Some novel aspects of DNA physical and chemical gels

DNA chemical and physical networks have been investigated with respect to their stability and swelling, and rheological properties. Chemical networks were prepared by cross-linking DNA with ethylene glycol diglycidyl ether (EGDE), and physical networks by association with cationic polymers. Both types of gels display strong elastic properties and have a shear thinning behaviour. Addition of cationic surfactants effectively collapses the chemical gels, de-swelling starting from a critical aggregation concentration (cac) much lower than the critical micelle concentration (cmc) but similar to that for binding of surfactant to DNA in solution. The swelling-deswelling process appears to be reversible; thus the addition of an anionic surfactant to a gel collapsed by cationic surfactant gives a gel volume close to that of the original gel. Physical networks prepared by mixing DNA (either single- or double-stranded) with cationic polyelectrolytes, both derivatives of hydroxyethyl cellulose, one of them carrying hydrophobic groups, show an intriguing asymmetric phase separation and a very different rheological response from that of the polymers alone. Phase maps of the mixtures show three distinctive regions, a two-phase region, a bluish one-phase region and a transparent one-phase region. Effects due to hydrophobic groups on the polymers are relatively minor.