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

Abstract

The rate constant, k d, 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 k d (approx. 3·10 7 M −1·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 ( k s,h) exceeds 1 cm·s −1 at 25°C and the formal potential ( E 0 c) vs. sce is − 1.155 V at 25°C and − 1.149 V at 1°C at pH 9.1, with an uncertainty of about ±0.005 V.