The Light-Harvesting Complex of PS II (LHCII) of Higher Plants
An array of several antenna units is functionally connected with one reaction center complex in the photosynthetic membrane, so that energy transfer between the antenna complexes and from the antenna to the RC occurs. Four or even more (six in Figure 18) trimeric LHCII structures are connected with one reaction center. It is outlined in (Lambrev et al., 2011) that the PSII-LHCII super-complexes can be arranged in large ordered domains in the granal thylakoids. Ordered LHCII domains with a diameter of hundreds of nanometers have been observed (Garab and Mustardy, 1999). These physical domains contain thousands of pigments, however, it is not clear whether and to what extent the excited singlet states can migrate in these large LHC domains during their lifetime. The size of the functional domain, i.e. the number of pigments connected via EET, depends on the excited state lifetime and on the time of transfer between the pigments. The EET inside the LHCII is found to take place on the fs-ps time scale (Hader, 1999). This dynamic strongly depends on the structural arrangement of the pigments, which is stabilized by the protein matrix that gives rise to a well-defined coupling of the embedded pigments. Structural arrangement and EET are strongly correlated. Even quantum entanglement might influence the ultrafast EET in LHCII (Calhoun et al., 2009).
LHC trimers tend to aggregate strongly depending on the suspension medium and the preparation protocol. Figure 19 shows that huge LHC aggregates can be formed in diluted media. The size of the homogeneous LHCII domains in Figure 19 reaches values of up to 2 pm. The extent of EET between photosynthetic units, termed connectivity, leads to fluorescence rise kinetics of the prompt fluorescence after excitation with short laser pulses. This connectivity has also been proposed to explain the sigmoidal shape of the fluorescence induction transient. Different models have been proposed, for instance the so called “lake model” in which the reaction centres are embedded in a network of interconnected antennae (Garab and Mustardy, 1999).
According to the concept of connected photosynthetic units, the functional domain size of the LHC antenna is much larger than the number of pigments in one pigment-protein complex. Several attempts have been made to estimate the domain size in thylakoids and isolated LHCII preparations. A common approach is based on studies of the excitation annihilation that results from the interaction between two excited (singlet or triplet) molecules which leads to the dissipation of one excitation
Figure 19. AFM picture of aggregated LHCII trimers prepared by detergent removal. The LHCII samples at original concentration of 10 pg Chl/ml were immobilized and dried on glass plates precoated with poly-L-lysine. The AFM images were taken in close-contact mode. For further information see (Lambrev et al., 2011). Image reproduced with permission.
according to the qualitative equation:
where Si and T denote the 1th excited singlet and triplet state, respectively. The coexistence of two or more excited electronic states in one domain therefore leads to quenching and a reduction of the average excited-state lifetime. The different dynamics of biexcitons and higher exciton states in comparison to single excitons in coupled quantum configurations primarily contribute to the decrease of the fluorescence quantum yield of LHCII complexes at high excitation intensities. However, the main physical reason for this effect is not singlet-singlet annihilation but Pauli blocking of radiative relaxation channels as described in (Richter et al., 2008).
The molecular structure of the LHCII trimer with a diameter of about 10 nm is shown in Figure 20 (Standfuss et al., 2005). Each monomeric unit contains six Chl b and eight Chl a molecules. In addition four carotenoid molecules are bound per monomeric subunit, i.e. two luteins, one neoxanthin and one violaxanthin (Liu et al., 2004). The carotenoid molecules have a special regulatory function. Violaxanthin exhibits a slightly higher energetically excited state than the first excited singlet state in Chl a (1Chl a*), while the first excited state in zeaxanthin, which has a larger delocalised n -electron system than violaxanthin, is lower
Figure 20. Molecular structure of the trimeric LHC complex according to (Standfuss et al., 2005). Protein structures are shown in grey. The carotenoids are coloured in pink and orange. Chl b is hold in light blue and Chl a is pictured in green. A) shows the front view of the complex and B) presents the side view of the complex embedded in the thylakoid membrane (see also Figure 18). Image reproduced with permission.
than :Chl a*. Violaxanthin can absorb light energy and transfer it to Chl molecules via the mechanism of Dexter transfer (Hader, 1999), whereas the energy flows from Chl to zeaxanthin. This process in the plant LHCII is regulated by the so-called “Xanthophyll cycle” that deepoxilates violaxanthin to zeaxanthin at high light conditions (Hader, 1999) by violaxan- thin-deepoxidase while this process is reversed by the zeaxanthin- epoxidase (Jahns et al., 2001; Morosinotto et al., 2002).
Solubilised LHC trimers in buffer containing Beta-DM were measured with a commercial absorption spectrometer (lambda 19). Figure 21 shows the absorption spectrum of solubilized LHCII; it exhibits bands of Chl a (Soret band (Bx) with peak at 437 nm, Qy with peak at 675 nm) and Chl b (soret band (Bx) with peak at 474 nm, Qy band with peak at 650 nm). Minor absorption around 400 nm and 600 nm is respectively assigned to the By and Qx transitions of the chlorophylls. The strong absorption between 480 nm and 500 nm originates from carotenoid molecules. Below 400 nm, the absorption is dominated by the contribution of the proteins with the typical maximum of tryptophan that peaks at 267 nm. Below 250 nm, the steep increase in the absorption results from the sum of all organic molecules in the complex.
The structure of the Chl molecules is shown in Figure 22. It is characterized by the closed porphyrin ring system complexing the Mg2+ ion with a binding motif of 50% covalent and 50% coordinative binding. The phytol chain is bound to ring IV as indicated in Figure 22. The difference between Chl a, Chl b and Chl d is given by different side groups found at the binding sites of R1 and R2 at ring I and II, respectively (see inset in Figure 22).
Chl a and Chl b are found in the LHCII of higher plants, while PS II complexes and PS I contain only Chl a molecules.