Ultrafast Triplet Generation
and its Sensitization
Drives Efficient Photoisomerization of Tetra-<i>cis</i>-lycopene
to All-<i>trans</i>-lycopene
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Abstract
Lycopene biosynthesis in photosynthetic
organisms controls the
metabolic flux of reaction center carotenoids like β-carotene
and lutein through the geometric four-step isomerization of 7,9,9′,7′-tetra<i>-cis</i>-lycopene (prolycopene) to its all-<i>trans</i> form. In plants and cyanobacteria, a redox-controlled flavoenzyme
carotenoid isomerase catalyzes the prolycopene isomerization although
its functional loss inside the chloroplast can be rescued by light.
In order to address the chloroplast-specificity and efficiency of
the light-induced isomerization reaction, we need to critically understand
the excited state dynamics of prolycopene and the nature of electronic
states that lead to the isomerization. Using broadband femtosecond
transient absorption spectroscopy, we observe ∼610 fs rise
of the long-lived triplet state from the photoexcited S<sub>2</sub> with a quantum yield of ∼0.19. The triplet state eventually
triggers the first CC bond isomerization at the symmetric
9 or 9′ position on the tetra-<i>cis</i> backbone
to yield the tri-<i>cis</i> product with 15% quantum efficiency.
However, direct sensitization of the photoreactive triplet state via <i>meso</i>-tetraphenyl porphyrin sensitizer under steady state
illumination leads to an efficient production of all-<i>trans</i>-lycopene with 58% quantum yield. Our work implies that chlorophyll-enriched
chloroplasts should form an optimized photoreaction vessel for prolycopene
isomerization, and synthetic utilization of such <i>cis</i>-carotenoids can lead to efficient triplet harvesting photon-conversion
devices