EFFECT OF DEACETYLATION ON THE SYNERGISTIC INTERACTION OF ACETAN WITHLOCUST BEAN GUM OR KONJAC MANNAN

It has been discovered that deacetylation of the bacterial polysacchar ide acetan promotes synergistic interactions with either locust bean g um (LBG) or konjac mannan (KM). Acetan is similar in structure to xant han, and adopts a similar 5-fold conformation in the solid state. Like xanthan, it shows a thermally reversible order (helix)-disorder (coil ) transition in solution. Both polymers have a cellulosic backbone wit h charged (anionic) sidechains attached at O-3 of alternate glucosyl r esidues, but the sidechains in acetan are longer (pentasaccharide rath er than trisaccharide) and do not contain pyruvic substituents. Acetan has two sites of acetylation, one at O-6 of the inner mannosyl residu e of the carbohydrate sidechains (as in xanthan) and the other on the polymer backbone (believed to be at O-6 of the branched glucosyl resid ues). Solutions of acetan or deacetylated acetan were equilibrated aga inst 10 mM potassium chloride (to stabilise the ordered conformation) and were mixed (at 25 degrees C) with solutions of LBG or KM, also equ ilibrated against 10 mM potassium chloride. Unlike xanthan, native ace tan showed no evidence of synergistic interaction with either LBG or K M. After deacetylation, however, large enhancements were observed in d ilute-solution viscosity, and thermoreversible gels were formed at hig her concentrations. With KM as co-synergist, gel melting was accompani ed by an intense endotherm in differential scanning calorimetry. The m agnitude of this endotherm increased with storage time at 25 degrees C , reaching a final value of Delta H approximate to 15.9 J/g (in compar ison with Delta H approximate to 5.0 J/g for the order-disorder transi tion of deacetylated acetan alone). It is suggested that interaction o ccurs by formation of heterotypic junctions between the acetan backbon e and unsubstituted regions of the plant polysaccharide, and that the acetate groups on native acetan promote solubility and hence inhibit a ssociation. (C) 1998 Elsevier Science Ltd.