Complementary Fluorescence Emission and Second Harmonic Spectra Improve Bilayer Characterization

Area of Science:

• Molecular spectroscopy
• Biophysics
• Materials science

Background:

• Laurdan is a fluorescent probe used to study lipid bilayers.
• Understanding lipid packing and domain formation is crucial for membrane function.

Purpose of the Study:

• To investigate Laurdan fluorescence emission and second harmonic (SH) spectra in various solvents and phospholipid bilayers.
• To elucidate the relationship between Laurdan conformation, lipid packing, and domain evolution.

Main Methods:

• Complementary analysis of Laurdan fluorescence and SH spectra.
• Computation of SH spectra using electro spin resonance spectroscopy methods.
• Fitting spectra to log-normal distributions.
• Temperature-dependent measurements.

Main Results:

• SH spectra revealed two emissions in nonpolar/protic solvents and bilayer phases, and a single line in aprotic polar solvents.
• A previously undetected 432 nm blue line was observed in the gel phase.
• Temperature trajectories indicated inhomogeneous lipid packing and domain evolution.
• The blue line corresponds to Laurdan in tightly packed regions, while the red line relates to Laurdan-water coupled states.

Conclusions:

• Laurdan conformation (planar/nonplanar) correlates with emission wavelengths.
• SH spectroscopy enhances the detection of subtle spectral features.
• Temperature-dependent spectral changes provide insights into lipid domain dynamics and membrane curvature preferences.