Abstract
The molecular mechanisms that regulate the asymmetric partitioning of the nitroxide spin-labeled lipids, 5-PC and 16-PC, in the coexistence of DLPC-rich liquid-disordered and DPPC-rich gel phases are investigated. In the two-phase coexistence region, our results of the ESR spectra show that the partition of 16-PC into the two phases strictly follows the “lever rule” as a function of composition by fitting the spectra with two conjugated phases. On the other hand, 5-PC spin label does not partition equally in the same region. This indicates the asymmetric partitioning of lipid spin labels that is due to the steric environment at which the spin label is attached. The mechanism of the asymmetric partitioning is rationalized in terms of the local free-surface-area of spin label, which is demonstrated the major factor that affects the partitioning of spin labels into the coexisting phases. The analysis result from the stochastic spectral simulations unravels the complex mixing behaviors and molecular incorporation of the labeled molecules in the membrane phases, irrespective of their asymmetric partition.