Side groups Schiff base

Mesomorphic order in polymers with side groups containing the cholesterol moiety has scarcely been studied in spite of the importance of cholesterol derivatives in the field of liquid crystals and biological materials. The main focus of attention has been 

A recent publication (13) which appeared while this work was in progress described the spontaneous development of layered order in polymers with the cholesteric moiety attached to the backbone via a long, flexible molecular spacer constituted by an Co-amino-carboxylic acid chain. Layered structures are developed with the long side groups roughly perpendicular to the main chain. The authors attributed the development of these structures to the flexibility of the bridge connecting the cholesterol moiety. 

Monomers. Cholesteryl p-acryloyloxybenzoate (ChAB) was prepared in three steps by the following synthetic route  

The product was recrystallized from chloroform/acetone and chloroform/ethanol to constant transition temperature. Elemental analysis Calcd. for C3 7Hr O C, 79.29% H, 9.29%. Found C, 79.51% H, 9.3I%7 The NMR spectra were consistent with the expected structure.  

Cholesterylmethacrylate (ChMA) was obtained by a procedure similar to the one described in the literature (15 ).  

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CTC formation is accompanied by a volume contraction129. Thus, poly(schiff base)s having no side groups give complexes with a density 10—15% higher than that calculated on an additivity assumption. 

Other examples of this synthetic strategy are known for example, a recent zirconium polymer by Illingsworth and Burke (8), who joined amine side groups of a zirconium bis(quadridentate Schiff-base) with an acid dianhydride to give amide linkages. Once again, caution is necesary, as Jones and Power (2) learned when they attempted to link metal bisO-diketonates) with sulfur halides that is, they obtained insoluble metal sulfides because the p-diketone complexes which they used were fairly labile and the insolubility drove the reactions to completion in the wrong direction. 

Figure 5.37 APG can be used to label specifically arginine residues in proteins, producing stable, cyclic Schiff base-like bonds with the side-chain guanidino groups. Photoactivation with UV light then causes ring expansion of the phenyl azide group, initiating covalent bond formation with amines.

FIGURE 46. Retinal chromophore in bR is attached via a protonated Schiff base to Lys-216 on helix G and is tilted toward the extracellular side. To determine its detailed structure, retinal was selectively deuteriated on the three methyl groups on the cyclohexene ring and incorporated into bR from H. Halobium. Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

Glycoproteins, such as horseradish peroxidase, are coupled selectively to other proteins or NH2-groups bearing molecules via oligosaccharide side chain oxidation. The vicinal OH groups of oligosaccharide residues are oxidized by periodate to aldehyde groups, which react with amines to form imines (Schiff bases). 

Under oxidative conditions, a side chain alkyl group is sometimes removed from a coordinated amino group. This has been observed in the case of the complexes of the Schiff base of salicylaldehyde with alanine esters (Equation 30) (60, 61). 

Transketolase requires the cofactor thiamine pyrophosphate (TPP), which stabilizes a two-carbon car-banion in this reaction (Fig. 14—26a), just as it does in the pyruvate decarboxylase reaction (Fig. 14-13). Transaldolase uses a Lys side chain to form a Schiff base with the carbonyl group of its substrate, a ketose,... 

FIGURE 14-26 Carbanion intermediates stabilized by covalent interactions with transketolase and transaldolase, (a) The ring of TPP stabilizes the two-carbon carbanion carried by transketolase see Fig. 14-13 for the chemistry of TPP action, (b) In the transaldolase reaction, the protonated Schiff base formed between the e-amino group of a Lys side chain and the substrate stabilizes a three-carbon carbanion. 

In Fig. 14-5 the reactions of PLP-amino acid Schiff bases are compared with those of (i-oxo-acids. Beta-hydroxy-a-oxo acids and Schiff bases of PLP with (i-hydroxy-a-amino acids can react in similar ways. The reactions fall naturally into three groups (a,b,c) depending upon whether the bond cleaved is from the a-carbon of the substrate to the hydrogen atom, to the carboxyl group, or to the side chain. A fourth group of reactions of PLP-dependent enzymes (d) also involve removal of the a-hydrogen but are mechanistically more complex. Some of the many reactions catalyzed by these enzymes are listed in Table 14-3. 

Side chain cleavage (Group c). In a third type of reaction the side chain of the Schiff base of Fig. 14-5 undergoes aldol cleavage. Conversely, a side chain can be added by (3 condensation. The best known enzyme of this group is serine hydroxymethyltransferase, which converts serine to glycine and formaldehyde.211-21313 The latter is not released in a free form but is transferred by the same enzyme specifically to tetrahydrofolic acid (Eq. 14-30), with which it forms a cyclic adduct. 

Observation of an abnormally large shift in the position of fluorescent emission of pyridoxal phosphate (PLP) in glycogen phosphorylase answered an interesting chemical question.187188 A 330 nm (30,300 cm ) absorption band could be interpreted either as arising from an adduct of some enzyme functional group with the Schiff base of PLP and a lysine side chain (structure A) or as a nonionic tautomer of a Schiff base in a hydrophobic environment (structure B, Eq. 23-24). For structure A, the fluorescent emission would be expected at a position similar to that of pyridoxamine. On the other hand, Schiff bases of the... 

The structure of opsin is unknown, but its prosthetic group (1 l-c/.v-retinal) is bonded to it through an imine (Schiff base) function formed between the aldehyde group of the retinal and the side-chain amino function of a lysine unit of opsin ... 

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