Effect of dicationic gemini surfactants 16-s-16 (s=4, 5, 6) on the ninhydrin-dipeptide (glycyl-tyrosine) reaction

The effect of dicationic gemini surfactants H33C16(CH3)2N+-(CH2)s-N+(CH3)2 C16H33, 2Br- (s= 4, 5, 6) on the reaction of a dipeptide glycyltyrosine (GlyTyr) with ninhydrin has been studied spectrophotometrically at 70 degrees C and pH 5.0. The reaction follows first- and fractional-order kinetics, respectively, in [GlyTyr] and [ninhydrin]. The gemini surfactant micellar media are comparatively more effective than their single chainsingle head counterpart cetyltrimethylammonium bromide (CTAB) micelles. Whereas typical rate constant (k?) increase and leveling-off regions, just like CTAB, are observed with geminis, the latter produces a third region of increasing k? at higher concentrations. This subsequent increase is ascribed to the change in the micellar morphology of the geminis. The pseudophase model of micelles was used to quantitatively analyze the k? - [gemini] data, wherein the micellar-binding constants KS for [GlyTyr] and KN for ninhydrin were evaluated. (C) 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 800809, 2012