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 ∼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

Asymmetric Electron Transfer in Cyanobacterial Photosystem I: Charge Separation and Secondary Electron Transfer Dynamics of Mutations Near the Primary Electron Acceptor A(0)

Point mutations were introduced near the primary electron acceptor sites assigned to A(0) in both the PsaA and PsaB branches of Photosystem I in the cyanobacterium Synechocystis sp. PCC 6803. The residues Met688(PsaA) and Met668(PsaB), which provide the axial ligands to the Mg(2+) 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 [Formula: see text] redox pair and result in a change in the intrinsic charge separation rate and secondary electron transfer kinetics from [Formula: see text] to A(1). 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

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 b(6)f 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 b(6)f complex

Sensitivity and specificity of commercially available rapid diagnostic tests for viral hepatitis B and C screening in serum samples.

Early diagnosis of chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infections is pivotal for optimal disease management. Sensitivity and specificity of 19 rapid diagnostic test (RDT) kits by different manufacturers (ABON, CTK Biotech, Cypress Diagnostics, Green Gross, Human Diagnostic, Humasis, InTec, OraSure, SD Bioline, Wondfo) were assessed on serum samples of 270 Mongolians (90 seropositive for hepatitis B surface antigen (HBsAg), 90 seropositive for hepatitis C antibody (HCV-Ab), 90 healthy subjects). All tested RDTs for detection of HBsAg performed with average sensitivities and specificities of 100% and 99%, respectively. Albeit, overall sensitivity and specificity of RDTs for detection of HCV-Ab was somewhat lower compared to that of HBsAg RDTs (average sensitivity 98.9%, average specificity 96.7%). Specificity of RDTs for detection of HCV-Ab was dramatically lower among HBsAg positive individuals, who were 10.2 times more likely to show false positive test results. The results of our prospective study demonstrate that inexpensive, easy to handle RDTs are a promising tool in effective HBV- and HCV-screening especially in resource-limited settings

Watching a signaling protein function in real time via 100-ps time-resolved Laue crystallography

To understand how signaling proteins function, it is crucial to know the time-ordered sequence of events that lead to the signaling state. We recently developed on the BioCARS 14-IDB beamline at the Advanced Photon Source the infrastructure required to characterize structural changes in protein crystals with near-atomic spatial resolution and 150-ps time resolution, and have used this capability to track the reversible photocycle of photoactive yellow protein (PYP) following trans-to-cis photoisomerization of its p-coumaric acid (pCA) chromophore over 10 decades of time. The first of four major intermediates characterized in this study is highly contorted, with the pCA carbonyl rotated nearly 90° out of the plane of the phenolate. A hydrogen bond between the pCA carbonyl and the Cys69 backbone constrains the chromophore in this unusual twisted conformation. Density functional theory calculations confirm that this structure is chemically plausible and corresponds to a strained cis intermediate. This unique structure is short-lived (?600 ps), has not been observed in prior cryocrystallography experiments, and is the progenitor of intermediates characterized in previous nanosecond time-resolved Laue crystallography studies. The structural transitions unveiled during the PYP photocycle include trans/cis isomerization, the breaking and making of hydrogen bonds, formation/relaxation of strain, and gated water penetration into the interior of the protein. This mechanistically detailed, near-atomic resolution description of the complete PYP photocycle provides a framework for understanding signal transduction in proteins, and for assessing and validating theoretical/computational approaches in protein biophysics

Spectral Resolution of the Primary Electron Acceptor A0 in Photosystem I

The reduced state of the primary electron acceptor of Photosystem I, A0, was resolved spectroscopically in its lowest energy Qy region for the first time without the addition of chemical reducing agents and without extensive data manipulation. To carry this out, we used the menB mutant of Synechocystis sp. PCC 6803 in which phylloquinone is replaced by plastoquinone-9 in the A1 sites of Photosystem I. The presence of plastoquinone-9 slows electron transfer from A0 to A1, leading to a long-lived A0– state. This allows its spectral signature to be readily detected in a time-resolved optical pump–probe experiment. The maximum bleaching (A0– – A0) was found to occur at 684 nm with a corresponding extinction coefficient of 43 mM–1 cm–1. The data show evidence for an electrochromic shift of an accessory chlorophyll pigment, suggesting that the latter Qy absorption band is centered around 682 nm

Design of an observational multi-country cohort study to assess immunogenicity of multiple vaccine platforms (InVITE).

In response to the COVID-19 pandemic, COVID-19 vaccines have been developed, and the World Health Oraganization (WHO) has granted emergency use listing to multiple vaccines. Studies of vaccine immunogenicity data from implementing COVID-19 vaccines by national immunization programs in single studies spanning multiple countries and continents are limited but critically needed to answer public health questions on vaccines, such as comparing immune responses to different vaccines and among different populations