Self-assembled lipidporphyrin bilayer vesicles. Microstructure and dioxygen binding in aqueous medium

An amphiphilic tetraphenylporphyrinatometal (Zn, Fe) derivative having four dialkylglycerophosphocholine groups on one side of the ring plane (lipidporphyrin) was easily dispersed in an aqueous medium to give a red, stable dispersion. Based on electron microscopy, lipidporphyrins themselves produced spherical unilamellar vesicles with a diameter of ca. 100 nm. The thickness of the membrane (9.5 ± 0.5 nm) corresponded to a bilayer of the lipidporphyrin (4.6 nm); the ratio of the lipidporphyrin molecules in the outer and inner layers was 1.57. The lipidporphyrinatozinc(II) vesicle displayed some characteristics of a J-aggregate: (i) a red-shifted Soret band (425 → 438 nm), (ii) no fluorescence quenching, and (iii) a reduced triplet-state lifetime. Simple exciton calculations indicate that the porphyrin squares located in the outer and inner layers formed a J-aggregate with an average angle of 47°. A vesicle composed of the lipidporphyrinatoiron(II) coordinated with an alkylimidazole reversibly formed a stable dioxygen adduct. The O₂-binding equilibrium and kinetic parameters were determined. This vesicle has the ability to act as a totally synthetic O₂ carrier under physiological conditions (pH 7.4, 37°C).