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

DMPC (dimyristoyl-L-alpha-phosphatidylcholine) liposomes are used as artificial photosynthetic media to study the behavior of carotenoids. 8'-Apo-beta-caroten-8'-al (I) and beta-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 S-1 state of I in DMPC liposomes is as expected for an alkane environment. The lifetime of the S-1 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 S-1 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 cr under strong irradiation. In liposomes, Chi a fluorescence quenching by I is observed when using either the Q(y) band or the Soret band of Chi a as the excitation line.