Dynamic molecular oxygen accessibility to a buried Mn2+ protein site:: A high-field EPR experiment

A high-field (W-band, 3.35 T, 95 GHz) electron paramagnetic resonance (EPR) experiment to measure dynamic molecular oxygen accessibility to manganese(II) ions in liquids is described. The method is based on the direct observation of magnetic interactions between molecular oxygen and the manganese(II) ion in solution. The effect is observed as a Lorentzian broadening of the EPR line. The observation of this effect is facilitated by narrowing the manganese(II) EPR signal at high magnetic field and utilizing elevated oxygen pressures of up to 3 atm. The magnitude of the broadening effect is dependent on both the oxygen permeability of the solvent and the coordination of the manganese(II) ion and is independent of the frequency of the EPR experiment. The latter indicates that Heisenberg spin exchange between the electronic spins of oxygen and manganese(II) during bimolecular collisions is the likely broadening mechanism. The method can be also used to study dynamic molecular oxygen accessibility to manganese(II) sites in biological macromolecules, as demonstrated by an example of a buried site in Concanavalin A lectin. It was found that the oxygen accessibility of this site is significantly smaller (by a factor of 4) than that of an aqua ion and is recovered upon denaturing of the protein.