This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. To expand the application of the SPM/DOPC/cholesterol model lipid system to real plasma membranes, the dynamic characterization of the Lo and Ld phases on the phase boundary by ESR spectra has been undertaken for comparison to real plasma membranes. The order parameter and dynamic parameters, such as rotational diffusion coefficients, can be obtained from fitting an ESR spectrum with a dynamic model, and these parameters can be used to characterize a lipid phase. If parameter from a model membrane phase exhibits similar characteristics to those parameters obtained from the same spin-probe in real plasma membranes, then we can say with confidence that the model membrane is similar to the real membrane. Many different membrane spin-probes can be employed to increase the specificity of the characterization as well as increasing the understanding of the phase state. This characterization combined with the "global" tie-line configuration for the Lo+Ld phase coexistence region will allow a more detailed model for lipid rafts.