Amine-containing linkers

Succinic anhydride also is a convenient extender for creating spacer arms on chromatography supports. Supports derivatized with amine-terminal spacers may be succinylated to totally block the amine functionalities and form terminal carboxylic acid linkers for coupling amine-containing affinity ligands (Cuatrecasas, 1970). 

Figure 129 DSG is a nondeavable cross-linker that can react with two amine-containing molecules to form amide bonds.

Figure 138 DFDNB is a small cross-linker able to form covalent bonds between amine-containing molecules. The aromatic fluorine atoms are readily displaced by nucleophiles.

Glutaraldehyde is the most popular bis-aldehyde homobifunctional cross-linker in use today. However, a glance at glutaraldehyde s structure is not indicative of the complexity of its possible reaction mechanisms. Reactions with proteins and other amine-containing molecules would be expected to proceed through the formation of Schiff bases. Subsequent reduction with sodium cyanoborohydride or another suitable re-ductant would yield stable secondary amine linkages (Chapter 2, Section 5.3, and Chapter 3, Section 4). This reaction sequence certainly is possible, but other cross-linking reactions also are feasible. 

Figure 172 SASD is a photoreactive cross-linker that can be used to modify amine-containing compounds through its NHS ester end and subsequently photolyzed to initiate coupling with nucleophiles (after ring expansion to an intermediate dehydroazepine derivative). The cross-links may be selectively cleaved at the internal disulfide group using DTT.

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