Effect of Curcumin on Peptide-Membrane Interaction: In Kinetics and in Equilibrium

Abstract

We performed a three-chamber aspirated GUV experiment to study the competitive binding between curcumin and melittin. GUVs were produced and aspirated in chamber 1. Chamber 2 contained 10 microM curcumin. Chamber 3 contained 10 microM curcumin and various concentrations of peptides: penetratin, melittin or magainin. Aspirated GUVs were first equilibrated in chamber 2 and then transferred to chamber 3, where we observed the response of the GUV to the addition of peptides. Without the peptides in chamber 3, the GUV should remain unchanged. The results can be described as the peptides replacing curcumin bound on the bilayer interface and shifting curcumin into the hydrocarbon region, except for magainin. The bound curcumin essentially blocked magainin from binding to the membranes. This can be explained as a kinetic effect because the effect size of a bound magainin is twice the effective size of a bound curcumin, whereas the effective sizes of penetratin and melittin are comparable to curcumin (this is an usual property of curcumin for its small molecular size). Thus for penetratin and melittin, the behavior of the peptides is the same with or without curcumin. On the other hand, the curcumin/peptide/lipid mixtures produce bilayers totally unlike either curcumin with lipid or peptide with lipid. Instead of steady membrane thinning by either melittin or curcumin alone, the addition of melittin to curcumin/DOPC actually thickening the bilayers. This is the first example of incompatible results from equilibrium experiment and from kinetic experiment. It implies that competitive binding of two different membrane-active molecules can be complicated, a caution for drawing conclusion about synergy effects.