The intrinsic structural complexity of proteins makes it hard to identify the contributions of each noncovalent interaction behind the remarkable rate accelerations of enzymes. Coulombic forces are evidently primary, but despite developments in artificial nanoreactor design, a picture of the extent to which these can contribute has not been forthcoming. Here we report on two supramolecular capsules that possess structurally identical inner-spaces that differ in the electrostatic potential (EP) field that envelops them: one positive and one negative. This architecture means that only changes in the EP field influence the chemical properties of encapsulated species. We quantify these influences via acidity and rates of cyclization measurements for encapsulated guests, and we confirm the primary role of Coulombic forces with a simple mathematical model approximating the capsules as Born spheres within a continuum dielectric. These results reveal the reaction rate accelerations possible under Coulombic control and highlight important design criteria for nanoreactors.