The ability of photosynthetic organisms to use the sun's light as a sole source of energy sustains life on our planet. Photosystems I (PSI) and II (PSII) are large, multi-subunit, pigment-protein complexes that enable photosynthesis, but this intriguing process remains to be explained fully. Currently, crystal structures of these complexes are available for thermophilic prokaryotic cyanobacteria. The mega-Dalton trimeric PSI complex from thermophilic cyanobacterium, Thermosynechococcus elongatus, was solved at 2.5 Å resolution with X-ray crystallography. That structure revealed the positions of 12 protein subunits (PsaA-F, PsaI-M, and PsaX) and 127 cofactors. Although mesophilic organisms perform most of the world's photosynthesis, no well-resolved trimeric structure of a mesophilic organism exists. Our research model for a mesophilic cyanobacterium was Synechocystis sp. PCC6803. This study aimed to obtain well-resolved crystal structures of [1] a monomeric PSI with all subunits, [2] a trimeric PSI with a reduced number of subunits, and [3] the full, trimeric wild-type PSI complex. We only partially succeeded with the first two structures, but we successfully produced the trimeric PSI structure at 2.5 Å resolution. This structure was comparable to that of the thermophilic species, but we provided more detail. The PSI trimeric supercomplex consisted of 33 protein subunits, 72 carotenoids, 285 chlorophyll a molecules, 51 lipids, 9 iron-sulfur clusters, 6 plastoquinones, 6 putative calcium ions, and over 870 water molecules. This study showed that the structure of the PSI in Synechocystis sp. PCC6803 differed from previously described PSI structures. These findings have broadened our understanding of PSI structure.

Asiatic acid (AsA), a naturally occurring pentacyclictriterpenoid found in Centella asiatica, plays a major role in neuroprotection, anticancer, antioxidant, and hepatoprotective activities. Human serum albumin (HSA), a blood plasma protein, participates in the regulation of plasma osmotic pressure and transports endogenous and exogenous substances. The study undertaken to analyze the drug-binding mechanisms of HSA is crucial in understanding the bioavailability of drugs. In this study, we analyzed the cytotoxic activity of AsA on HepG2 (human hepatocellular carcinoma) cell lines and its binding, conformational, docking, molecular simulation studies with HSA under physiological pH 7.2. These studies revealed a clear decrease in the viability of HepG2 cells upon exposure to AsA in a dose-dependent manner with an IC50 of 45 μM. Further studies showed the quenching of intrinsic fluorescence of HSA by AsA with a binding constant of KAsA = 3.86 ± 0.01 × 10(4) M(-1), which corresponds to the free energy of (ΔG) -6.3 kcal M(-1) at 25 °C. Circular dichroism (CD) studies revealed that there is a clear decrease in the α-helical content from 57.50 ± 2.4 to 50% ± 2.3 and an increase in the β-turns from 25 ± 0.65 to 29% ± 0.91 and random coils from 17.5% ± 0.95 to 21% ± 1.2, suggesting partial unfolding of HSA. Autodock studies revealed that the AsA is bound to the subdomain IIA with hydrophobic and hydrophilic interactions. From molecular dynamics, simulation data (RMSD, Rg and RMSF) emphasized the local conformational changes and rigidity of the residues of both HSA and HSA-AsA complexes.

The green alga Chlamydomonas (C.) reinhardtii is a model organism for photosynthesis research. State transitions regulate redistribution of excitation energy between photosystem I (PS I) and photosystem II (PS II) to provide balanced photosynthesis. Chlorophyll (Chl) a fluorescence induction (the so-called OJIPSMT transient) is a signature of several photosynthetic reactions. Here, we show that the slow (seconds to minutes) S to M fluorescence rise is reduced or absent in the stt7 mutant (which is locked in state 1) in C. reinhardtii. This suggests that the SM rise in wild type C. reinhardtii may be due to state 2 (low fluorescence state; larger antenna in PS I) to state 1 (high fluorescence state; larger antenna in PS II) transition, and thus, it can be used as an efficient and quick method to monitor state transitions in algae, as has already been shown in cyanobacteria (Papageorgiou et al. 1999, 2007; Kaňa et al. 2012). We also discuss our results on the effects of (1) 3-(3,4-dichlorophenyl)-1,4-dimethyl urea, an inhibitor of electron transport; (2) n-propyl gallate, an inhibitor of alternative oxidase (AOX) in mitochondria and of plastid terminal oxidase in chloroplasts; (3) salicylhydroxamic acid, an inhibitor of AOX in mitochondria; and (4) carbonyl cyanide p-trifluoromethoxyphenylhydrazone, an uncoupler of phosphorylation, which dissipates proton gradient across membranes. Based on the data presented in this paper, we conclude that the slow PSMT fluorescence transient in C. reinhardtii is due to the superimposition of, at least, two phenomena: qE dependent non-photochemical quenching of the excited state of Chl, and state transitions.

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BackgroundNon photochemical reduction of PQ pool and mobilization of LHCII between PSII and PSI are found to be linked under abiotic stress conditions. The interaction of non photochemical reduction of PQ pool and state transitions associated physiological changes are critically important under anaerobic condition in higher plants.Methodology/FindingsThe present study focused on the effect of anaerobiosis on non-photochemical reduction of PQ pool which trigger state II transition in Arabidopsis thaliana. Upon exposure to dark-anaerobic condition the shape of the OJIP transient rise is completely altered where as in aerobic treated leaves the rise is unaltered. Rise in F o and F J was due to the loss of oxidized PQ pool as the PQ pool becomes more reduced. The increase in Fo′ was due to the non photochemical reduction of PQ pool which activated STN7 kinase and induced LHCII phosphorylation under anaerobic condition. Further, it was observed that the phosphorylated LHCII is migrated and associated with PSI supercomplex increasing its absorption cross-section. Furthermore, evidences from crr2-2 (NDH mutant) and pgr5 mutants (deficient in non NDH pathway of cyclic electron transport) have indicated that NDH is responsible for non photochemical reduction of the PQ pool. We propose that dark anaerobic condition accelerates production of reducing equivalents (such as NADPH by various metabolic pathways) which reduce PQ pool and is mediated by NDH leading to state II transition.Conclusions/SignificanceAnaerobic condition triggers non photochemical reduction of PQ pool mediated by NDH complex. The reduced PQ pool activates STN7 kinase leading to state II transition in A. thaliana.

Photosystem I (PSI), found in all oxygenic photosynthetic organisms, uses solar energy to drive electron transport with nearly 100% quantum efficiency, thanks to fast energy transfer among antenna chlorophylls and charge separation in the reaction center. There is no complete consensus regarding the kinetics of the elementary steps involved in the overall trapping, especially the rate of primary charge separation. In this work, we employed twodimensional coherent electronic spectroscopy to follow the dynamics of energy and electron transfer in a monomeric PSI complex from Synechocystis PCC 6803, containing only subunits A−E, K, and M, at 77 K. We also determined the structure of the complex to 4.3 Å resolution by cryoelectron microscopy with refinements to 2.5 Å. We applied structure-based modeling using a combined Redfield−Forster theory to compute the excitation dynamics. The absorptive 2D electronic spectra revealed fast excitonic/vibronic relaxation on time scales of 50−100 fs from the high-energy side of the absorption spectrum. Antenna excitations were funneled within 1 ps to a small pool of chlorophylls absorbing around 687 nm, thereafter decaying with 4−20 ps lifetimes, independently of excitation wavelength. Redfield−Forster energy transfer computations showed that the kinetics is limited by transfer from these red-shifted pigments. The rate of primary charge separation, upon direct excitation of the reaction center, was determined to be 1.2−1.5 ps −1 . This result implies activationless electron transfer in PSI.

Herein, the amino‐acid based glycine (Gly) solubility in different solvents (ethanol, pyridine, and n‐hexane) exhibited striking interlocking behavior with mesoporous silica nanoparticles (MP‐SiO2NPs) has been reported and discussed in detail. The synthesis of MP‐SiO2NPs carried out employing the modified Stober's ‘Sol‐Gel’ method. The yielded MP‐SiO2NPs size ranges from ∼20–80 nm, with an average particle size of ca. 36 nm. The morphology of Gly bound with MP‐SiO2NPs was analyzed through electron microscopic imaging (SEM, TEM), followed by characterizations (BET, PXRD, DSC, TGA, EDAX) in various solvents. Interestingly, Gly dissolved in particular solvents demonstrated remarkable binding and interlocking properties with the well‐dispersed MP‐SiO2NPs to form a foamy surface. The developed [(Gly)‐(MPSiO2NPs)] based aggregate is stable at room temperature (∼25 °C). Further, developed [(Gly)‐(MP‐SiO2NPs)] aggregate used to load the anticancer drug (DOX) and it shows ∼80 % loading efficacy. Whereas, the DOX release from [(Gly)‐(MP‐SiO2NPs)] is calculated as ∼59 % after 24 hr. The designed nanoformulation [(Gly)‐(MP‐SiO2NPs)] aggregate along with DOX shows significant inhibition (i. e. 74 %) on K562 (chronic myeloid leukemia) blood cancer cells. Such low‐density foamy materials are believed to be utilized in industrial and pharmaceutical applications.

Herein, the synthesis of an amino-acid-based di-block copolymer (di-BCP) in-between an l-glutamic acid-5-benzyl ester and L-aspartic acid-4-benzyl ester [(l-GluA-5-BE)-b-(l-AspA-4-BE)] has been reported.