Chlorophyll a and carotenoid triplet states in light-harvesting complex II of higher plants

Abstract

Laser-flash-induced transient absorption measurements were performed on trimeric light-harvesting complex II to study carotenoid (Car) and chlorophyll (Chl) triplet states as a function of temperature. In these complexes efficient transfer of triplets from Chl to Car occurs as a protection mechanism against singlet oxygen formation. It appears that at room temperature all triplets are being transferred from Chl to Car; at lower temperatures (77 K and below) the transfer is less efficient and chlorophyll triplets can be observed. In the presence of oxygen at room temperature the Car triplets are partly quenched by oxygen and two different Car triplet spectral species can be distinguished because of a difference in quenching rate. One of these spectral species is replaced by another one upon cooling to 4 Ki demonstrating that at least three carotenoids are in close contact with chlorophylls. The triplet minus singlet absorption (T-S) spectra show maxima at 504-506 nm and 517-523 nm, respectively. In the Chl Qy region absorption changes can be observed that are caused by Car triplets. The T-S spectra in the Chl region show an interesting temperature dependence which indicates that various Car's are in contact with different Chl a molecules. The results are discussed in terms of the crystal structure of light-harvesting complex II.

Carotenoid-binding Sites of the Major Light-harvesting Complex II of Higher Plants

“…Therefore, only Chl a triplets need to be quenched and therefore in close contact with xanthophyll molecules. The above assignment is not in agreement with studies of the triplet activity in LHCII (7)(8)(9)(10), suggesting the involvement of additional xanthophyll molecules. Accordingly, direct energy transfer between xanthophylls and Chl b was observed (3), suggesting that at least some of the Chl b sites are in close contact with xanthophylls.…”

“…We therefore conclude that zeaxanthin is not a genuine component of LHCII, at least not as a tightly bound chromophore. We confirm that zeaxanthin is a good quencher of 3 Chl*; in fact, despite incomplete equilibration, it is only slightly less efficient than lutein, the major xanthophyll component of native LHCII and the best 3 Chl* a quencher (10,28). Accordingly, refolding in vitro in the presence of lutein as the only xanthophyll available yielded a fully equilibrated and functional complex.…”

“…Protection from photobleaching, which is the effect of either of the two processes or of both, clearly shows that xanthophylls in both the L1 and L2 sites and the N1 site are active in photoprotection. Triplet minus singlet spectra of LHCII showed that 3 Chl* quenching was afforded by lutein, but not by neoxanthin (10,28). On this basis, we propose that the role of neoxanthin in photoprotection of LHCII is to scavenge the 1 O 2 * diffusing from Chl a chromophores to the Chl b-rich domain where neoxanthin is located.…”

A comparison of the three isoforms of the light-harvesting complex II using transient absorption and time-resolved fluorescence measurements

“…Native trimeric LHC II was purified from spinach (Peterman et al 1995) using anion-exchange chromatography and the detergent n-dodecyl-b-D-maltoside (Sigma) for solubilization of the complexes. Samples were diluted in a medium containing 20 mM Hepes buffer at pH 7.8 and 0.03% (w/v) n-dodecyl-b-D-maltoside, whereas the medium for the Lhcb1, Lhcb2 and Lhcb3 isoforms of LHC II contained 40 mM Hepes buffer at pH 7.8 and 0.05% (w/v) n-dodecyl-b-D-maltoside.…”

Design principles of natural light-harvesting as revealed by single molecule spectroscopy

“…These molecules extend the absorption spectral window by harvesting solar energy in the blue-green spectral region, which is then rapidly transferred to the Chls [12]. Even more important is their photoprotective role whereby Chl singlet and triplet states are efficiently quenched: Chl triplets would otherwise react with oxygen to produce highly reactive (and therefore lethal) singlet oxygen [13]; Chl singlets are quenched when the excitation rate of the photosystem becomes too high (vide infra) [14,15].…”