Proton relaxation times and hydration numbers of manganese ions bound to enzymes

The frequency dependence of the proton relaxation times in aqueous solutions of manganese carboxypeptidase A and pyruvate kinase has been measured and analysed. The electronic spin longitudinal relaxation time was found to be frequency dependent and to control the proton relaxation times at low frequencies. The number of exchangeable water molecules in the coordination shell of bound manganese was found to be one for carboxypeptidase A and two for pyruvate kinase.