Abstract
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.