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 soi l bacterium. To describe proton binding to this material both salt eff ects and pH effects must be taken into account. A stepwise analysis, m ade by first eliminating the influence of the salt level and then anal yzing the intrinsic protonation behavior, leads to a very good descrip tion of the original data. The specific volume of the cell walls is de rived from charging curves measured at different salt levels using a m aster-curve approach in combination with a Donnan model to account for electrostatic effects, Results indicate that swelling becomes signifi cant 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 potenti als obtained with this approach are close to potentials derived from e lectrophoretic mobility measurements. The intrinsic proton affinity di stribution of the protonation constants is obtained. Results indicate the presence of chemical heterogeneity, At least three different group s are distinguished. On the basis of this affinity distribution analys is 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 co nstants obtained are physically realistic. (C) 1995 Academie Press, In c