Probing the micellar solubilisation and inter-micellar exchange of polyphenols using the DPPH· free radical.

Encapsulation of polyphenols can be used for improving their stability and targeting. We present here a spectrophotometric method to probe the micellar solubilisation and inter-micellar exchange of polyphenols using the 2,2-diphenyl-1-picrylhydrazyl (DPPH·) free radical as a visible probe. Our method relies on the partitioning of DPPH· into micelles, on the reduction of DPPH· by polyphenols, and on the change in absorbance of DPPH· when reduced/oxidised. Hence, an absorbance drop at 528 nm gives evidence of the co-localisation of polyphenols and DPPH· in micelles. Using catechin and sodium dodecyl sulfate (SDS) as model molecules, we have shown that the reduction stoichiometry increases up to the critical micelle concentration (CMC) of SDS, where it reaches a plateau: this is due to the solubilisation of catechin in pre-micellar aggregates and then in micelles. The initial rate of reduction increases with increasing SDS concentration up to the CMC and then decreases due to a dilution effect.