Spontaneous crosslinking

Richard at Rh6ne-Poulenc [67] and Zosel at BASF [68] have carried out extensive studies on the dynamic mechanical properties of latex films. They examined latex made from monomers (like butyl acrylate and butadiene) that crosslink spontaneously during emulsion polymerization. Here crosslink density was reduced through the use of chain transfer agents. Alternatively, with monomers like BMA which form non-crosslinked latex, crosslinking was induced through the addition of bifimctiona] monomers such as methallyl methacrylate (MAMA). Richard has recently published a review of his work in this area [75]. 

Stretching a polymer sample tends to orient chain segments and thereby facilitate crystallization. The incorporation of different polymer chains into small patches of crystallinity is equivalent to additional crosslinking and changes the modulus accordingly. Likewise, the presence of finely subdivided solid particles, such as carbon black in rubber, reinforces the polymer in a way that imitates the effect of crystallites. Spontaneous crystal formation and reinforcement... 

Fig. 4 Structures and formation routes of crosslinks in elastin. In the first step, lysine is catalyti-cally converted to allysine by lysyl oxidase all subsequent condensation steps are spontaneous...

Similarly to the phospholipid polymers, the MPC polymers show excellent biocompatibility and blood compatibility [43—48]. These properties are based on the bioinert character of the MPC polymers, i.e., inhibition of specific interaction with biomolecules [49, 50]. Recently, the MPC polymers have been applied to various medical and pharmaceutical applications [44-47, 51-55]. The crosslinked MPC polymers provide good hydrogels and they have been used in the manufacture of soft contact lenses. We have applied the MPC polymer hydrogel as a cell-encapsulation matrix due to its excellent cytocompatibility. At the same time, to prepare a spontaneously forming reversible hydrogel, we focused on the reversible covalent bonding formed between phenylboronic acid and polyol in an aqueous system. 

PMBV and PVA can spontaneously form a hydrogel without using any crosslinkers. Even in cell culture conditions, the gelation can be confirmed. Therefore, it was possible to encapsulate cells in the PMBV/PVA hydrogel. The encapsulation method is illustrated in Fig. 6. 

Insoluble polystyrene crosslinked with divinylbenzene can easily be converted by sulfonation to a usable ion exchanger. For this purpose a mixture of 0.2 g of silver sulfate and 150 ml of concentrated sulfuric acid are heated to 80-90 °C in a 500 ml threenecked flask fitted with stirrer, reflux condenser, and thermometer. 20 g of a bead polymer of styrene and divinylbenzene (see Example 3-41) are then introduced with stirring the temperature climbs spontaneously to 100-105 °C.The mixture is maintained at 100 C for 3 h,then cooled to room temperature and allowed to stand for some hours. Next the contents of the flask are poured into a 11 conical flask that contains about 500 ml of 50% sulfuric acid. After cooling, the mixture is diluted with distilled water, and the gold-brown colored beads are filtered off on a sintered glass filter and washed copiously with water. 

Rusakov 107 108) recently proposed a simple model of a nematic network in which the chains between crosslinks are approximated by persistent threads. Orientional intermolecular interactions are taken into account using the mean field approximation and the deformation behaviour of the network is described in terms of the Gaussian statistical theory of rubber elasticity. Making use of the methods of statistical physics, the stress-strain equations of the network with its macroscopic orientation are obtained. The theory predicts a number of effects which should accompany deformation of nematic networks such as the temperature-induced orientational phase transitions. The transition is affected by the intermolecular interaction, the rigidity of macromolecules and the degree of crosslinking of the network. The transition into the liquid crystalline state is accompanied by appearence of internal stresses at constant strain or spontaneous elongation at constant force. 

Formaldehyde is a toxic substance that reacts spontaneously with amino groups of proteins and nucleic acids, hydroxymethylating them and forming methylene-bridge crosslinks between them. Free formaldehyde therefore wreaks havoc in living cells and could not serve as a useful hydroxymethylating agent. In the form of A5,A10-methylenetetrahydrofolate, however, its chemical reactivity is attenuated but retained in a potentially available form where needed for specific enzymatic action. Formate, how-... 

Tropoelastin molecules are crosslinked in the extracellular space through the action of the copper-dependent amine oxidase, lysyl oxidase. Specific members of the lysyl oxidase-like family of enzymes are implicated in this process (Liu etal, 2004 Noblesse etal, 2004), although their direct roles are yet to be demonstrated enzymatically. Lysyl oxidase catalyzes the oxidative deamination of e-amino groups on lysine residues (Kagan and Sullivan, 1982) within tropoelastin to form the o-aminoadipic-6-semialdehyde, allysine (Kagan and Cai, 1995). The oxidation of lysine residues by lysyl oxidase is the only known posttranslational modification of tropoelastin. Allysine is the reactive precursor to a variety of inter- and intramolecular crosslinks found in elastin. These crosslinks are formed by nonenzymatic, spontaneous condensation of allysine with another allysine or unmodified lysyl residues. Crosslinking is essential for the structural integrity and function of elastin. Various crosslink types include the bifunctional crosslinks allysine-aldol and lysinonorleucine, the trifunctional crosslink merodes-mosine, and the tetrafunctional crosslinks desmosine and isodesmosine (Umeda etal, 2001). 

AP sites, left unrepaired, impede the progression of DNA polymerases during DNA replication and can be mutagenic in the synthesis of both DNA and RNA. AP sites can also be converted to toxic lesions by spontaneous rearrangements that produce structures that can crosslink with proteins and lipids. They can also create toxic lesions by the activities of topoisomerase I and topoisomerase II. In addition, the processing of AP sites by the BER proteins can produce structures that are toxic. Hence, to prevent these deleterious events from occurring, the BER process is likely coordinated and each step involves a handing off from one enzyme in the pathway to the next. 

For purposes of this manuscript, we wish to concentrate only on the steps leading to the formation of desmosines, amino acids found predominantly in elastin. With respect to their formation, the following suggests their spontaneous formation from peptidyl lysine and the oxidation product, peptidyl allysine. Narayanan et al. (28,29) have shown that when purified lysyl oxidase and non-crosslined elastin, specifically tropoelastin, are incubated together, the desmosines are formed. Desmosine formation, however, only occurs at temperatures that favor fibrillar arrangements of tropoelastin. Subsequently, it is felt that the maturation of non-crosslinked elastin into cross-linked elastin appears to involve only two major steps, namely insolublization through the formation of fibrils and fixation of the fibrils by crosslinking. 

The preparation of disulfide-stabilized nanogel PEG/DNA complexes with a diameter of around 100 nm was reported by Mok et al. [128], A thiol-functionalized six-arm branched PEG was used for DNA solubilization in DMSO, which led to spontaneous PEG/DNA nanocomplex formation. Subsequent addition of the thiol-reactive dithio-bis-maleimidoethane (DTME) disulfide crosslinker led to stabilized nanogels (Fig. 6) that showed high gene transfection efficiency. 

---