Catalytic Behavior of a Series of Cationic Gemini (16-s-16 Type, s = 4, 5, 6) and CTAB Surfactants on the Reaction of Ninhydrin with [Ni(II)–Gly-Phe]⁺

Abstract

The influence of cationic CTAB/gemini surfactants on the reaction of ninhydrin with the nickel–glycylphenylalanine ([Ni(II)–Gly-Phe]⁺) complex was investigated spectrophotometrically, at 70 °C and pH = 5.0. The kinetics were found to follow a pseudo-first- and fractional-order reaction path with respect to concentrations of [Ni(II)–Gly-Phe]⁺ complex and ninhydrin, respectively. The values of rate constant (k _ψ) increase with increase in CTAB concentration and reach a maximum value, and then, with further increase in the CTAB concentration, a slow decreasing effect is observed. In the case of geminis, k _ψ increase and leveling-off regions (just like the monocationic CTAB) are obtained with [geminis], in addition to a third region of increasing k _ψ at higher concentrations. On the basis of the observed data a probable mechanism has been proposed.

Appendix

The rate equation for Eqs. 4–7 is given by:

$$ \frac{{ - {\text{d}}(([{\text{Ni(II)}}{-}{\text{Gly-Phe}}]^{ + } )_{\text{W}} + ( [ {\text{Ni(II)}}{-}{\text{Gly-Phe}}]^{ + } )_{\text{m}} )}}{{\text{d}}t} = \frac{{ - {\text{d}}([{\text{Ni(II)}}{-}{\text{Gly-Phe}}]^{+})_{{t}} }}{{\text{d}}t} = \frac{{ - {\text{d[Product]}}}}{{\text{d}}t} = k_{\text{W}}^{{\prime }}( [ {\text{Ni(II)}}{-}{\text{Gly-Phe}}]^{+})_{\text{W}} + k_{\text{m}}^{{\prime }} ( [ {\text{Ni(II)}}{-}{\text{Ni(II)}}{-}{\text{Gly-Phe}}]^{+})_{\text{m}}$$

(10)

where ([Ni(II)–Gly-Phe]⁺) _t is the stoichiometric concentration of the metal–peptide complex at time t. The observed rate constant for the product formation, k _ψ, is given by:

$$ k_{\varPsi } \, = \frac{{ - {\text{d([Ni(II)}}{-}{\text{Gly-Phe]}}^{ + } )_{{t}} }}{{\text{d}}t} / ( [ {\text{Ni(II)}}{-}{\text{Gly-Phe}}]^{ + } )_{{t}} = k_{\text{W}}^{{\prime }} A_{\text{W}} + k_{\text{m}}^{{\prime }} A_{\text{m}} $$

(11)

where A _w and A _m are the fractions of the uncomplexed and complexed substrates, respectively. Often, for a pseudo-first-order process [D _n] ≫ ([Ni(II)–Gly-Phe]⁺)_m and A _m is constant. The equilibrium constant, K _S, can be expressed in terms of the concentrations and in terms of the fractions of the complexed and uncomplexed substrates:

$$ K_{\text{S}} = \frac{{ ( [ {\text{Ni(II)}}{-}{\text{Gly-Phe}}]^{ + } )_{\text{m}} }}{{ ( ( [ {\text{Ni(II)}}{-}{\text{Gly-Phe}}]^{ + } )_{{t}} - ( [ {\text{Ni(II)}}{-}{\text{Ni(II)}}{-}{\text{Gly-Phe}}]^{ + } )_{\text{m}} ) [D_{\text{n}} ]}} = \frac{{A_{\text{m}} }}{{ [D_{\text{n}} ]A_{\text{W}} }} = \frac{{A_{\text{m}} }}{{ [D_{\text{n}} ] ( 1- A_{\text{m}} )}} $$

(12)

Combination of Eqs. 11 and 12 and rearranging leads to:

$$ k_{\psi } = \frac{{k_{\text{W}}^{{\prime }} + k_{\text{m}}^{{\prime }} K_{\text{S}} [D_{\text{n}} ]}}{{ 1+ K_{\text{S}} [D_{\text{n}} ]}} $$

(13)