Phase transitions in phosphatidylcholine dispersion observed with an interference refractometer.

Area of Science:

• Biophysics
• Materials Science
• Physical Chemistry

Background:

• Phosphatidylcholines undergo crystal-to-liquid crystal phase transitions, affecting their physical properties.
• Understanding these transitions is crucial for lipid membrane behavior and function.
• Turbidity changes observed during these transitions are linked to alterations in membrane properties.

Purpose of the Study:

• To accurately measure the refractive index change during the phosphatidylcholine crystal-to-liquid crystal phase transition.
• To confirm the role of refractive index changes in the observed turbidity variations.
• To assess the relationship between refractive index and density changes in lipid molecules.

Main Methods:

• Utilized an interferometer with high accuracy (6 x 10^-6) to measure refractive index.
• Employed two distinct methods for refractive index determination: spatial filtering and a piezoelectric compensator.
• Used very dilute phosphatidylcholine samples, requiring minimal material (milligrams).

Main Results:

• Two independent measurement methods yielded consistent results for refractive index change.
• The high accuracy allowed detection of phase transitions in dilute samples.
• Confirmed that altered refractive index of lipid membranes is the primary cause of turbidity change at the transition temperature.

Conclusions:

• Refractive index change is the dominant factor contributing to turbidity changes in phosphatidylcholine dispersions during phase transitions.
• The observed fractional change in refractive index does not correlate well with the fractional density change of lipid molecules within vesicles.