Aggregation of LHCII Leads to a Redistribution of the Triplets over the Central Xanthophylls in LHCII

Abstract

We present laser flash-induced triplet-minus-singlet (TmS(flash)) and absorbance-detected-magnetic-resonance (TmS(ADMR)) measurements on the light-harvesting chlorophyll a/b pigment-protein complex (LHCII) from pea. We investigated the influence of LHCII aggregation on xanthophyll triplet formation. The effect of aggregation was previously studied using TmS(ADMR) [van der Vos et al. (1994) Biochim. Biophys. Acta 1208, 243-250] for LHCII from spinach, and it was concluded that aggregation leads to a large increase of the amount of intertrimer triplet transfer. However, a similar study on LHCII from pea with the use of TmS(flash) measurements [Barzda et al. (1998) Biochemistry 37, 546-561] showed much smaller effects. To resolve this apparent discrepancy and to compare the results of TmS(ADMR) and TmS(flash) measurements, we used both techniques to study LHCII from pea, applying an identical aggregation procedure in both cases. It appears that aggregation does not lead to an increase of intertrimer triplet transfer as thought before but to a redistribution of the triplets over the two central xanthophylls (mainly lutein) that are present in each monomeric subunit of LHCII. Moreover, it is argued that the TmS band at 525 nm is due to lutein instead of violaxanthin as was reported in earlier studies. It is concluded that aggregation leads to a change in chlorophyll-xanthophyll interactions, which might explain the large change in excited-state lifetime of chlorophyll a in LHCII upon aggregation. This change in lifetime is possibly related to the phenomenon of nonphotochemical quenching in green plants, which is an important protective regulatory mechanism, that lowers the probability of photoinhibition.

Molecular Basis of Light Harvesting and Photoprotection in CP24

“…This value is half of the value of Chl a in solution (ϳ6 ns), thus indicating that the complex is in a partially quenched state (50% of the fluorescence yield). This effect, although less strong, has been observed previously on other antenna complexes (52)(53)(54)(55), and it has been suggested to be due to mixing of the excited state of Chls and carotenoids, which opens a dissipation channel via the short lived carotenoid S 1 state (45,56).…”

“…A shorter lifetime than Chl in solution was observed also for other antenna complexes (52)(53)(54)(55), indicating that the Chls in protein environment are partially quenched. It was proposed that strong interactions between Chls and carotenoids, which can act as a fast decay channel, are responsible for the observed quenching (45,56). Analysis of the CP24 mutants allows us to check for the involvement in this process of the xanthophylls in sites L1 and L2.…”

Light-harvesting and structural organization of Photosystem II: From individual complexes to thylakoid membrane

“…Once in a while a Chl a singlet excitation is transformed into a Chl a triplet that could easily lead to the formation of destructive singlet oxygen molecules. Fortunately, these dangerous Chl triplets are nearly all (up to 95%) scavenged by the carotenoid molecules that are in Van der Waals contact with the Chl a molecules [22][23][24][25][26].…”

Light-Harvesting in Photosystem II

“…Some of the Chl a singlet excitations are transformed into Chl a triplets, which can lead to the formation of destructive singlet oxygen molecules. Fortunately, most of these dangerous Chl triplets ([95 %) are scavenged by the carotenoids that are in Van der Waals contact with Chl a (Barzda et al 1998;Lampoura et al 2002;Mozzo et al 2008a;Carbonera et al 1992;van der Vos et al 1991).…”