Kinetics and mechanism of electron transfer reaction of single and double chain surfactant cobalt(III) complex by Fe²⁺ ions in liposome (dipalmitoylphosphotidylcholine) vesicles: effects of phase transition

Abstract

In this study, we report the kinetics of reduction reactions of single and double chain surfactant cobalt(III) complexes of octahedral geometry, cis-[Co(en)₂(4AMP)(DA)](ClO₄)₃ and cis-[Co(dmp)₂(C₁₂H₂₅NH₂)₂](ClO₄)₃ (en = ethylenediamine, dmp = 1,3-diaminopropane, 4AMP = 4-aminopropane, C₁₂H₂₅NH₂ = dodecylamine) by Fe²⁺ ion in dipalmitoylphosphotidylcholine (DPPC) vesicles at different temperatures under pseudo first-order conditions. The kinetics of these reactions is followed by spectrophotometry method. The reactions are found to be second order and the electron transfer is postulated as outer sphere. The remarkable findings in the present investigation are that, below the phase transition temperature of DPPC, the rate decreases with an increase in the concentration of DPPC, while above the phase transition temperature the rate increases with an increase in the concentration of DPPC. The main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction between vesicles surface and hydrophobic part of the surfactant complexes. Besides, comparing the values of rate constants of these outer-sphere electron transfer reactions in the absence and in the presence of DPPC, the rate constant values in the presence of DPPC are always found to be greater than in the absence of DPPC. This is ascribed to the double hydrophobic fatty acid chain in the DPPC that gives the molecule an overall tubular shape due to the intervesicular hydrophobic interaction between vesicles surface and hydrophobic part of the surfactant complexes more suitable for vesicle aggregation which facilitates lower activation energy, and consequently higher rate is observed in the presence of DPPC. The activation parameters (ΔS^# and ΔH^#) of the reactions at different temperatures have been calculated which corroborate the kinetics of the reaction.

Keywords:

• surfactant complex
• electron transfer
• vesicles
• hydrophobicity
• phase transition
• outer sphere

Acknowledgments

Karuppiah Nagaraj sincerely acknowledges the University Grants Commission (UGC), New Delhi for UGC-SAP RFSMS fellowship.

Additional information

Funding

Financial assistance from the CSIR [grant number 01(2461)/11/EMR-II)]; DST [grant number SR/S1/IC-13/2009]; UGC [grant number 41-223/2012(SR)] sanctioned to Prof. S. Arunachalam is also gratefully acknowledged.