IMPACT OF THE DONNAN EFFECT ON THE ACTION OF XYLANASES ON FIBER SUBSTRATES

Carboxylic acid bound to xylans in the fibre matrix cause a pH gradien t between the fibre and the surrounding solution, known as the Donnan effect. The gradient is dependent on the ionic strength of the fibre s olution. When metal-free kraft pulp was used as a substrate for the Tr ichoderma reesei xylanase, the apparent pH optimum of the xylanase at low ionic strengths was found to be significantly higher, around 9, wh en a monovalent hydroxide was used for pH adjustment, as compared to t he pH optimum obtained with a divalent metal hydroxide, i.e. pH 5-6. B y increasing the ionic strength of the pulp solution by salt addition, the pH optimae of the xylanase became 5-6 with all the hydroxides. Th is difference was caused by the Donnan effect. Thus, at low ionic stre ngths the actual pH is lower than that measured in the solution, resul ting in an increased apparent pH optimum of the xylanase. In practice, these results are important when applying xylanases on practical, fib re bound substrates, such as kraft pulps. (C) 1997 Elsevier Science B. V.