chlorobactene and okenone

Photophysical properties of two typical aryl carotenoids, okenone and chlorobactene, were studied with application of femtosecond and microsecond time-resolved absorption spectroscopies. These carotenoids are structurally similar and differ only by keto-group and character of the aryl ring. The studies have concentrated on aspects of the photochemistry of these carotenoids as possibility of solvent polarity induced formation of intramolecular charge transfer state in okenone, which contains a keto-group directly attached to the carbon-carbon double bond conjugation, estimating the energy of the forbidden first excited singlet electronic state, S1 (2(1)Ag(-)) and testing the photoprotective capabilities of okenone and chlorobactene in real biological systems. The energies of the S1 (2(1)Ag(-)) state obtained for these carotenoids are 12 750 cm(-1) for okenone and 13 450 cm(-1) for chlorobactene and are not affected either by temperature or solvent polarity. The effect of cryogenic temperature on the excited states lifetimes and energies was also studied at 77 K in 2-methyltetrahydrofuran, which forms a transparent glass upon freezing. The ability to quench bacteriochlorophylls triplets was studied on model bacteriochlorophyll a-carotenoid mixtures with application of flash photolysis. The triplet state lifetime obtained from the anticipated kinetic modelling of the rise and decay of the pool of carotenoid triplets are 2.1 μs for okenone and 2.8 μs for chlorobactene.

Topics: Absorption, Physicochemical; Bacteriochlorophylls; Carotenoids; Electron Transport; Energy Transfer; Solvents; Temperature

Organic biomarkers in marine sedimentary rocks hold important clues about the early history of Earth's surface environment. The chemical relicts of carotenoids from anoxygenic sulfur bacteria are of particular interest to geoscientists because of their potential to signal episodes of marine photic-zone euxinia such as those proposed for extended periods in the Proterozoic as well as brief intervals during the Phanerozoic. It is therefore critical to constrain the environmental and physiological factors that influence carotenoid production and preservation in modern environments. Here, we present the results of coupled pigment and nucleic acid clone library analyses from planktonic and benthic samples collected from a microbially dominated meromictic lake, Fayetteville Green Lake (New York). Purple sulfur bacteria (PSB) are abundant and diverse both in the water column at the chemocline and in benthic mats below oxygenated shallow waters, with different PSB species inhabiting the two environments. Okenone (from PSB) is an abundant carotenoid in both the chemocline waters and in benthic mats. Green sulfur bacteria and their primary pigment Bchl e are also represented in and below the chemocline. However, the water column and sediments are devoid of the green sulfur bacteria carotenoid isorenieratene. The unexpected absence of isorenieratene and apparent benthic production of okenone provide strong rationale for continued exploration of the microbial ecology of biomarker production in modern euxinic environments.

Topics: Biomarkers; Carotenoids; Fresh Water; Geologic Sediments; New York; Phototrophic Processes; Phytoplankton; RNA, Ribosomal, 16S