Characterization of the liquid-ordered state by proton MAS NMR

We investigated if magic angle spinning (MAS) H-1 NMR can be used as a tool for detection of liquid-ordered domains ( rafts) in membranes. In experiments with the lipids SOPC, DOPC, DPPC, and cholesterol we demonstrated that H-1 MAS NMR spectra of liquid-ordered domains (l(o)) are distinctly different from liquid-disordered (l(d)) and solid-ordered (s(o)) membrane regions. At a MAS frequency of 10 kHz the methylene proton resonance of hydrocarbon chains in the l(d) phase has a linewidth of similar to 50 Hz. The corresponding linewidth is similar to 1 kHz for the l(o) phase and several kHz for the s(o) phase. According to results of H-1 NMR dipolar echo spectroscopy, the broadening of MAS resonances in the lo phase results from an increase in effective strength of intramolecular proton dipolar interactions between adjacent methylene groups, most likely because of a lower probability of gauche/trans isomerization in lo. In spectra recorded as a function of temperature, the onset of lo domain ( raft) formation is seen as a sudden onset of line broadening. Formation of small domains yielded homogenously broadened resonance lines, whereas large lo domains (diameter > 0.3 mu m) in an l(d) environment resulted in superposition of the narrow resonances of the l(d) phase and the much broader resonances of lo. 1 H MAS NMR may be applied to detection of rafts in cell membranes.