K. V. Lakshmi

Ligand Environment of the S2 State of Photosystem II: A Study of the Hyperfine Interactions of the Tetranuclear Manganese Cluster by 2D 14N HYSCORE Spectroscopy

The solar water-splitting protein complex, photosystem II, catalyzes the light-driven oxidation of water to dioxygen in Nature. The four-electron oxidation reaction of water occurs at the tetranuclear manganese-calcium-oxo catalytic cluster that is present in the oxygen-evolving complex of photosystem II. The mechanism of light-driven water oxidation has been a subject of intense interest, and the oxygen-evolving complex of photosystem II has been studied extensively by structural and biochemical methods. While the recent X-ray crystal structures and single-crystal EXAFS investigations provide a model for the geometry of the tetranuclear manganese-calcium-oxo catalytic cluster, there is limited knowledge of the protein environment that surrounds the catalytic cluster. In this study, we demonstrate the application of two-dimensional hyperfine sublevel correlation spectroscopy to determine the magnetic couplings of the catalytic cluster with the (14)N atoms of surrounding amino acid residues in the S(2) state of the oxygen-evolving complex of photosystem II. We utilize two-dimensional difference spectroscopy to facilitate unambiguous assignments of the spectral features and identify at least three separate (14)N atoms that are interacting with the catalytic cluster in the S(2) state. The results presented here, for the first time, identify previously unknown ligands to the catalytic cluster of photosystem II and provide avenues for the assignment of residues by site-directed mutagenesis and the refinement of computational and mechanistic models of photosystem II.

Eco-friendly synthesis of metal dichalcogenides nanosheets and their environmental remediation potential driven by visible light.

Exfoliated transition metal dichalcogenides (TMDs) such as WS2 and MoS2 have shown exciting potential for energy storage, catalysis and optoelectronics. So far, solution based methods for scalable production of few-layer TMDs usually involve the use of organic solvents or dangerous chemicals. Here, we report an eco-friendly method for facile synthesis of few-layer WS2 and MoS2 nanosheets using dilute aqueous solution of household detergent. Short time sonication of varying amount of bulk samples in soapy water was used to scale up the production of nanosheets. Thermal stability, optical absorption and Raman spectra of as-synthesized WS2 and MoS2 nanosheets are in close agreement with those from other synthesis techniques. Efficient photocatalytic activity of TMDs nanosheets was demonstrated by decomposing Brilliant Green dye in aqueous solution under visible light irradiation. Our study shows the great potential of TMDs nanosheets for environmental remediation by degrading toxic industrial chemicals in wastewater using sunlight.

The structure and activation of substrate water molecules in the S2 state of photosystem II studied by hyperfine sublevel correlation spectroscopy

The water-splitting protein, photosystem II, catalyzes the light-driven oxidation of water to dioxygen. The solar water oxidation reaction takes place at the catalytic center, referred to as the oxygen-evolving complex, of photosystem II. During the catalytic cycle, the oxygen-evolving complex cycles through five distinct intermediate states, S0–S4. In this study, we trap the oxygen-evolving complex in the S2 intermediate state by low temperature illumination of photosystem II isolated from three different species, Thermosynechococcus vulcanus, the PsbB variant of Synechocystis PCC 6803 and spinach. We apply two-dimensional hyperfine sublevel correlation spectroscopy to detect weak magnetic interactions between the paramagnetic tetra-nuclear manganese cluster of the S2 state of the OEC and the surrounding protons. We identify five groups of protons that are interacting with the tetra-nuclear manganese cluster. From the values of hyperfine interactions and using the recently reported 1.9 A resolution X-ray structure of the OEC in the S1 state [Umena et al., Nature, 2011, 473, 55], we discuss the assignments of the five groups of protons and draw important conclusions on the structure of the oxygen-evolving complex in the S2 state. In addition, we conclude that the structure of OEC is nearly identical in photosystem II from Thermosynechococcus vulcanus, the PsbB variant of Synechocystis PCC 6803 and spinach.

Dicopper Cu(I)Cu(I) and Cu(I)Cu(II) Complexes in Copper-Catalyzed Azide–Alkyne Cycloaddition

A discrete, dicopper μ-alkynyl complex, [Cu2(μ-η1:η1-C≡C(C6H4)CH3)DPFN]NTf2 (DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine; NTf2- = N(SO2CF3)2-), reacts with p-tolylazide to yield a dicopper complex with a symmetrically bridging 1,2,3-triazolide, [Cu2(μ-η1:η1-(1,4-bis(4-tolyl)-1,2,3-triazolide))DPFN]NTf2. This transformation exhibits bimolecular reaction kinetics and represents a key step in a proposed, bimetallic mechanism for copper-catalyzed azide-alkyne cycloaddition (CuAAC). The μ-alkynyl and μ-triazolide complexes undergo reversible redox events (by cyclic voltammetry), suggesting that a cycloaddition pathway involving mixed-valence dicopper species might also be possible. Synthesis and characterization of the mixed-valence μ-alkynyl dicopper complex, [Cu2(μ-η1:η1-C≡C(C6H4)CH3)DPFN](NTf2)2, revealed an electronic structure with an unexpected partially delocalized spin, as evidenced by electron paramagnetic resonance spectroscopy. Studies of the mixed-valence μ-alkynyl complexs reactivity suggest that a mixed-valence pathway is less likely than one involving intermediates with only copper(I).

Aryl Group Transfer from Tetraarylborato Anions to an Electrophilic Dicopper(I) Center and Mixed-Valence μ-Aryl Dicopper(I,II) Complexes

The synthesis of discrete, cationic binuclear μ-aryl dicopper complexes [Cu2(μ-η(1):η(1)-Ar)DPFN]X (Ar = C6H5, 3,5-(CF3)2C6H3, and C6F5; DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine; X = BAr4(-) and NTf2(-); Tf = SO2CF3) was achieved by treatment of a dicopper complex [Cu2(μ-η(1):η(1)-NCCH3)DPFN]X2 (X = PF6(-) and NTf2(-)) with tetraarylborates. Structural characterization revealed symmetrically bridging aryl groups, and (1)H NMR spectroscopy evidenced the same structure in solution at 24 °C. Electrochemical investigation of the resulting arylcopper complexes uncovered reversible redox events that led to the synthesis and isolation of a rare mixed-valence organocopper complex [Cu2(μ-η(1):η(1)-Ph)DPFN](NTf2)2 in high yield. The solid-state structure of the mixed-valence μ-phenyl complex exhibits inequivalent copper centers, despite a short Cu···Cu distance. Electronic and variable-temperature electron paramagnetic resonance spectroscopy of the mixed-valence μ-phenyl complex suggest that the degree of spin localization is temperature-dependent, with a high degree of spin localization observed at lower temperatures. Electronic structure calculations agree with the experimental results and suggest that the spin is localized almost entirely on one metal center.

The Structure and Function of Quinones in Biological Solar Energy Transduction: A Cyclic Voltammetry, EPR, and Hyperfine Sub-Level Correlation (HYSCORE) Spectroscopy Study of Model Naphthoquinones

Quinones function as electron transport cofactors in photosynthesis and cellular respiration. The versatility and functional diversity of quinones is primarily due to the diverse midpoint potentials that are tuned by the substituent effects and interactions with surrounding amino acid residues in the binding site in the protein. In the present study, a library of substituted 1,4-naphthoquinones are analyzed by cyclic voltammetry in both protic and aprotic solvents to determine effects of substituent groups and hydrogen bonds on the midpoint potential. We use continuous-wave electron paramagnetic resonance (EPR) spectroscopy to determine the influence of substituent groups on the electronic properties of the 1,4-naphthoquinone models in an aprotic solvent. The results establish a correlation between the presence of substituent group(s) and the modification of electronic properties and a corresponding shift in the midpoint potential of the naphthoquinone models. Further, we use pulsed EPR spectroscopy to determine the effect of substituent groups on the strength and planarity of the hydrogen bonds of naphthoquinone models in a protic solvent. This study provides support for the tuning of the electronic properties of quinone cofactors by the influence of substituent groups and hydrogen bonding interactions.

Effect of Hydrogen Bond Strength on the Redox Properties of Phylloquinones: A Two-Dimensional Hyperfine Sublevel Correlation Spectroscopy Study of Photosystem I

The phylloquinones of photosystem I (PS I), A(1A) and A(1B), exist in near-equivalent protein environments but possess distinct thermodynamic and kinetic properties. Although the determinants responsible for the different properties of the phylloquinones are not completely understood, the strength and geometry of hydrogen bond interactions are significant factors in tuning and control of function. This study focuses on characterizing the hydrogen-bonding interactions of the phylloquinone acceptor, A(1A), by (1)H and (14)N HYSCORE spectroscopy. Photoaccumulation of PS I complexes at pH 8.0 results in the trapping of the phyllosemiquinone anion, A(1A)(-), on the A-branch of cofactors. The experiments described here indicate that A(1A)(-) forms a single H-bond. Using a simple point dipole approximation, we estimate its length to be 1.6 ± 0.1 Å. The value of the (1)H isotropic hyperfine coupling constant suggests that the H-bond has significant out-of-plane character. The (14)N HYSCORE spectroscopy experiments support the assignment of a H-bond wherein, the (14)N quadrupolar coupling constant is consistent with a backbone amide nitrogen as the hydrogen bond donor.

The assembly of a multisubunit photosynthetic membrane protein complex: a site-specific spin labeling EPR spectroscopic study of the PsaC subunit in photosystem I.

The assembly of the PsaC subunit in the photosystem I (PS I) complex was studied using site-specific spin labeling electron paramagnetic resonance (EPR) spectroscopic techniques. The binding was monitored from the perspective of a reporter spin label attached to either the native C34(C) or the engineered C75(C) residue of wild-type PsaC (PsaC(WT)). Three distinct stages of PsaC assembly were analyzed: unbound PsaC, the P(700)-F(X)/PsaC complex, and the P(700)-F(X)/PsaC/PsaD complex. The changes in the EPR spectral line shape and the rotational correlation time of the spin label when PsaC(WT) binds to the PS I core are consistent with the conformational changes that are expected to occur during the assembly process. The addition of the PsaD subunit to the P(700)-F(X)/PsaC(WT-C34) complex induces further EPR spectral changes, which indicate that the presence of PsaD affects the orientation of the PsaC subunit on the PS I core. The binding of several PsaC variants, each lacking one or more key binding contacts with the PsaA/PsaB heterodimer, was monitored using a reporter spin label at C34(C). Our results indicate that the absence of the PsaC-PsaA/PsaB binding contacts causes PsaC to bind in an altered configuration on the PS I core. In particular, the removal of the entire C-terminus (PsaC(C-term)) causes PsaC to dock in a significantly different orientation when compared to the wild-type protein, as indicated by the EPR spectrum of the P(700)-F(X)/PsaC(C-term-C34) complex. Because the PsaC(C-term) variant retains only the symmetric network of PsaC-PsaA/PsaB ionic contacts, the altered EPR spectrum could, in principle, reflect a fraction of reaction centers that contain PsaC bound in the 180 degrees-rotated, C(2)-symmetry-related configuration. The results of this study are used to provide a comprehensive, stepwise mechanism for the binding of PsaC on the PS I core.

A Stable Seven-Membered Heterocycle, Containing B, C, N, O, and P Atoms, inside a Smaragdyrin Macrocycle

Unprecedented examples of smaragdyrin macrocycles containing seven membered heterocyclic rings were synthesized under simple reaction conditions in high yields. The heterocycle formed inside smaragdyrin macrocycle is rare example of heterocycle containing five different atoms, such as B, C, N, O, and P atoms. The mixed B(III) and P(V) complexes of smaragdyrin macrocycles showed new structural, spectral, and electrochemical properties.

Mixed Boron(III) and Phosphorous(V) Complexes of meso‐Triaryl 25‐Oxasmaragdyrins

Two unprecedented mixed B(III) /P(V) complexes of meso-triaryl 25-oxasmaragdyrins were synthesized in appreciable yields under mild reaction conditions. These unusual 25-oxasmaragdyrin complexes containing one or two seven-membered heterocyclic rings comprised of five different atoms (B, C, N, O, and P) were prepared by reacting B(OH)(Ph)-smaragdyrin and B(OH)2 -smaragdyrin complexes, respectively, with POCl3 in toluene at reflux temperature. The products were characterized by HRMS and 1D- and 2D-NMR spectroscopy. X-ray crystallography of one of the mixed B(III) /P(V) smaragdyrin complexes indicated that the macrocycle is significantly distorted and contains a stable seven-membered heterocyclic ring within the macrocycle. The bands in the absorption and emission spectra were bathochromically shifted with reduced quantum yields and singlet-state lifetimes relative to the free base, meso-triaryl 25-oxasmaragdyrin. The mixed B(III) /P(V) complexes were difficult to oxidize but easier to reduce than the free base. The DFT-optimized structure of the 25-oxasmaragdyrin complex with two seven-membered heterocycles indicated that it was a bicyclic spiro compound with two half-chair-like conformers. This was in contrast to the chair-like conformation of the complex with a single seven-membered heterocyclic ring. Moreover, incorporation of a second phosphate group in the former case stabilized the bonding geometry and resulted in higher stability, which was reflected in the bathochromic shift of the absorption spectra, more-positive oxidation potential, and less-negative reduction potential.