2 research outputs found
Molecular Origin of Photoprotection in Cyanobacteria Probed by Watermarked Femtosecond Stimulated Raman Spectroscopy
Photoprotection
is fundamental in photosynthesis to avoid oxidative
photodamage upon excess light exposure. Excited chlorophylls (Chl)
are quenched by carotenoids, but the precise molecular origin remains
controversial. The cyanobacterial HliC protein belongs to the Hlip
family ancestral to plant light-harvesting complexes, and binds Chl <i>a</i> and Ī²-carotene in 2:1 ratio. We analyzed HliC by
watermarked femtosecond stimulated Raman spectroscopy to follow the
time evolution of its vibrational modes. We observed a 2 ps rise of
the Cī»C stretch band of the 2A<sub>g</sub><sup>ā</sup> (S<sub>1</sub>) state of Ī²-carotene upon Chl <i>a</i> excitation, demonstrating energy transfer quenching and fast excess-energy
dissipation. We detected two distinct Ī²-carotene conformers
by the Cī»C stretch frequency of the 2A<sub>g</sub><sup>ā</sup> (S<sub>1</sub>) state, but only the Ī²-carotene whose 2A<sub>g</sub><sup>ā</sup> energy level is significantly lowered
and has a lower Cī»C stretch frequency is involved in quenching.
It implies that the low carotenoid S<sub>1</sub> energy that results
from specific pigmentāprotein or pigmentāpigment interactions
is the key property for creating a dissipative energy channel. We
conclude that watermarked femtosecond stimulated Raman spectroscopy
constitutes a promising experimental method to assess energy transfer
and quenching mechanisms in oxygenic photosynthesis
Photoadduct Formation from the FMN Singlet Excited State in the LOV2 Domain of Chlamydomonas reinhardtii Phototropin
The
two light, oxygen, and voltage domains of phototropin are blue-light
photoreceptor domains that control various functions in plants and
green algae. The key step of the light-driven reaction is the formation
of a photoadduct between its FMN chromophore and a conserved cysteine,
where the canonical reaction proceeds through the FMN triplet state.
Here, complete photoreaction mapping of CrLOV2 from Chlamydomonas reinhardtii phototropin and AsLOV2
from Avena sativa phototropin-1 was
realized by ultrafast broadband spectroscopy from femtoseconds to
microseconds. We demonstrate that in CrLOV2, a direct photoadduct
formation channel originates from the initially excited singlet state,
in addition to the canonical reaction through the triplet state. This
direct photoadduct reaction is coupled by a proton or hydrogen transfer
process, as indicated by a significant kinetic isotope effect of 1.4
on the fluorescence lifetime. Kinetic model analyses showed that 38%
of the photoadducts are generated from the singlet excited state