Cross-links, intermolecularly formed

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. 

The second requirement for an elastomeric material is that the chains be cross-linked to form a loose three-dimensional intermolecular network. This cross-linking may be either by covalent chemical bonds or by strong non-covalent interactions such as hydrogen bonds or hydrophobic interactions. Cross-linking is essential for the restoring mechanism once the stress is removed from the material. If the material is not cross-linked, low stresses will straighten the polymer chains, but at higher sResses they will slip past each other and irreversibly deform. 

The best known property of pectin is that it can gel under suitable conditions. A gel may be regarded as a system in which the polymer is in a state between fully dissolved and precipitated. In a gel system, the polymer molecules are cross-linked to form a tangled, interconnected three-dimensional network that is immersed in a liquid medium (Flory, 1953). In pectin and most other food gels, the cross-linkages in the network are not point interactions as in covalently linked synthetic polymer gels, but involve extended segments, called junction zones, from two or more pectin molecules that are stabilized by the additive effect of weak intermolecular forces. 

Outside the dilute regime, chain overlap occurs, which implies that polymer molecules are mutually entangled. This greatly increases viscosity, as well as the dependence of viscosity on concentration and the extent of strain rate thinning. Moreover, the solution shows elastic besides viscous behavior, and if intermolecular cross-links are formed, a gel is obtained. The chain overlap concentration decreases with increasing molecular size, [j value, and stiffness. For overlapping chains, the solution is characterized by a correlation length, which does not depend on molecular size, and which is... 

Intermediates occurring in these mechanisms have been identified by ESR measurements and by flash photolysis studies using optical absorption detection. For example, ESR measurements on wool keratins revealed the formation of sulfur-centered radicals of the structure RCH2S, which, in this case, are assumed to result from a reaction of electronically excited tyrosine moieties with cystine residues [11]. In many proteins, cross-links are formed. In the case of keratin and collagen, the cross-links are of the tryptophan-histidine and dityrosine types [11]. Cross-links formed by the combination of R-S or R-S-S radicals, both intermolecularly and intramolecularly, with incorrect sites are considered to be an important source of photoaggregation effects [8]. ESR measurements have also yielded evidence of C-H and C-N bond ruptures [8]. 

The general properties of the resins are much as to be expected. They have very good heat resistance but are mechanically much weaker than the corresponding organic cross-linked materials. This weakness may be ascribed to the tendency of the polymers to form ring structures with consequent low cross-linking efficiency and also to the low intermolecular forces. 

The cross-linking method relies on bifimctional reagents to form intermolecular linkages between the enzyme molecules to render them insoluble. Often albumin is added as an extender and glutaraldehyde is most commonly employed. This material can then be either formed as a free standing membrane or applied to the inner surface of the dialysis membrane... 

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