Amide, coupling linkage

Baker and Saleem [51] have reported on the reactive compatibilization of oxazoline modified PS and carbox-ylated polyethylene. The coupling reaction results in amide-ester linkages at the time of melt mixing. A schematic representation of the reaction is shown in Scheme 2. 

Figure 3.3 EDC may be used in tandem with sulfo-NHS to create an amine-reactive protein derivative containing active ester groups. The activated protein can couple with amine-containing compounds to form amide bond linkages.

Cascade Blue cadaverine and Cascade Blue ethylenediamine both contain a carboxamide-linked diamine spacer off the 8-methoxy group of the pyrene trisulfonic acid backbone. The cadaverine version contains a 5-carbon spacer, while the ethylenediamine compound has only a 2-carbon arm. Both can be coupled to carboxylic acid-containing molecules using a carbodiimide reaction (Chapter 3, Section 1). Since Cascade Blue derivatives are water-soluble, the carbodiimide EDC can be used to couple these fluorophores to proteins and other carboxylate-containing molecules in aqueous solutions at a pH range of 4.5-7.5. The reaction forms amide bond linkages (Figure 9.39). 

Figure 9.61 QDs containing carboxylate groups can be coupled to amine-containing proteins or other molecules using the EDC/sulfo-NHS reaction to form amide bond linkages. The intermediate sulfo-NHS ester is negatively charged and will help maintain particle stability due to like charge repulsion between particles.

Figure 11.2 D-Biotin can be directly coupled to amine-containing molecules using the water-soluble carbodiimide EDC to form an amide bond linkage.

Figure 17.28 EPL reactions can be used to couple a fusion protein to a surface containing a thioester derivative. After cells are grown and the fusion protein expressed, a pH and temperature shift causes intein cleavage with release of the expressed protein with an N-terminal cysteine residue. Reaction with the thioester surface results in a native chemical ligation reaction that forms an amide bond linkage with the expressed protein.

Figure 19.15 The carbodiimide EDC can be used in the presence of sulfo-NHS to create reactive sulfo-NHS ester groups on a carrier protein. Subsequent coupling with an amine-containing hapten can be done to create amide bond linkages.

The problem of the nucleophilicity of amides in glycosylation reactions is not limited to the sulfoxide method and has been shown to result in the formation of glycosyl imidates from intermolecular reaction with activated donors. It appears that this problem may be suppressed by the prior silylation of the amide [348,349]. Accordingly, it may be sufficient to operate the sulfoxide method with an excess of triflic anhydride when amides are present so as to convert all amides into O-triflyl imidates, which are then hydrolyzed on work-up. Despite these problems, several examples have been published of successful sulfoxide glycosylation reactions with acceptors carrying remote peptide bonds [344,345] and with donors coupled to resins via amide-based linkages [346,347], with no apparent problems reported. Sulfonamides and tertiary amides appear to be well tolerated by the sulfoxide method [340,350],... 

An alternative to this route involves 4-vinylaniline or 4-vinylbenzoic acid, relatively easily coupled to various acid counterparts to produce amide linkages. A multitude of amide coupling protocols may in this case in principle be used, for example, by initial activation of the acid by thionyl- or oxalyl chloride and subsequent addition of the corresponding amine in the presence of base (pyridine, triethylamine, etc.) [5,6], or by use of coupling reagents such as dicyclohexylcarbodii-mide (DCC). 

Another approach was to use amide (peptide) linkage as a substitute for interglycosidic bond. Amide formations are well exemplified in peptide S3Tithesis (extensive work on coupling reagents, advantage of solid phase synthesis and potential automation) and present a major advantage, the compatibility with various functions such as alcohol functions. 

A substantial number of bioactive molecules, such as polypeptides, N-acetyl-DL-penicillamine, p-(dipropylsulfamoyl)benzoic acid, and nicotinic acid, contain a carboxylic acid function, and this provides a site for linkage to a polyphosphazene chain. A number of prototype polymers have been synthesized in which pendent amino groups provide coupling sites for the carboxylic acid (34). The amide linkages so formed are potentially bioerodible, but the use of a hydrolytic sensitizing cosubstituent would be expected to accelerate the process. 

Dopamine, a neurotransmitter, was covalently coupled, via an amide bond, to a modified polystyrene having A-(2-(3,4-dihydroxyphenyl)ethyl) isonicotinamide units. The dopamine-coupled polymer was coated onto glassy carbon electrodes. In aqueous electrolyte solutions (pH 7), cathodic current caused cleavage of the amide linkage and release of dopamine at potentials more negative than 0.9 V [41]. The chemical scheme for the amide bond cleavage is presented in Figure 18. 

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