Order parameters in membranes: Following Joachim Seelig's path

Area of Science:

• Biophysics
• Membrane Biophysics
• Solid-State Nuclear Magnetic Resonance (ssNMR)

Background:

• Biological membrane models were established in the 1970s.
• The dynamic nature of membranes was experimentally demonstrated by Joachim Seelig using ssNMR.

Purpose of the Study:

• To measure order parameters of deuterium-labeled lipids using ssNMR.
• To characterize membrane dynamics across intramolecular, molecular, and collective scales.
• To investigate the influence of lipid structure and sterols on membrane fluidity.

Main Methods:

• Solid-state Nuclear Magnetic Resonance (ssNMR) spectroscopy.
• Measurement of order parameters for C-D (²H), C-H, and C-C bonds in lipids.
• Application of statistical mechanics to describe order parameter profiles.

Main Results:

• Demonstrated a fluid membrane interior through ssNMR order parameter measurements.
• Quantified membrane dynamics on intramolecular (Å/ns-ps), molecular (nm/1-100 ns), and collective (μm/μs) scales.
• Revealed that membranes are rigid at glycerol/chain levels but fluid at the center/surface, with sterols acting as dynamic regulators.

Conclusions:

• Lipid order parameters provide a comprehensive description of membrane dynamics.
• Statistical mechanics analysis of order parameters allows calculation of chain length, bilayer thickness, and elastic constants.
• The surface area per lipid can be accurately calculated from order parameter plateaus.