Carotenoids in liposomes: Photodegradation, excited state lifetimes, and energy transfer

DMPC (dimyristoyl-L-a-phosphatidylcholine) liposomes are used as artificial photosynthetic media to study the behavior of carotenoids. 8′-Apo-β-caroten-8′-al (I) and β-carotene (II) degrade faster under irradiation in DMPC liposomes than in organic solvents, which is possibly because vibrational deactivation of carotenoid excited states is less efficient in rigid lipid membranes. The lifetime of the first excited singlet state (Si) of I in DMPC liposomes is 27.2 ps, very close to that in 3-methylpentane (26.4 ps), but longer than its lifetime in EtOH (17.1 ps) or CH₂Cl₂ (14.1 ps). The lifetime of the Si state of I in DMPC liposomes is as expected for an alkane environment. The lifetime of the Si state of II in DMPC liposomes is 10.3 ps, very close to its lifetimes in 3-methylpentane (8.1 ps), EtOH (9.2 ps), and CH₂Cl₂ (8.5 ps). This independence of the Si state lifetime of II from the matrix agrees with earlier conclusions. Carotenoid I can suppress the photodegradation of chlorophyll a (Chl a) in liposomes, which shows the protection role of I on Chl a under strong irradiation. In liposomes, Chl a fluorescence quenching by I is observed when using either the Q_y band or the Soret band of Chl a as the excitation line.