DYNAMICS AND ORDERING IN MIXED-MODEL MEMBRANES OF DIMYRISTOYLPHOSPHATIDYLCHOLINE AND DIMYRISTOYLPHOSPHATIDYLSERINE - A 250-GHZ ELECTRON-SPIN-RESONANCE STUDY USING CHOLESTANE
Jp. Barnes et Jh. Freed, DYNAMICS AND ORDERING IN MIXED-MODEL MEMBRANES OF DIMYRISTOYLPHOSPHATIDYLCHOLINE AND DIMYRISTOYLPHOSPHATIDYLSERINE - A 250-GHZ ELECTRON-SPIN-RESONANCE STUDY USING CHOLESTANE, Biophysical journal, 75(5), 1998, pp. 2532-2546
We report here on a 250-GHz electron spin resonance (ESR) study of mac
roscopically aligned model membranes composed of mixtures of dimyristo
ylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS),
utilizing the nixtroxide-labeled cholesterol analog cholestane (CSL).
Two clearly resolved spectral components, distinct in both their orde
ring and dynamics, are resolved. The major component in membranes comp
osed mostly of DMPC shows typical characteristics, with the long axis
of CSL parallel to the bilayer normal with slow (10(6) less than or eq
ual to R less than or equal to 10(7) s(-1)) rotational diffusion rates
, as expected for cholesterol. The second component grows in as the mo
le fraction of DMPS increases. A detailed analysis shows that CSL sens
es a local, strongly biaxial environment. Our results imply that the i
nefficient packing between cholesterol and DMPS occurs probably becaus
e of the strong interactions between the PS headgroups, which provide
the local biaxiality. Such a packing of the headgroups has been predic
ted by molecular dynamics simulations but had not been observed experi
mentally. The analysis of these spectral components was greatly aided
by the excellent orientational resolution provided by the 250-GHz spec
tra. This enabled the key qualitative features of this interpretation
to be ''read'' off the spectra before the detailed analysis.