Acid catalysed decomposition of NADH

Braun et al. [49] showed that NADH decomposes much more rapidly under acidic than neutral conditions. They found that the decomposition was first-order in both cases with the respective rate constants being 5.9 x 10 4s l at pH 4.1 and 0.11 x 10-4s at pH 7.1 for a 0.62 mmol dm-3 NADH solution in the Tris buffer. The authors propose that the 

Analytical models of modified electrodes for NADH oxidation 

To determine the effect of acid catalysed decomposition of NADH on the electrochemical response in our experiments, the decrease in oxidation current for NADH was recorded as a function of time. The results of this experiment were compared with the decrease in NADH concentration as spectrophotometrically determined. The rates of decrease of the current and the concentration of NADH are both first-order and occur on similar timescales (Fig. 2.14). Analysis of the data for the two experiments provide first-order rate constants of 1.68 and 1.16 x 10-4 s-1 for the electrochemical and spectrophotometric measurements, respectively. The small difference between these two constants can be explained by the additional consumption of NADH by reaction at the electrode during the electrochemical measurement. This electrochemical process is also a first-order rate process, and the extent of the effect can be determined by using the treatment of Hitchman and Albery [50] for electrolysis using a rotating disc electrode. The results are consistent with the observed difference in the two rate constants. 

To determine whether, or not, it would be possible to remove the effect of acid catalysed decomposition of NADH by working at neutral pH, the decomposition rate was measured at pH 7. The decomposition was again found to be first-order, but with a rate constant of 8.6 x 0-6 s-1. Comparing this to the value of 1.16 x 1CT4 s-1, determined for the decomposition in pH 5 buffer, shows that the decomposition rate has decreased by an order of magnitude. 

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