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

Area of Science:

• Biophysics
• Materials Science
• Spectroscopy

Background:

• Electron spin resonance (ESR) experiments on lipid bilayers often show lower order parameters compared to deuterium-labeling experiments.
• This discrepancy was previously attributed to the physical distortion of the bilayer by the spin probe's doxyl ring.

Purpose of the Study:

• To investigate the influence of spin-label motion on ESR spectra of anisotropically immobilized spin labels in lipid bilayers.
• To determine if motional effects, rather than probe-induced distortion, account for the observed lower order parameters.

Main Methods:

• Rigorous simulation of ESR spectra using a formalism that explicitly incorporates spin-label motion.
• Application of a time-independent effective Hamiltonian formalism with inclusion of slow motional corrections.

Main Results:

• Simulations demonstrate that neglecting motional effects in previous analyses leads to an underestimation of order parameters.
• The apparent decrease in order parameters is explained by the increased importance of slow motional effects, particularly with larger doxyl rings.
• The true order parameters, when motional effects are properly accounted for, may be consistent between ESR and deuterium-labeling experiments.

Conclusions:

• The discrepancy in order parameters between ESR and deuterium-labeling experiments in lipid bilayers is likely due to the neglect of spin-label motion in prior analyses.
• Accurate interpretation of ESR spectra requires incorporating slow motional corrections into the effective Hamiltonian formalism.
• The size of the spin probe's doxyl ring influences the apparent order parameter by affecting the significance of motional effects, not by causing a real decrease in bilayer order.