KINETIC DETERMINATION OF CYSTEINE BASED ON ITS INHIBITORY EFFECT ON THE SILVER-CATALYSED LIGAND SUBSTITUTION REACTION BETWEEN HEXACYANOFERRATE(II) AND DPQ

Cysteine is a biologically important sulfur-containing amino acid that plays a vital role in numerous physiological processes, including protein synthesis, antioxidant defence, detoxification, and cellular redox regulation. Owing to its strong affinity towards soft metal ions such as silver(I), cysteine readily forms stable metal-thiol complexes that significantly influence metal-catalysed reactions. Exploiting this property, the present study describes a simple, rapid, and highly sensitive kinetic spectrophotometric method for the determination of trace amounts of cysteine based on its inhibitory effect on the silver(I)-catalysed ligand substitution reaction between hexacyanoferrate(II) and dipyrido[3,2-f:2′,3′-h]quinoxaline (DPQ). The reaction was monitored spectrophotometrically at 547 nm through the formation of the dark green complex, [Fe(CN)_5 DPQ]^(3-), under optimized experimental conditions of pH 3.0 ± 0.02, temperature 25.0 ± 0.1°C, and ionic strength 0.05 M maintained with potassium nitrate. Linear calibration plots were obtained over a wide concentration range of cysteine using fixed-time kinetic measurements. The proposed method exhibited excellent sensitivity with a detection limit in the sub-micromolar range and demonstrated good precision and reproducibility. The kinetic data were interpreted using a modified Michaelis–Menten model to evaluate the Michaelis constant (Km), catalyst–substrate stability constant (Ks), and catalyst–inhibitor formation constant (KCI). A plausible reaction mechanism explaining the inhibitory behaviour of cysteine through catalyst complexation is also proposed. The developed method provides an inexpensive alternative to sophisticated instrumental techniques for the determination of trace cysteine in analytical and environmental samples.