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Abstract

The effect of microbial pectin esterases (Aspergillus foetidus) and higher plants (tomatoes, alfalfa) on high-esterified pectin was investigated. Pectin derivatives of esterification degree E 42-66% were prepared by a partial deesterification catalyzed by the above-mentioned enzymes. The distribution pattern of free and esterified carboxyl groups in the linear pectin macromolecule was determined on the basis of activity coefficients of counterions γ_Ca²⁺ as found in solutions of the proper calcium pectinates. The activity coefficients γ_Ca²⁺ were compared with the corresponding γ_Ca²⁺ values determined in pectins partially deesterified with alkali, with a random distribution pattern of free carboxyl groups. The plant pectin esterases under investigation lead to a block-wise arrangement of the free carboxyl groups in the pectin molecule, whereas A. foetidus pectin esterase cleaves randomly the ester groups in the same mode as do the alkali hydroxides. Significant differences were observed with final products of enzyme deesterification of pectin even in the selectivity of Ca²⁺ 2K⁺ cation exchange, as documented by the change of electrostatic free enthalpy of exchange of these cations. The different mechanism of action of plant and A. foetidus pectin esterases was thus unambiguously proved by a physico-chemical method.