Nicotinamide adenine dinucleotide (NAD+). Formal potential of the NA+/NAD couple and NAD dimerization rate.

The rate constant, kd, for the dimerization of the free radical (NAD), produced on the initial one-electron reduction of NAD+, was measured by double potential-step chronoamperometry, fast-scan cyclic voltammetry (cathodic-anodic peak current ratio) and slow-scan cyclic voltammetry (peak potential shift) for a medium in which neither NAD+ nor its reduction products are adsorbed at the solution/electrode interface. All three methods give concordant values of kd (approx. 3 X 10(7) M-1 X s-1), which are in reasonable accord with the values determined by pulse radiolysis but are considerably greater than values previously determined electrochemically. For the NAD+/NAD couple, the heterogeneous rate constant (ks,h) exceeds 1 cm X s-1 at 25 degrees C and the formal potential (E0c) vs. sce is -1.155 V at 25 degrees C and -1.149 V at 1 degree C at pH 9.1, with an uncertainty of about +/- 0.005 V.