Determining the location of hydrophobic spin traps within liposomes

We have recently reported on an initial study of free radical penetration into liposomal membrane using two new hydrophobic spin traps, namely 2,2-dimethyl-4-phenyl-2H-imidazole 1-oxide (DMPIO) and its 2-methyl-2-nonyl analogue (MNPIO). The radical trapping data was most readily resolved by positing that only MNPIO is lodged completely within the lipid bilayer; the N-oxide group of DMPIO, on the other hand, is actually located at or near the lipid/water interface, available for interaction with aqueous phase radicals. We have been able to confirm these results by using the ¹³C chemical shifts of these spin traps as a gauge with which to measure the polarity of the microenvironment experienced by the carbons. As expected, the spin traps experience a gradient of solvent polarities, increasing as one goes from deep within the lipid bilayer out toward the aqueous phase.