The photochemical properties of carotenoids in solution and in pigment-protein complexes

Date of Completion

January 2001


Chemistry, Physical|Biophysics, General




The spectroscopic properties of peridinin in solution, and the efficiency and dynamics of energy transfer from peridinin-to-chlorophyll a in the peridinin-chlorophyll-protein (PCP) from Amphidinium carterae were studied by steady-state absorption, fluorescence, fluorescence excitation, and fast transient optical spectroscopy. Steady-state and kinetics measurements of singlet energy transfer from peridinin-to-chlorophyll revealed an 94 ± 3% efficiency. Nanosecond time-resolved transient optical spectroscopy revealed that chlorophyll triplet states are efficiently quenched by peridinin whose triplet state subsequently decays with a lifetime of 10 ± 1 μs in the PCP complex. Close association between the peridinins and chlorophylls, which is clearly evident in the 3-D structure of the PCP complex, along with proper alignment of pigments and energy state matching are responsible for the high efficiencies of the photochemical processes. ^ The lifetime of the lowest excited singlet state of peridinin is found to be strongly dependent on solvent polarity and ranges from 7 ± 1 ps in the strongly polar solvent trifluoroethanol to 172 ± 3 ps in the nonpolar solvents cyclohexane and benzene. The wavelengths of emission maxima, the quantum yields of fluorescence, and the transient absorption spectra are also affected by the solvent environment. To elucidate the molecular features of peridinin responsible for the solvent dependence of the spectroscopic properties and dynamic behavior of several related carotenoids were studied. It is observed that carotenoids which contain carbonyl functional groups in conjugation with the carbon-carbon π-electron system display broader absorption spectral features, have more complex transient absorption spectra and show a pronounced dependence of the excited singlet state lifetime on solvent environment compared to those which do not contain carbonyl functional groups. It is suggested that these effects are related to the presence of an intramolecular charge transfer state in the excited state manifold of the molecules containing conjugated carbonyls. The structural variations in the series of carotenoids studied here make it possible to focus on what molecular features control the spectroscopic and dynamic properties of carotenoids in general. ^