Influence of cationic and anionic micelles on the reaction of ninhydrin with different amino acids: A kinetic study

Abstract

The kinetics of the reaction of ninhydrin with six amino acids in the presence of cationic micelles of cetyltrimethyl ammonium bromide (crAB) and anionic micelles of sodium dodecylsulphate (SDS) have been studied spectrophotometrically at pH 5.0 at 575 om. Both the micelles strongly inhibit the reaction due to the association of the substrate with the micelles. The results are best accounted for by the distribution of substrate into micellar and aqueous pseudo-phases as well as the combination of substrate molecules with surfactant molecules. From the kinetic data, micelle-substrate association constants <em>(K_s), </em>rate constant (k_m), fraction of substrate associated with micelles <em>(</em><em>a</em><em>), </em>kinetic erne value, index of cooperativity <em>(n) </em>and activation parameters have been calculated. The negative km values suggest that the reaction does not occur in the micellar reaction zone (Stern-layer). The nonpolar side chain of amino acids play no significant kinetic role. Association of a dehydrated form of ninhydrin at the surface of the micelles is negligible.