kinetics was studied with EPR and optical stopped-flow techniques. The analysis showed that formation of Cys-DNIC was first order in ferrous iron and second order in Cys-NO
The calculation of the third-order rate of formation gave k = (1.0 ± 0.2) x 105M-2s-1 (estimated by EPR method) or (2.0 ± 0.1) x s 1 (estimated by optical method). The possible reason for the small discrepancy was considered in . It should be noted that DNIC is formed from Cys-NO at a fourfold faster rate than from free NO molecules . This is significant because free NO has a far higher diffusive mobility than Cys-NO due to its smaller size.
Once formed, the Cys-DNIC is not rigorously stable, but has a finite lifetime that depends on the levels of its reaction partners in the chemical equilibria. The excess concentration of free thiol ligands is a particularly important parameter: At lower concentration of free cysteine, the lifetime of paramagnetic Cys-DNIC forming from Cys-NO and iron notably diminished
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