High-field EPR Study of the Effect of Chloride on Mn2+ Ions in Frozen Aqueous Solutions

The effect of chloride concentration on Mn2+ (S = 5/2, I = 5/2) ions in frozen aqueous solutions is studied by high-field high-frequency electron paramagnetic resonance (HFEPR). The usually six sharp lines characteristic of Mn2+ ions, arising from the ms = −1/2 → 1/2 transition, is modified by the addition of Cl− anions and the six resonances become much broader and more complex. This new feature likely arises from the ligation of one Cl− anion to a hydrated Mn2+ ion forming a [Mn(H2O)5Cl]− complex. This complex increases linearly with Cl− concentration with an association constant of Ka, apparent = 61 M−1. The structure of the putative chloride complex was studied using density functional theory calculations and the expected zero-field interaction of such a manganese center was calculated using the superposition model. The predicted values were similar to those determined from the simulation of the spectrum of the ms = −5/3 → −3/2 transition of the chloride complex. This effect of Cl− anions occurs at biologically relevant concentration and can be used to probe the Mn2+ ions in cellular and protein environments.