Electrochromic shift of chlorophyll absorption in photosystem I from Synechocystis sp. PCC 6803: a probe of optical and dielectric properties around the secondary electron acceptor

Nanosecond absorption dynamics at approximately 685 nm after excitation of photosystem I (PS I) from Synechocystis sp. PCC 6803 is consistent with electrochromic shift of absorption bands of the Chl a pigments in the vicinity of the secondary electron acceptor A(1). Based on experimental optical data and structure-based simulations, the effective local dielectric constant has been estimated to be between 3 and 20, which suggests that electron transfer in PS I is accompanied by considerable protein relaxation. Similar effective dielectric constant values have been previously observed for the bacterial photosynthetic reaction center and indicate that protein reorganization leading to effective charge screening may be a necessary structural property of proteins that facilitate the charge transfer function. The data presented here also argue against attributing redmost absorption in PS I to closely spaced antenna chlorophylls (Chls) A38 and A39, and suggest that optical transitions of these Chls, along with that of connecting chlorophyll (A40) lie in the range 680-695 nm.     

The single chlorophyll a molecule in the cytochrome b6f complex: unusual optical properties protect the complex against singlet oxygen

The cytochrome b(6)f complex of oxygenic photosynthesis mediates electron transfer between the reaction centers of photosystems I and II and facilitates coupled proton translocation across the membrane. High-resolution x-ray crystallographic structures (Kurisu et al., 2003; Stroebel et al., 2003) of the cytochrome b(6)f complex unambiguously show that a Chl a molecule is an intrinsic component of the cytochrome b(6)f complex. Although the functional role of this Chl a is presently unclear (Kuhlbrandt, 2003), an excited Chl a molecule is known to produce toxic singlet oxygen as the result of energy transfer from the excited triplet state of the Chl a to oxygen molecules. To prevent singlet oxygen formation in light-harvesting complexes, a carotenoid is typically positioned within approximately 4 A of the Chl a molecule, effectively quenching the triplet excited state of the Chl a. However, in the cytochrome b(6)f complex, the beta-carotene is too far (> or =14 Angstroms) from the Chl a for effective quenching of the Chl a triplet excited state. In this study, we propose that in this complex, the protection is at least partly realized through special arrangement of the local protein structure, which shortens the singlet excited state lifetime of the Chl a by a factor of 20-25 and thus significantly reduces the formation of the Chl a triplet state. Based on optical ultrafast absorption difference experiments and structure-based calculations, it is proposed that the Chl a singlet excited state lifetime is shortened due to electron exchange transfer with the nearby tyrosine residue. To our knowledge, this kind of protection mechanism against singlet oxygen has not yet been reported for any other chlorophyll-containing protein complex. It is also reported that the Chl a molecule in the cytochrome b(6)f complex does not change orientation in its excited state.     

Genomic patterns in the widespread Eurasian lynx shaped by Late Quaternary climatic fluctuations and anthropogenic impacts

Disentangling the contribution of long‐term evolutionary processes and recent anthropogenic impacts to current genetic patterns of wildlife species is key to assessing genetic risks and designing conservation strategies. Here, we used 80 whole nuclear genomes and 96 mitogenomes from populations of the Eurasian lynx covering a range of conservation statuses, climatic zones and subspecies across Eurasia to infer the demographic history, reconstruct genetic patterns, and discuss the influence of long‐term isolation and/or more recent human‐driven changes. Our results show that Eurasian lynx populations shared a common history until 100,000 years ago, when Asian and European populations started to diverge and both entered a period of continuous and widespread decline, with western populations, except Kirov, maintaining lower effective sizes than eastern populations. Population declines and increased isolation in more recent times probably drove the genetic differentiation between geographically and ecologically close westernmost European populations. By contrast, and despite the wide range of habitats covered, populations are quite homogeneous genetically across the Asian range, showing a pattern of isolation by distance and providing little genetic support for the several proposed subspecies. Mitogenomic and nuclear divergences and population declines starting during the Late Pleistocene can be mostly attributed to climatic fluctuations and early human influence, but the widespread and sustained decline since the Holocene is more probably the consequence of anthropogenic impacts which intensified in recent centuries, especially in western Europe. Genetic erosion in isolated European populations and lack of evidence for long‐term isolation argue for the restoration of lost population connectivity.     

Asymmetric electron transfer in cyanobacterial Photosystem I: charge separation and secondary electron transfer dynamics of mutations near the primary electron acceptor A0

Point mutations were introduced near the primary electron acceptor sites assigned to A0 in both the PsaA and PsaB branches of Photosystem I in the cyanobacterium Synechocystis sp. PCC 6803. The residues Met688PsaA and Met668PsaB, which provide the axial ligands to the Mg2+ of the eC-A3 and eC-B3 chlorophylls, were changed to leucine and asparagine (chlorophyll notation follows Jordan et al., 2001). The removal of the ligand is expected to alter the midpoint potential of the A0/A0- redox pair and result in a change in the intrinsic charge separation rate and secondary electron transfer kinetics from A0- to A1. The dynamics of primary charge separation and secondary electron transfer were studied at 690 nm and 390 nm in these mutants by ultrafast optical pump-probe spectroscopy. The data reveal that mutations in the PsaB branch do not alter electron transfer dynamics, whereas mutations in the PsaA branch have a distinct effect on electron transfer, slowing down both the primary charge separation and the secondary electron transfer step (the latter by a factor of 3-10). These results suggest that electron transfer in cyanobacterial Photosystem I is asymmetric and occurs primarily along the PsaA branch of cofactors.     

Coat Polymorphism in Eurasian Lynx: Adaptation to Environment or Phylogeographic Legacy?

Journal of Mammalian Evolution - We studied the relationship between the variability and contemporary distribution of pelage phenotypes in one of most widely distributed felid species and an array...     

An anomalous distance dependence of intraprotein chlorophyll-carotenoid triplet energy transfer

In the light-harvesting chlorophyll pigment-proteins of photosynthesis, a carotenoid is typically positioned within a distance of ~4 Å of individual chlorophylls or antenna arrays, allowing rapid triplet energy transfer from chlorophyll to the carotenoid. This triplet energy transfer prevents the formation of toxic singlet oxygen. In the cytochrome b₆f complex of oxygenic photosynthesis that contains a single chlorophyll a molecule, this chlorophyll is distant (14 Å) from the single β-carotene, as defined by x-ray structures from both a cyanobacterium and a green alga. Despite this separation, rapid (<8 ns) long-range triplet energy transfer from the chlorophyll a to β-carotene is documented in this study, in seeming violation of the existing theory for the distance dependence of such transfer. We infer that a third molecule, possibly oxygen trapped in an intraprotein channel connecting the chlorophyll a and β-carotene, can serve as a mediator in chlorophyll-carotenoid triplet energy transfer in the b₆f complex.     

On the structural role of the aromatic residue environment of the chlorophyll a in the cytochrome b6f complex

Because light is not required for catalytic turnover of the cytochrome b 6 f complex, the role of the single chlorophyll a in the structure and function of the complex is enigmatic. Photodamage from this pigment is minimized by its short singlet excited-state lifetime ( approximately 200 ps), which has been attributed to quenching by nearby aromatic residues ( Dashdorj et al., 2005). The crystal structure of the complex shows that the fifth ligand of the chlorophyll a contains two water molecules. On the basis of this structure, the properties of the bound chlorophyll and the complex were studied in the cyanobacterium, Synechococcus sp. PCC 7002, through site-directed mutagenesis of aromatic amino acids in the binding niche of the chlorophyll. The b 6 f complex was purified from three mutant strains, a double mutant Phe133Leu/Phe135Leu in subunit IV and single mutants Tyr112Phe and Trp125Leu in the cytochrome b 6 subunit. The purified b 6 f complex from Tyr112Phe or Phe133Leu/Phe135Leu mutants was characterized by (i) a loss of bound Chl and b heme, (ii) a shift in the absorbance peak and increase in bandwidth, (iii) multiple lifetime components, including one of 1.35 ns, and (iv) relatively small time-resolved absorbance anisotropy values of the Chl Q y band. A change in these properties was minimal in the Trp125Leu mutant. In vivo, no decrease in electron-transport efficiency was detected in any of the mutants. It was concluded that (a) perturbation of its aromatic residue niche influences the stability of the Chl a and one or both b hemes in the monomer of the b 6 f complex, and (b) Phe residues (Phe133/Phe135) of subunit IV are important in maintaining the short lifetime of the Chl a singlet excited state, thereby decreasing the probability of singlet oxygen formation.