As expected due to the application of a phosphate buffer, experimental
results showed auto-oxidation of the cofactor NADH over time, as
previously reported in literature. In order to model the reaction system
as precisely as possible, the auto-oxidation of NADH was measured at
wavelengths of λ1 = 340 nm and
λ2 = 292 nm in a time range of 48 hours using a
VWRTM UV-1600PC Spectrophotometer. Additionally,
absorption values at λ3 = 260 nm increased analogous to
NADH-oxidation during these experiments, indicating that NADH is
oxidized to NAD+. In our mathematical model it is
therefore assumed, that NADH auto-oxidizes to NAD+.
The resulting NADH-oxidation for this system is described through a
function of exponential decay as shown in equation (5):