Desulfation of algal galactans

The desulfation of galactans was shown to be acid catalysed, and most likely to proceed through an intermediate complex in which SO3 is transferred to an acceptor molecule. The reaction is inhibited by molecules such as methanol and water, frequently included as 'catalysts'. The reaction for l-carrageenan appears to be stepwise, to be dependent on the nature of the acid and the sulfate acceptor, and is essentially independent of the initial counterion, unless these are the actual reagents. A useful preparative procedure is to dissolve the oven-dried sodium salt of the polysaccharide in anhydrous dimethyl sulfoxide and react it at about 120 degrees C for approximately three hours in the presence of a pyridinium salt of pyromellitic acid, excess pyridine, and an amphoteric nonhydrogen-bonding sulfate acceptor, of which arsenous oxide and antimony trioxide are preferable for removing all forms of sulfate ester in good yield. The procedure appears to work satisfactorily for each of the types of sulfate ester present in algal galactans, without significant degradation occurring of the underlying polysaccharide skeletons. (C) 1998 Elsevier Science Ltd. All rights reserved.