A comparative study of the effect of cholesterol on bicelle model membranesusing X-band and Q-band EPR spectroscopy

摘要

X-band and Q-band electron paramagnetic resonance (EPR) spectroscopic techniques were used to inves-tigate the structure and dynamics of cholesterol containing phospholipid bicelles based upon molecularorder parameters (S_(mol)), orientational dependent hyperfine splittings and line shape analysis of the corre-sponding EPR spectra. The nitroxide spin-label 3-β-doxyl-5-α-cholestane (cholestane) was incorporatedinto DMPC/DHPC bicelles to report the alignment of bicelles in the static magnetic field. The influence ofcholesterol on aligned phospholipid bicelles in terms of ordering, the ease of alignment, phase transitiontemperature have been studied comparatively at X-band and Q-band. At a magnetic field of 1.25T (Q-band), bicelles with 20 mol% cholesterol aligned at a much lower temperature (313 K), when compared to318 K at a 0.35 T field strength for X-band, showed better hyperfine splitting values (18.29 G at X-band vs.18.55 G at Q-band for perpendicular alignment and 8.25 G at X-band vs. 7.83 G at Q-band for the parallelalignment at 318 K) and have greater molecular order parameters (0.76 at X-band vs. 0.86 at Q-band at318 K). Increasing cholesterol content increased the bicelle ordering, the bicelle-alignment temperatureand the gel to liquid crystalline phase transition temperature. We observed that Q-band is more effec-tive than X-band for studying aligned bicelles, because it yielded a higher ordered bicelle system for EPRspectroscopic studies.