Astaxanthin Protecting Membrane Integrity against Photosensitized Oxidation through Synergism with Other Carotenoids

Incorporation of astaxanthin or zeaxanthin in giant unilamellar vesicles (GUVs) of phosphatidylcholine resulted in a longer lag phase than incorporation of β-carotene or lycopene for the onset of budding induced by chlorophyll a photosensitization and quantified by a dimensionless entropy parameter using optical microscopy and digital image heterogeneity analysis. The lowest initial rate of GUV budding after the lag phase was seen for GUVs with astaxanthin as the least reducing carotenoid, while the lowest final level of entropy appeared for those with lycopene or β-carotene as a more reducing carotenoid. The combination of astaxanthin and lycopene gave optimal protection against budding with respect to both a longer lag phase and lower final level of entropy by combining good electron acceptance and good electron donation. Quenching of singlet oxygen by carotenoids close to chlorophyll a in the membrane interior in parallel with scavenging of superoxide radicals by astaxanthin anchored in the surface may explain the synergism between carotenoids involving both type I and type II photosensitization by chlorophyll a.