Gels are elastic, solid-like coherent systems of continuous structure and composed of at least two components'. Polysaccharides form several different types of gel with water. Most commonly, long-chain polysaccharides produce hydrogels through complex junction-zones, thermoreversible helical regions, or other coaggregation phenomena213 . However, more-stable hydrogels are obtained by covalently crosslinking polysaccharide chains. Sephadex" G (Pharmacia AB, Uppsala, Sweden) are such covalently crosslinked gels based on partially degraded fractions of dextran, a linear, (l-6)-linked a-D-glUCan having little (l-+3) branch-ing4. They are made by mixing epichlorohydrin with dextran in concentrated sodium hydroxide solution; the resulting, three-dimensional network consists of ldeoxyglycerol-l-y1 ("glyceryl")-linked dextran chains having free glyceryl side chains4 (1). The swelling and water content of the gels depend on the degree of crosslinking'. Crosslinking itself depends on the molecular weight of the dextran and on the relative proportions of epichlorohydrin to dextran during gelation4. For the Sephadex G gels, the water content increases from the most tightly crosslinked gel, G-10, having 1 g of water/g of dry gel through a series of gels (G-15, G-25, G-50, G-75, and G-100) of ever increasing water content to the most loosely crosslinked gel, G-200, which has a water content of 20 g of water/g of dry ge14.
The structure, types and degrees of crosslinking, and physical properties of these Sephadex G gels are only partially known. This report describes gel characteristics obtained by liquid-state '3C-n.m.r. spectroscopy. Liquid-state 13C-n.m.r. spectroscopy, commonly used for analysis of polysaccharide structure5, has been applied to covalently crosslinked synthetic gel&', hydrogen-bonded polysaccharide hydrogels3*8y9, and" Sephadex G-75. Solid-state, magic-angle spinning 13C-n.m.r. spectroscopy has also been used for gels", although ultracentrifugal gel compression'2 could decrease the solvent content of the gel.