Abstract
Lhca4 is one of the peripheral antennae of higher plant photosystem I and it is characterized by the presence of chlorophyll a with absorption and emission bands around 30 nm red-shifted compared to those of the other chlorophylls associated with plant complexes. In this work we have investigated the origin of this red shift by using the recent structure of Lhca4 (Qin et al., Science, 2015, 348, 989) to build an exciton model that includes a charge-transfer (CT) state mixed with the excited-state manifold. A simultaneous quantitative fit of absorption, linear dichroism, fluorescence, and Stark absorption spectra of the wild-type Lhca4 and NH mutant (where the sites involved in CT are affected) enables us to determine the origin of the CT state and explore its spectral signatures. A huge borrowing of dipole strength by the CT, accompanied by anomalous broadening and red-shifting of the fluorescence as well as dramatic changes in the Stark spectrum, can be accounted for by a model implying an exciton-type mixing between excited states and CT states.
Highlights
In plants Photosystem I (PSI) is composed of a core complex, containing about 100 chlorophylls (Chls) including the reaction center, where charge separation occurs, and an outer antenna (LHCI), composed of 4 subunits (Lhca1–4) assembled on one side of the PSI-core[4] in the form of two heterodimers, Lhca[1,2,3,4] and Lhca2–3.5 Lhcas belong to the light-harvesting complex multigenic family[6] and the recent crystal structures show that they share a very similar folding and coordinate 13–15 Chls and 3–4 carotenoids each in highly conserved binding sites.[7,8]
From the fit of the spectra we have found that the l0 and lj values determined for Chls a should be multiplied by 0.8, and 3.2 for Chls b, and CT states, respectively
The main challenge in explaining the functioning of the complex is the choice of an adequate physical model for the coupling between excited states and CT states
Summary
Photosystem I (PSI) is one of the two multi-pigment–protein complexes responsible for the first steps of oxygenic photosynthesis.[1,2,3] In plants PSI is composed of a core complex, containing about 100 chlorophylls (Chls) including the reaction center, where charge separation occurs, and an outer antenna (LHCI), composed of 4 subunits (Lhca1–4) assembled on one side of the PSI-core[4] in the form of two heterodimers, Lhca[1,2,3,4] and Lhca2–3.5 Lhcas belong to the light-harvesting complex multigenic family[6] and the recent crystal structures show that they share a very similar folding and coordinate 13–15 Chls (a and b) and 3–4 carotenoids each in highly conserved binding sites.[7,8] their spectroscopic properties are largely different.[2]The red-most emission (with fluorescence (FL) peaking at B730 nm) originates from the Lhca[4] subunit. The character of the exciton mixing between the pigments is determined by the pigment–pigment couplings, and strongly dependent on the site energies (including the spread of the unperturbed energies plus additional random shifts induced by the disorder).
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