Thermodynamics and Kinetic Investigation of Reaction of Acriflavine with L-cysteine in Aqueous Medium

Abstract

Graphic abstract The kinetic investigation of reaction of 3,6-diamino-10-methylacridin-10-ium chloride (acriflavine) with l-cysteine in aqueous acidic medium at maximum absorption = 460 nm, ionic strength (I) = 0.1 mol dm −3 and temperature (T) = 307 K has been carried out spectrophotometrically. Using a pseudo first order approach, the rate of the redox reaction resulted to first order with respect to the [acriflavine, (AF)] and [l-cysteine, (CSH)] and second order total. Stoichiometric determination confirms that a mole of the acriflavine is consumed by a mole of l-cysteine at a time for the attainment of product formation. The reaction followed a one-way acid independence path. The adjustment in ionic strength (NaCl) and solvent polarity (water/acetone mixture) of the reaction system showed no reasonable effect and there was a fairly decrease in the reaction rate, respectively. The reaction rate is also characterised by neither catalysis nor inhibition of added ions. The negative magnitude of entropy of activation, S ‡ , (− 151.39 ± 031 J mol −1 K −1 ) and positive value of enthalpy of activation, H ‡ , (+ 31.378 ± 030 kJ mol −1 ) suggest that the reaction proceeds via an associative pathway and reasonable energy are needed to lower the activation energy for a tangible products formation. Two acridine unit products polymerised afterward forming a dimer as a result of an intramolecular coupling. A determinable intermediate has been observed through a spectroscopic test and confirmed by Michaelis–Menten’s plot. Based on Taube’s inorganic electron transfer reaction, an inner-sphere mechanistic pathway is implicated with a stable intermediate complex formation as shown below;

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