Thermodynamic and structural properties of lipid-photosensitizer conjugates mixed with phospholipids: Impact on the formation and stability of nano-assemblies

The photosensitizer Phenalenone (PN) was grafted with one or two lipid (C-18) chains to form pure nano assemblies or mixed lipid vesicles suitable for photodynamic therapy. Mixtures of PN-C-18 conjugates with stearoyl-oleoyl phosphatidylcholine (SOPC) form vesicles that disintegrate into bilayer sheets as the concentration of PN-C18 conjugates increases. We hypothesized that PN-C-18 conjugates control the thermodynamic and structural properties of the mixtures and induce the disintegration of vesicles due to PN pi-pi-interactions. Monolayers were analyzed by surface pressure and grazing incidence X-ray diffraction (GIXD) measurements, and vesicles by differential scanning calorimetry and cryo-TEM. The results showed that PN-triazole-C-18 (1A) and PN-NH-C-18 (1B) segregate from the phospholipid domains. PN-(C-18)(2) (conjugate 2) develops favorable interactions with SOPC and distearoyl-phosphatidylcholine (DSPC). GIXD demonstrates the contribution of SOPC to the structuring of conjugate 2 and the role of the major component in controlling the structural properties of DSPC-conjugate 2 mixtures. Above 10 mol% conjugate 2 in SOPC vesicles, the coexistence of domains with different molecule packing leads to conjugate segregation, vesicle deformation, and the formation of small bilayer discs stabilized by the inter-bilayer pi-pi stacking of PN molecules.

Automated summary

This study investigates the suitability of photosensitizer PN grafted with lipid chains to form nano assemblies or mixed lipid vesicles for photodynamic therapy. The results show that PN-C-18 conjugates can control the thermodynamic and structural properties of mixtures, leading to the disintegration of vesicles due to pi-pi interactions between PN molecules.