ANALYSIS OF FACTORS DETERMINING EPR-SPECTRA OF SPIN PROBES THAT PARTITION BETWEEN AQUEOUS AND LIPID PHASES

Electron paramagentic resonance spectra of spin probes that partition between lipid and water were examined in water-phospholipid mixtures, fatty acids, and long-chain hydrocarbons. The concentrations of the probes in the phospholipid and aqueous phases were determined by several methods. Rigorous calculations of correlation times were performed. The analysis of the two parameters—concentration and correlation time in each phase—indicated that breaks in plots of partition parameters determined from measurements of line heights do not always reflect accurately the actual probe distribution in lipid-water phases, primarily as a consequence of the different activation energies for probe rotation in each phase. The delineation of the various processes underlying the spectra of partitioning molecules should allow a more reliable use of these probes in complex systems. Unresolved proton hyperfine splittings, ring interconversion, molecular ordering, probe location, and probe-probe interactions affect the spectral line shapes. The contribution of each factor depends upon the probe and the system. Neglect of ordering of the probes where this occurs leads to wrong evaluation of correlation times, and hence to erroneous values for the microviscosity. © 1978, American Chemical Society. All rights reserved.