Electron Paramagnetic Resonance and Electron Spin Echo Studies of Co2+ Coordination by Nicotinamide Adenine Dinucleotide (NAD+) in Water Solution

Abstract Co2? binding to the nicotinamide adenine dinucleotide (NAD?) mole-cule in water solution was studied by electron paramagnetic resonance (EPR) and electron spin echo at low temperatures. Cobalt is coordinated by NAD? when the metal is in excess only, but even in such conditions, the Co/NAD? complexes coexist with Co(H2O)6 complexes. EPR spin-Hamiltonian parameters of the Co/NAD? complex at 6 K are gz = 2.01, gx = 2.38, gy = 3.06, Az = 94 9 10-4 cm-1, Ax = 33 9 10-4 cm-1 and Ay = 71 9 10-4 cm-1. They indicate the low-spin Co2? configuration with S = 1/2. Electron spin echo envelope modulation spectroscopy with Fourier transform of the modulated spin echo decay shows a strong coordination by nitrogen atoms and excludes the coordination by phosphate and/or amide groups. Thus, Co2? ion is coordinated in pseudo-tetrahedral geometry by four nitrogen atoms of ade-nine rings of two NAD? molecules.