Niedzwiedzki DM, Hunter CN, Blankenship RE. Evaluating the Nature of So-Called S*-State Feature in Transient Absorption of Carotenoids in Light-Harvesting Complex 2 (LH2) from Purple Photosynthetic Bacteria.
J Phys Chem B 2016;
120:11123-11131. [PMID:
27726397 PMCID:
PMC5098231 DOI:
10.1021/acs.jpcb.6b08639]
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Abstract
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Carotenoids
are a class of natural pigments present in all phototrophic
organisms, mainly in their light-harvesting proteins in which they
play roles of accessory light absorbers and photoprotectors. Extensive
time-resolved spectroscopic studies of these pigments have revealed
unexpectedly complex photophysical properties, particularly for carotenoids
in light-harvesting LH2 complexes from purple bacteria. An ambiguous,
optically forbidden electronic excited state designated as S* has
been postulated to be involved in carotenoid excitation relaxation
and in an alternative carotenoid-to-bacteriochlorophyll energy transfer
pathway, as well as being a precursor of the carotenoid triplet state.
However, no definitive and satisfactory origin of the carotenoid S*
state in these complexes has been established, despite a wide-ranging
series of studies. Here, we resolve the ambiguous origin of the carotenoid
S* state in LH2 complex from Rba. sphaeroides by
showing that the S* feature can be seen as a combination of ground
state absorption bleaching of the carotenoid pool converted to cations
and the Stark spectrum of neighbor neutral carotenoids, induced by
temporal electric field brought by the carotenoid cation–bacteriochlorophyll
anion pair. These findings remove the need to assign an S* state,
and thereby significantly simplify the photochemistry of carotenoids
in these photosynthetic antenna complexes.
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