Calorimetric measurements of proton adsorption onto Pseudomonas putida

Abstract

Having an understanding of the reactive nature of the bacterial surface is enhanced when its reactivity is considered in a thermodynamic framework. Towards this end, isothermal titration calorimetry was used to measure heats of proton adsorption onto Pseudomonas putida, a common Gram negative soil bacterium. Proton adsorption generated large exothermic heats and proton uptake continued down to pH 2.5. Applying a surface complexation model to the calorimetric data allowed for the derivation of site-specific enthalpies and entropies of proton adsorption. The 4-site non-electrostatic model of Borrok et al. [D.M. Borrok, J.B. Fein, J. Colloid Interface Sci. 286 (2005) 110] was chosen to describe proton adsorption and enabled derivation of site-specific enthalpies of −2.4 ± 0.3, −3.7 ± 0.2, −9.0 ± 0.6, and −36.0 ± 1.2 kJ/mol for Sites 1–4, respectively. Entropies of proton adsorption were calculated to be 51 ± 3, 75 ± 1, 91 ± 2, and 55 ± 4 J/mol K, for Sites 1–4, respectively. Enthalpies and entropies of Sites 1 and 3 are consistent with that of multifunctional organophosphonic acids, Site 2 is consistent with multifunctional carboxylic acids, and Site 4 is consistent with an amine. Temperature dependence of the acidity constants for Sites 1–3 is predicted to be minimal; however, Site 4 is predicted to more substantially affected by temperature.