Photo-Isomerization of Prolycopene in Chloroplast Occurs through the Triplet State: Implications on Carotenoid Biosynthesis

Abstract

The carotenoid lycopene in photosynthetic organisms forms the critical precursor to the biosynthesis of reaction center carotenoids beta-carotene and lutein. In plants and cyanobacteria, a redox-controlled flavoenzyme carotenoid isomerase (CRTISO) catalyzes the four-step geometric isomerization of 7,9,9′,7′-tetra-cis-lycopene (prolycopene) to all-trans-lycopene. In chloroplasts, the functional loss of CRTISO has been shown to be rescued by a light-mediated isomerization pathway. In order to address the chloroplast-specificity and compare the efficiency of the photoisomerization reaction against redox-controlled enzyme catalysis, we need to track the excited state dynamics of prolycopene, and evaluate the nature of electronic states that lead to the photoisomerization. Using broadband femtosecond transient absorption spectroscopy, we observe ∼610 fs rise of the triplet state from the photoexcited S2 with a quantum yield of ∼0.19. The triplet state mediates the first C=C bond isomerization at symmetric 9 or 9′ position on the tetra-cis backbone to yield the tri-cis-lycopene with 15% quantum yield. However, direct sensitization of the photoreactive triplet state via meso-tetraphenyl porphyrin sensitizer under steady state illumination leads to all-trans-lycopene with 58% quantum efficiency. Our work implies that long-lived chlorophyll triplets activate the efficient isomerization of prolycopene in chloroplasts in absence of CRTISO activity. In presence of CRTISO, the role of redox equivalents in dark isomerization of prolycopene, and the light induced signaling mechanisms for the regulation of carotenoid biosynthesis will be discussed.