Temperature dependence of elastic properties of the phospholipid vesicles in aqueous suspension probed by Brillouin spectroscopy

Abstract

The aqueous suspension of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles with different hydration levels α (water-to-lipid mass ratio) have been studied by Brillouin spectroscopy in the temperature range from −190 °C to 70 °C. The samples with different hydration levels demonstrate similar temperature behavior of their sound velocity in the temperature range from −190 °C to −25 °C. There is a strong correlation between the hydration level of the sample and the character of the sound velocity temperature dependence at higher temperatures. Nevertheless, all hydrated samples demonstrate a jump in the sound velocity at the gel–fluid phase transition temperature. The amplitude of this jump depends on the hydration level α of the sample. It has a maximum value in the sample with minimal α necessary for the phospholipid membrane's full hydration. To evaluate the sound velocity in the phospholipid membrane, we applied the two-component model to analyze the experimental data obtained in the sample with α = 0.25 (close to the minimal necessary value for the full DPPC membrane hydration). It was found that for temperatures higher than 0 °C, the two-component model works well if we consider that sound velocity in water between vesicle layers is approximately a factor of two higher than in bulk water.