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⁺) 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(H₂O)₆ complexes. EPR spin-Hamiltonian parameters of the Co/NAD⁺ complex at 6 K are <i>g</i>_<i>z</i> = 2.01, <i>g</i>_<i>x</i> = 2.38, <i>g</i>_<i>y</i> = 3.06, <i>A</i>_<i>z</i> = 94 × 10^-4 cm^-1, <i>A</i>_<i>x</i> = 33 × 10^-4 cm^-1 and <i>A</i>_<i>y</i> = 71 × 10^-4 cm^-1. They indicate the low-spin Co²⁺ configuration with <i>S</i> = 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 adenine rings of two NAD⁺ molecules.