Conjugation in amides

Bile acids are usually conjugated in amide linkage with the amino acid glycine or taurine, giving bile salts. The cholic acid conjugates with glycine and taurine are called glycocholate and taurocholate, respectively. 

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

An investigation of acylaziridines was carried out by comparison of IR, NMR and MS data and included some 1,2-dibenzoylaziridines as well as 2-p-nitrobenzoyl-3-phenyl-oxaziridine (68IZV1530). Amide conjugation in acylated nitrogen-containing three-membered rings is weaker than in open chain acid amides. 

Consider a nucleus that can partition between two magnetically nonequivalent sites. Examples would be protons or carbon atoms involved in cis-trans isomerization, rotation about the carbon—nitrogen atom in amides, proton exchange between solute and solvent or between two conjugate acid-base pairs, or molecular complex formation. In the NMR context the nucleus is said to undergo chemical exchange between the sites. Chemical exchange is a relaxation mechanism, because it is a means by which the nucleus in one site (state) is enabled to leave that state. 

Lipoic acid exists as a mixture of two structures a closed-ring disulfide form and an open-chain reduced form (Figure 18.33). Oxidation-reduction cycles interconvert these two species. As is the case for biotin, lipoic acid does not often occur free in nature, but rather is covalently attached in amide linkage with lysine residues on enzymes. The enzyme that catalyzes the formation of the lipoamide nk.2Lg c requires ATP and produces lipoamide-enzyme conjugates, AMP, and pyrophosphate as products of the reaction. 

Figure 17.6 The reaction of SANH with amine-containing proteins or other molecules results in amide bond modifications containing terminal hydrazine groups. The reaction of SFB with amine-containing proteins or other molecules results in amide bond modifications containing terminal aldehyde groups. Subsequently, the two modified molecules can be reacted together to create a conjugate via hydrazone bond formation.

In amides, the nitrogen electron pair is n-r conjugated with the carbonyl group and this electronic delocalization is normally expressed by resonance structures 1, 2, and 3. As a result, the amide function is essentially planar and it is assumed that the three atoms (C, ii, and 0) of this function are sp hybridized. The amide function can be illustrated in three dimensions by structure 4. The electronic distribution can also be viewed as the result of the delocalization of two n electron pairs, one from the oxygen atom and one from the nitrogen atom (cf. 1 and 3 versus 2) and on that basis, it is referred to here as the primary electronic delocalization of the amide function. 

Although to a lesser extent than amides (p. 165), the ester group is also stabilized by conjugation. In this case, the eth oxide part of the ester is electron-releasing. This explains the pi[Pg.197]

The aliphatic H2N- very rarely acts as acceptor and the conjugated amino group found in amide groups (asparagine and glutamine) and in nucleic acid bases (adenine, guanine, cytosine) is never an acceptor. 

Alam SN, Roberts RJ, Fischer LJ (1977) Age-related differences in salicyl-amide and acetaminophen conjugation in man./ Pediatr 90 130-135. 

These two polymers are characterized by a stable trans-configuration of the amide bond and by a high barrier of the rotation around the aryl carbon bond. The directions of rotational axes in all the chain units almost coincide. As a result, according to Tsvetkov the entire macromolecule acquires the shape of a crankshaft (Fig. 1). In the case of PBA, the retardation of rotation of the units may be also caused by a transfer of conjugation in the carbonyl-nitrogen system via a phenylene ring. 

---