PH DEPENDENT CHARGING BEHAVIOR OF ISOLATED CELL-WALLS OF A GRAM-POSITIVE SOIL BACTERIUM

The cell walls of Gram-positive bacteria are highly porous structures. Peptidoglycan, the main component of these cell walls, contains many acidic groups, leading to a pH- and salt-dependent charge. This charge is involved in many processes, such as the attachment to surfaces and the binding of metal ions, Acid-base titrations are performed on cell wall material from Rhodococcus erythropolis A177, a Gram-positive soil bacterium. To describe proton binding to this material both salt effects and pH effects must be taken into account. A stepwise analysis, made by first eliminating the influence of the salt level and then analyzing the intrinsic protonation behavior, leads to a very good description of the original data. The specific volume of the cell walls is derived from charging curves measured at different salt levels using a master-curve approach in combination with a Donnan model to account for electrostatic effects, Results indicate that swelling becomes significant for salt levels below 0.1 M, and that above pH 5 at a given salt level the volume is independent of the particle charge, Donnan potentials obtained with this approach are close to potentials derived from electrophoretic mobility measurements. The intrinsic proton affinity distribution of the protonation constants is obtained. Results indicate the presence of chemical heterogeneity, At least three different groups are distinguished. On the basis of this affinity distribution analysis a binding model, the Langmuir-Freundlich equation, is selected. The combination of the proton binding model and the Donnan model gives a very close reproduction of the observed charging behavior, AU model constants obtained are physically realistic. (C) 1995 Academie Press, Inc