Iodide penetration into lipid bilayers as a probe of membrane lipid organization

Area of Science:

• Biophysics
• Membrane Biophysics
• Physical Chemistry

Background:

• Lipid bilayers form the fundamental structure of cell membranes.
• Understanding lipid packing and organization is crucial for membrane function.
• Fluorescence quenching is a valuable tool for probing molecular environments.

Purpose of the Study:

• To utilize iodide as a penetrating fluorescence quencher to measure lipid bilayer molecular packing.
• To investigate the temperature-dependent behavior of iodide quenching in lipid bilayers.
• To assess the sensitivity of iodide quenching to lipid phase transitions and membrane defects.

Main Methods:

• Employing iodide (KI) as a fluorescence quencher for membrane-associated fluorophores.
• Measuring tryptophan fluorescence quenching of melittin in DMPC bilayer vesicles.
• Assessing pyrene-phosphatidylcholine fluorescence quenching in DMPC vesicles.
• Analyzing quenching efficiency variations with temperature, particularly around the lipid phase transition.

Main Results:

• Iodide quenching efficiency of melittin fluorescence peaked at the DMPC phase transition temperature (24°C).
• Acrylamide quenching efficiency showed no temperature dependence, unlike iodide.
• Iodide quenching of pyrene-phosphatidylcholine also exhibited a maximum at the lipid phase transition.
• These results suggest iodide penetrates the bilayer hydrocarbon region, especially at phase transitions.

Conclusions:

• Iodide penetrates the membrane hydrocarbon region at phase transition due to increased bilayer defects.
• The iodide quenching efficiency's change during lipid phase transition offers a sensitive method to probe lipid organization.
• This technique provides insights into membrane fluidity and structural dynamics.