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

Co 2+ binding to the nicotinamide adenine dinucleotide (NAD + ) molecule 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(H 2 O) 6 complexes. EPR spin-Hamiltonian parameters of the Co/NAD + complex at 6 K are g z = 2.01, g x = 2.38, g y = 3.06, A z = 94 × 10 −4 cm −1 , A x = 33 × 10 −4 cm −1 and A y = 71 × 10 −4 cm −1 . They indicate the low-spin Co 2+ 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, Co 2+ ion is coordinated in pseudo-tetrahedral geometry by four nitrogen atoms of adenine rings of two NAD + molecules.