Spin-label and deuterium order parameter discrepancies in bilayers: one possible explanation.

We have simulated electron spin resonance spectra of anisotropically immobilized spin labels of the type seen in lipid and soap-like bilayers using a rigorous formalism which explicitly includes the effects of spin-label motion. In most bilayer systems, spin-label experiments have shown lower order parameters then deuterium-label experiments. In the past this apparent decrease in the order parameters was thought to reflect the distortion of the bilayer by the doxyl ring of the spin probes. We wish to report that this type of discrepancy may be due to the neglect of important motional effects in the time-independent effective Hamiltonian formalisms used in previous interpretations of anisotropically immobilized spin label spectra. That the true order parameters may be the same can be shown by including slow motional corrections in the effective Hamiltonian formalism. The larger volume of the doxyl ring may change the apparent order parameter by increasing the importance of the slow motional effects, as opposed to causing a real decrease in the order parameter, as previously proposed.