'Spin-labelling' with the second hydrogen radioisotope: Radicals formed from squalene by muonium addition

By radio-labelling with muonium (the second hydrogen radioisotope), a free radical species has been clearly identified in squalene (2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene) with a muon- electron hyperfine coupling of 240.7 MHz, as measured by the muon spin rotation (MuSR) technique. The radical is undetectable in pure squalene due to its high viscosity and the large molecular size which leads to extreme line broadening, but its signals may be resolved on reducing the viscosity of the medium by dilution with diethyl ether. The potential of the MuSR method is thus demonstrated as a means for spin-labelling radical species such as are formed from cellular antioxidants; ESR spectroscopy is unsuitable for studies of radicals formed from this molecule, due to extreme line-broadening and spectral complexity.