The field-dependence of the solid-state photo-CIDNP effect in two states of heliobacterial reaction centers

Solid state NMR/Biophysical Organic Chemistr

15N photo-CIDNP MAS NMR analysis of reaction centers of Chloracidobacterium thermophilum

-OH Chl a have been shown to be the primary electron acceptors in green sulfur bacteria and heliobacteria, respectively, and thus a Chl a molecule serves this role in all known homodimeric type-1 RCs.Solid state NMR/Biophysical Organic Chemistr

Type I reaction center from the green sulfur bacterium Chlorobium tepidum: is Chl a a primary electron acceptor?

The green sulfur bacterium Chlorobium tepidum has one of the simplest type I reaction center (RC) complexes. While its structure is still unknown, biochemical and protein sequence analyses suggest that it is similar to photosystem I (PS I), with two BChl a forming a special pair P840, four Chl a serving as pairs of accessory and primary electron acceptor (A0) pigments and 14 BChl a constituting as an immediate RC antenna. This is a dramatic simplification compared to PS I RC, where 90 Chl a antenna pigments serve as antenna and 6 additional Chl a molecules function as electron transfer cofactors. The resulting spectral congestion has prevented direct visualization of ultrafast electron transfer processes within PS I RC and even the sequence of primary electron transfer processes in PS I is still under debate. The suggested presence of two types of pigments in RC from Chlorobium tepidum removes spectral congestion and opens a way to directly visualize electron transfer steps in type I RC using ultrafast spectroscopy, since the Chl a and BChl a pigments absorb at ∼670 nm and ∼800 nm, respectively. To confirm the proposed functional role of Chl a as electron transfer cofactor we performed extensive ultrafast optical pump-probe experiments on different preparations of RC complexes from Chlorobium tepidum, revealing energy/electron transfer rates between different groups of pigments. Surprisingly, we found that ∼3 out of 4 Chl a pigments do not transfer excitation energy to the BChl a antenna or to P840, which indicates that these pigments must be >20Å away from any other BChl a pigment and thus argues against the suggested presence of 4 Chl a in the reaction center core complex

Association of Photosystem I and Light-Harvesting Complex II during State Transitions

Green plant photosystem I (PS I) not only binds a chlorophyll a/b-binding, membrane-intrinsic antenna complex (LHCI) that is associated with the PS I core complex under almost all physiological conditions, but it can also transiently bind the major chlorophyll a/b-binding light-harvesting complex (LHCII), when the light conditions favor excitation of photosystem II (PS II) and the photosynthetic apparatus is in the so-called state 2. Recently, a low-resolution structure was obtained of a PS I–LHCII supercomplex from Arabidopsis thaliana. We describe here some of the structural features of this transient complex, and discuss the role of small PS I subunits that are involved in the binding of LHCII. We also discuss structural features of the PS I complex of the green algae Chlamydomonas reinhardtii, which has a larger LHCI antenna and shows a more pronounced difference between state 1 and state 2.

A Provenance-Based Trust Model for Delay Tolerant Networks

Part 1: Full PapersInternational audienceManaging trust efficiently and effectively is critical to facilitating cooperation or collaboration and decision making tasks in tactical networks while meeting system goals such as reliability, availability, or scalability. Delay tolerant networks are often encountered in military network environments where end-to-end connectivity is not guaranteed due to frequent disconnection or delay. This work proposes a provenance-based trust framework for efficiency in resource consumption as well as effectiveness in trust evaluation. Provenance refers to the history of ownership of a valued object or information. We adopt the concept of provenance in that trustworthiness of an information provider affects that of information, and vice-versa. The proposed trust framework takes a data-driven approach to reduce resource consumption in the presence of selfish or malicious nodes. This work adopts a model-based method to evaluate the proposed trust framework using Stochastic Petri Nets. The results show that the proposed trust framework achieves desirable accuracy of trust evaluation of nodes compared with an existing scheme while consuming significantly less communication overhead

Temporal and spectral characterization of the photosynthetic reaction center from Heliobacterium modesticaldum

A time-resolved spectroscopic study of the isolated photosynthetic reaction center (RC) from Heliobacterium modesticaldum reveals that thermal equilibration of light excitation among the antenna pigments followed by trapping of excitation and the formation of the charge-separated state P800+A0– occurs within ~25 ps. This time scale is similar to that reported for plant and cyanobacterial photosystem I (PS I) complexes. Subsequent electron transfer from the primary electron acceptor A0 occurs with a lifetime of ~600 ps, suggesting that the RC of H. modesticaldum is functionally similar to that of Heliobacillus mobilis and Heliobacterium chlorum. The (A0– − A0) and (P800+ − P800) absorption difference spectra imply that an 81-OH-Chl aF molecule serves as the primary electron acceptor and occupies the position analogous to ec3 (A0) in PS I, while a monomeric BChl g pigment occupies the position analogous to ec2 (accessory Chl). The presence of an intense photobleaching band at 790 nm in the (A0– − A0) spectrum suggests that the excitonic coupling between the monomeric accessory BChl g and the 81-OH-Chl aF in the heliobacterial RC is significantly stronger than the excitonic coupling between the equivalent pigments in PS I