Only a few of you answered question 1, perhaps you were discouraged by the molecular structure given, though this was in fact only there to help you. The answers however were of a generally high standard and the marks awarded reflected this.

The answer to this question can best be considered in 3 parts. In part 1 we can look at the occurrence of 3,6 AHG,2S, and this was usually correctly stated as being in the polysaccharides of the rhodophyta or red seaweed. The two polysaccharide families in question are of course the Carrageenans and the Agars though most responses tended to concentrate on the Carrageenans and everyone correctly stated that AHG was only found in Iota andKappa and not in Lambda Carrageenans. Very little mention was made of how it occurs however and it would have been appropriate to mention here that AHG arises after synthesis of the polysaccharide by the action of a modifying enzyme on sulphated galactose residues.

The second part should cover the consequences of the presence of AHG for the polysaccharide. The presence of AHG permits the adoption of a helical conformation which can then lead to gel formation through the formation of double-helical junction zones. A diagram could also have been useful to explain this point. Again you should have pointed out here that for gel formation it is essential to have non-bonding regions of the chain and these are provided by the remaining sulphated galactoses that have not been converted to AHG but no-one mentioned this! However most replies correctly used a comparison between Iota and Kappa Carrageenans to cover the differences in gel formation and rheology related to the composition of the two polysaccharides. For the discussion of Agar structure and helix formation , this would have been the place to include the graph I showed you of the relationship between sulphate content & AHG content on one hand and gel strength on the other which would have neatly summarised all the relevant information relating to variations in agar structure.

Other information related to the structure could also have been included here such as the interactions of the different polysaccharides with cations, the potential for synergy with galactomannans and the hysteresis observed with agar. Though not directly AHG per se these considerations are of relevance to the commercial applications.

The final part should have dealt with the commercial applications of these polysaccharides and how these are related to their properties. Most of you had no problem in including sufficient relevant examples of applications with explanation of how these were related to the rheology and structure. One very important example that most of you mentioned is the application of Carrageenans in the stabilisation of milk products.

Dr. L. Ramsden, Botany, 8/6/99