Characterization of phospholipid mixed micelles by translational diffusion

Area of Science:

• Biophysics
• Physical Chemistry
• Materials Science

Background:

• Translational diffusion is key to understanding molecular aggregation.
• Accurate measurement of diffusion rates is essential for characterizing micelles and other aggregates.
• Existing methods for measuring diffusion in complex systems have limitations.

Purpose of the Study:

• To measure the concentration dependence of translational self-diffusion rates in micelle and mixed micelle systems.
• To determine the hydrodynamic volume of micelles and aggregates.
• To investigate the behavior of detergents and phospholipids in mixed micelles.

Main Methods:

• Utilized bipolar gradient pulse pairs in longitudinal eddy current delay experiments for enhanced sensitivity.
• Measured the concentration dependence of translational diffusion rates (D(s)).
• Applied the D(s) = D(o)(1-3.2λΦ) relationship to calculate hydrodynamic volume at low volume fractions (Φ ≤ 15% v/v).

Main Results:

• Hydrodynamic volume for proteins was found to be approximately 2.6 times their unhydrated molecular volume.
• Dihexanoyl phosphatidylcholine (DHPC) micelles showed approximately 27 molecules per micelle with a critical micelle concentration of 14 mM.
• Free detergent concentration in mixed micelles was determined and found to depend strongly on the detergent to phospholipid ratio.

Conclusions:

• The developed method accurately determines hydrodynamic volume and micelle properties.
• Translational diffusion measurements provide insights into the exchange dynamics of components in mixed micelles.
• Hydrodynamic volume derived from diffusion data can predict rotational correlation times for peptide-loaded bicelles.