Contributions to membrane-embedded-protein diffusion beyond hydrodynamic theories.

Area of Science:

• Biophysics
• Membrane Biophysics
• Soft Matter Physics

Background:

• Traditional Saffman-Delbrück theory describes protein diffusion in lipid membranes with weak radius dependence (D∼lnR).
• Recent experiments show a stronger dependence (D∼1/R), suggesting non-hydrodynamic drag sources like lipid chain interactions.
• This discrepancy challenges the universality of existing models for membrane protein dynamics.

Purpose of the Study:

• To investigate the influence of protein-membrane interactions on diffusion coefficients.
• To explore a generic model of protein coupling to scalar order parameters in lipid membranes.
• To re-evaluate the role of hydrodynamic effects versus other drag sources.

Main Methods:

• Development of a generic model for protein coupled to a nonconserved scalar order parameter.
• Analysis of drag forces considering different protein-order parameter coupling mechanisms.
• Inclusion of order parameter advection to model fluid membrane dynamics.
• Comparison of model predictions with experimental observations and Saffman-Delbrück theory.

Main Results:

• The model shows that drag dependence on protein radius is sensitive to the coupling mechanism.
• A D∼1/R scaling is achievable if the protein is much larger than the order parameter correlation length.
• Including advection in fluid membranes restores Saffman-Delbrück-like scaling (D∼lnR).
• The effective protein radius becomes dependent on the order parameter's correlation length.

Conclusions:

• Hydrodynamic effects remain crucial for accurately computing drag on membrane proteins.
• Non-hydrodynamic interactions alone may not fully explain observed diffusion behaviors.
• The dynamics of the membrane's order parameter significantly influence protein mobility.
• Existing models may need refinement to incorporate fluid membrane advection effects.