Micropolarities of lipid bilayers and micelles: II. Application limits of dielectric constant determinations with the polarity probe α-tocopherol

Abstract

The reaction of α-tocopherol (αT) with 1,1-diphenyl-2-picrylhydrazyl (DPPH) has been used previously (F. Bellemare and M. Fragata, J. Colloid Interface Sci. 77, 243, 1980) to probe the polarity (dielectric constant, ϵ) of the head group/hydrocarbon core interface of phospholipid bilayers. To extend these studies to other regions of the bilayers we have now used a number of protic and aprotic solvents as models. The results revealed a critical dielectric constant (cdc) range below which the αT-DPPH reaction is only slightly dependent on the dielectric constant of the solvent medium. These values are ϵ ≈ 22 for polar aprotic solvents (moderately H-bonded solvents) and ϵ ≈ 27 for nonpolar aprotic solvents (poorly H-bonded solvents). On the one hand, the data show that the αT-DPPH chemical reaction method in its present form is not applicable to low dielectric constant environments. On the contrary, we found that it yields reliable values of ϵ for the polar head group of the membranes. One important suggestion is that it might be possible to construct a polarity diagram of the lipid bilayer head group and the head group/hydrocarbon core interface if one could place the chromanol ring of the αT molecule at different depths inside the bilayer.