Temperature Dependence of Chlorophyll Triplet State Quenching in Three Photosynthetic Light-Harvesting Complexes

Abstract

Chlorophyll triplet states generated in light-harvesting complexes (LHCs) can be quenched by oxygen with resulting in generation of harmful singlet oxygen state. To prevent itherefore, the chlorophyll triplets are quenched by carotenoids to prevent generation of the long-lived chlorophyll triplets.

Thise photoprotective mechanism is known to be highly efficient at physiological temperatures. [1,2] However, an observation of population of chlorophyll triplets was reported at low temperature (< 100 K). [3] In this work, tThe temperature dependence reason of the lower efficiency of photoprotection was studied in three LHCs: LHC II of higher plants, a chlorophyll a-chlorophyll c2-peridinin-protein complex (acpPC) of Dinoflagellate Amphidinium carterae and chlorosomes of Chloroflexus auranticus, by transient absorption spectroscopy with nano-microsecond resolution at temperatures between 77 and 295 K. The emphasis iwas put on acpPC and LHC II antennas, which belong to the same superfamily of LHCs.

In contrast to LHC II, which shows a high efficiency of quenching of core chlorophylls independently ofn temperature, two of three subpopulations of carotenoids peridinins in acpPC exhibit a significant rate decrease of a triplets quenching rate. The lifetime of the slowly quenched chlorophyll triplets was estimated to be 133 ns at 77K.

This is most likely caused by intrinsic temperature dependence of a Dexter energy transfer mechanism. The overall efficiency of photoprotection decreases at low temperatures in both complexes as long-lived (~ms) chlorophyll triplets are detected.

We presume that the decrease of temperature leads to conformation changes are involved, which detach these peripheral chlorophylls from the energy transfer chain and protection by carotenoids, and therefore trap the excitation on these chlorophylls, which are not neighbouring with any active carotenoid. Moreover Surprisingly, a part of chlorophyll triplets is quencheddeexcitated by triplet-triplet annihilation in acpPC an intrinsic LHC of Dinoflagellate.

Chlorosomes show a similar temperature dependence of chlorophyll triplet quenching rate of triplets by carotenoids same as acpPC. However, no bacteriochlorophylls triplets with a millisecond ~ms lifetime were detected at low temperatures. [1] Z.

Kvíčalová, J. Alster, E.

Hofmann, P. Khoroshyy, R.

Litvín, D. Bína, T.

Polívka, and J. Pšenčík.

Triplet-triplet energy transfer from chlorophylls to carotenoids in two antenna complexes from dinoflagellate Amphidinium carterae. Biochimica et Biophysica Acta - Bioenergetics, 57(4):341-349, 2016. [2] E.

J. G.

Peterman, F. M.

Dukker, R. van Grondelle, and H. van Amerongen. Chlorophyll a and Carotenoid Triplet States in Light-Harvesting Complex II of Higher Plants.

Biophys. J., 69(6):2670-2678, 1995. [3] D.

M. Niedzwiedzki, J.

Jiang, C. S.

Lo, and R. E.

Blankenship. Spectroscopic properties of the Chlorophyll a-Chlorophyll c2-Peridinin-ProteinComplex (acpPC) from the coral symbiotic dinoflagellate Symbiodinium.

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