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It is well known that many surface-active molecules aggregate into micelles at concentrations exceeding a certain value, the critical micelle concentration (cmc).1 Studies of micelles are of interest because these relatively simple aggregates are model systems for biological membranes.' Useful new information about the solubilizing properties of micelles can be obtained using molecular probes capable of associating with the micelles. One such approach would be to introduce into solution a stable free radical probe which can be observed by electron spin resonance (ESR). Encouragement for using a free radical probe was provided by several earlier optical experiments which also used probes (in each case a dye).1' 2 Nevertheless, our early attempts to investigate micelles using stable free radical probes were not successful because of the complex ESR spectra of the probes then available.3 Recently Hoffmalln and Henderson,4 and Rozantzev and co-workers5 6 have synthesized a remarkable new class of nitroxide free radicals and we wish to report here the first investigation of micelles7 using two of these stable free radicals, I and II, as probes. CH2 CH3 CH3 CH3N10 02N NH-N -0 N CH3 CH3 H CH3 NO2 CH3 I II Experimental.-The nitroxide free radical I is the 2,4-dinitrophenyl hydrazolle of 2,2,6,6-tetramethyl-4-piperidone nitrogen oxide, and it was prepared as described previously.5' 8 The free radical II, 2,2,4,4-tetramethyl-1,2,3,4-tetrahydro-'y-carboline-3-oxyl, was prepared according to the procedure of Rozantzev and Shapiro.9 The sodium dodecyl sulfate (99+% NaDS) was obtained from -Mann Research Laboratories and was used without further purification. The question of adequate purity of a surface-active molecule is always a difficult one. However, nearly all of the measurements were repeated using the average commercial 95 per cent NaDS and there was no appreciable difference in the data. Apparently the magnetic resonance measurements are not extremely sensive to impurities in the NaDS and further purification of the 99+ per cent NaDS for this work was not warranted. All ESR spectra were recorded on a Varian V4502 X-band spectrometer. A Varian V-4.532 dual cavity was used to determine the solubilities of I and II. In obtaining the rotational correlation time data, the probe concentration was 2 X 10-5 31 when the NaDS coneentration was above approximately 0.2 per cent. Below this point the concentration of probe was reduced because of solubility limitations and each solution was saturated with probe before use. The optical and nuclear magnetic resonance (NMIR) data were obtained using a Cary model 15 spectrophotometer and a Varian AG0 spectrometer, respectively.

MAGNETIC RESONANCE OF NITROXIDE PROBES IN MICELLE-CONTAINING SOLUTIONS