membered and higher lactams, the planar trans conformation can be strainless so that the contribution of conformational changes of the amide group to the AGp value will be minimum. Only the 10-membered lactam contains about 5% of cis form and a corresponding amount of energy will be released during polymerization. 

Substituents affect both the heat and entropy of polymerization, mainly through conformational effects [58—61]. Steric repulsions between substituents as well as between atoms of the monomer unit and substituents usually do not change the enthalpy of the cyclic lactam. Only 

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Krimm, 1968a,b Mattice and Mandelkern, 1971 Krimm and Tiffany, 1974). This conformation is similar to that of a single strand from collagen, with average backbone dihedrals of (0,0) = ( 75°, +145°). These dihedrals lead to an extended left-handed helical conformation with precisely three residues per turn and 9 A between residues i and i + 3 (measured Cft to C/3). A cartoon of a seven-residue alanine peptide in this conformation is shown in Figure 1. Notably, backbone carbonyl and amide groups point perpendicularly out from the helical axis into the solvent and are well-exposed. 

It was found that in spite of the large excess of modifying amine (N-isopropyl-, -diethyl, -dipropyl, -diisopropyl, -n-hexyl, -cyclohexyl, -n-octyl), the extent of substitution did not exceed 5-10 molar %. For the case of the N-isopropyl derivative, i.e. [poly(AAm-co-NiPAAm)], the authors connected such results with the temperature-induced conformational transformation of partially hydrophobized copolymer acquiring the contracted conformation, "... which made it difficult for N-isopropylamine to react further with the amide groups [22], Unfortunately, no data on the solution behaviour of these interesting copolymers have been reported to date, although there is a high probability that they would demonstrate certain properties of the protein-like macromolecules. At least, in favour of similar supposition is supported by the results of our studies [23] of somewhat different PAAm partially hydrophobized derivative, whose preparation method is depicted in Scheme 3. 

In aromatic compounds bearing two rotationally restricted amide groups, diastereoiso-meric atropisomers can arise because of the relative orientation of the amides. Ortholithi-ation can therefore lead to diastereoselectivity if the ortholithiation forms one of the two diastereoisomers selectively. A simple case is 169, whose double lithiation-ethylation leads only to the C2-symmetric diamide 170, indicating the probable preferred conformation of the starting material (Scheme 85) . 

The EFF calculations yielded a single Cs-symmetric conformation for each type of ferrichrome analog (Figure 4), both with a A-cis configuration of the hydroxamates about the metal when L-amino acids were used. Taken together with the spectroscopic data, pronounced differences were observed for the conformations of these iron complexes. Inspection of the calculated conformations showed that the backbone amide groups may... 

A secondary amide group is nearly planar and has a hindered rotation around the C-N bond which exists predominantly in the transoid conformation. This provides a dipole which is accessible for inter- and intramolecular hydrogen donor and acceptor interactions and dipole stacking. The silanol groups (Si-OH) of the adsorbent are ideal partners not only for hydrogen bonding but also, due to their acidity, for electrostatic interactions. 

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