Schiff base cross-linking

Schiff base cross-linking involves the reaction between macromolecules containing alcoholic, aminic, or hydrazide functional group with aldehydes to obtain a hydrogel... 

Affinity microparticles (AMPs) were obtained by cross-linking the S-layer lattice on S-layer-carrying cell wall fragments with glutaraldehyde, reducing Schiff bases with sodium borohydride, and immobilizing protein A as an IgG-specific ligand [92]. Thus, AMPs rep-... 

Formation of intra- and interchain cross-links via Schiff bases and aldol condensation products... 

The immobilization of metal catalysts onto sohd supports has become an important research area, as catalyst recovery, recycling as well as product separation is easier under heterogeneous conditions. In this respect, the iron complex of the Schiff base HPPn 15 (HPPn = iVA -bis(o-hydroxyacetophenone) propylene diamine) was supported onto cross-linked chloromethylated polystyrene beads. Interestingly, the supported catalyst showed higher catalytic activity than the free metal complex (Scheme 8) [50, 51]. In terms of chemical stability, particularly with... 

Formaldehyde fixes proteins in tissue by reacting with basic amino acids— such as lysine,5 7—to form methylol adducts. These adducts can form crosslinks through Schiff base formation. Both intra- and intermolecular cross-links are formed,8 which may destroy enzymatic activity and often immunoreactiv-ity. These formaldehyde-induced modifications reduce protein extraction efficiency and may also lead to the misidentification of proteins during proteomic analysis. 

Figure 15.1 Reactions of formaldehyde with peptides and amino acids. Shown are the four types of reaction products seen when peptides or amino acids are treated with formaldehyde in aqueous solution. These reaction products are methylol (hydroxymethyl) adduct (reaction 15.1), Schiff-base (reaction 15.2), 4-imidazolidinone adduct (reaction 15.3), and one type of methylene bridge [cross-link] (reaction 15.4).

One might expect that dehydration of a methylol-aducted protein would lead to the formation of Schiff bases, which might be expected to serve as highly reactive groups capable of undergoing further reactions—particularly cross-linking reactions, as the water content of the surrounding solution is reduced. As yet, however, there is little direct evidence that such intermediates are... 

Formaldehyde reacts with proteins to form adducts and cross-links.31516 Metz et al.3 have identified three types of chemical modifications after treatment of proteins with formaldehyde (a) methylol (hydroxymethyl) adducts, (b) Schiff bases, and (c) methylene bridges. The reaction of formaldehyde with proteins is summarized in Figure 19.1, but briefly, formaldehyde reacts primarily with lysine and cysteine to form methylol adducts. The methylol adduct can subsequently undergo a dehydration reaction to form a Schiff base. Adducted primary amine and thiol groups can undergo a second reaction with arginine,... 

The final structure of the formed cross-link proposed in this article might seem similar to that depicted in the current theory in the aspect that it is not monomeric but polymerized GA, although some differences surely exit for example, unsaturation at a, (3 - position occurs only at locations adjacent to Schiff base imine according to our theory. The most important difference between the two theories would be the timing when GA polymerization occurs. This difference would be crucial when we attempt to control the structure — for example, the chain length — of the formed cross- links in the future. 

Actual cross- linking reaction is not based on the simple mechanism that Schiff base linkages are formed at both ends of monomeric GA. Instead, the polymerization of GA, via aldol condensation, proceeds in parallel with the cross- linking reaction. 

Amino groups on proteins may be reacted with the bis-aldehyde compound glutaraldehyde to form activated derivatives able to cross-link with other proteins. The reaction mechanism for this modification proceeds by one of several possible routes. In the first option, one of the aldehyde ends can form a Schiff base linkage with e-amines... 

Schiff base interactions between aldehydes and amines typically are not stable enough to form irreversible linkages. These bonds may be reduced with sodium cya-noborohydride or a number of other suitable reductants (Chapter 3, Section 4) to form permanent secondary amine bonds. However, proteins cross-linked by glutaraldehyde without reduction nevertheless show stabilities unexplainable by simple Schiff base... 

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. 

Hapten molecules containing aldehyde residues may be cross-linked to carrier molecules by use of reductive amination (chapter 3, Section 4). At alkaline pH values, the aldehyde groups form intermediate Schiff bases with available amine groups on the carrier. Reduction of the resultant Schiff bases with sodium cyanoborohydride or sodium borohydride creates a stable conjugate held together by secondary amine bonds. 

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