Diphenylhexatriene membrane probes DPH and TMA-DPH: A comparative molecular dynamics simulation study.

Area of Science:

• Membrane biophysics
• Lipid bilayer dynamics
• Fluorescent probe behavior

Background:

• Phospholipids lack intrinsic fluorescence, necessitating extrinsic probes for spectroscopy and microscopy.
• 1,6-diphenylhexatriene (DPH) and TMA-DPH are widely used membrane probes.
• TMA-DPH's charged group is thought to anchor it at the lipid/water interface, distinct from DPH.

Purpose of the Study:

• To characterize the behavior of DPH and TMA-DPH in POPC and POPC/cholesterol bilayers using atomistic MD simulations.
• To elucidate the factors influencing the distinct dynamics and localization of these probes.

Main Methods:

• Atomistic Molecular Dynamics (MD) simulations.
• Simulations performed on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and POPC/cholesterol (4:1) bilayers.
• Analysis of probe dynamics, localization, and interactions within the lipid bilayers.

Main Results:

• TMA-DPH exhibits more hindered dynamics than DPH in both membrane types.
• The average location of TMA-DPH is only slightly shallower (~3-4Å) than DPH.
• Hindered motion of TMA-DPH is primarily due to electrostatic interactions, not significant depth differences.

Conclusions:

• The perceived distinct localization of TMA-DPH versus DPH may be less pronounced than previously assumed.
• Electrostatic interactions significantly influence TMA-DPH's membrane dynamics.
• These findings provide crucial insights for interpreting past and planning future fluorescence studies of lipid bilayers.