Different micellar packing and hydrophobicity of the membrane probes TEMPO and TEMPOL influence their partition between aqueous and micellar phases rather than location in the micelle interior

We report the investigation of the distribution of two stable nitroxyl radicals, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and 4-hydroxy-TEMPO (TEMPOL), between aqueous and micellar phases and in the micelle interior by quenching of fluorescence of dipyridamole (DIP) and by H-1 NMR, ESR, and optical absorption spectroscopies. Cationic cetyltrimethylammonium chloride (CTAC), anionic sodium dodecylsulfate (SDS), zwitterionic N-hexadecyl-N,N-dimethyl 3-ammonio-1-propanesulfonate (HPS), and non-ionic octylphenoxypolyethoxyethanol (Triton X-100) micelles were used. In all types of micelles the binding constants for more hydrophobic TEMPO appear higher than those for TEMPOL. At the same time the H-1 NMR and optical absorption data show that the distribution of both radicals in the micelle interior is practically the same: The paramagnetic fragments are localized in the interface between the polar headgroup region and the hydrophobic interior of micelles. There is no clear correlation between K-b and the micelle charge. Indeed, K-b for TEMPO with SIDS (2150 M-1) is lower than with CTAC (3250 M-1), but higher than with Triton X-100 (1300 M-1) or HPS (1000 M-1). The K-b values correlate quite well with the packing of the monomers in the micelle, being higher for the loosely packed micelles (higher cmc). Besides, the micelle headgroup hydration water seems to decrease the binding constant. (C) 1998 Academic Press.