Time-resolved UV-visible Spectroscopy Research Articles

Spontaneous Water-Promoted Self-Aggregation of a Hydrophilic Gold(I) Complex Due to Ligand Sphere Rearrangement.

Aggregating gold(I) complexes in solution through short aurophilic contacts promotes new photoluminescent deactivation pathways (aggregation-induced emission, AIE). The time dependence of spontaneous AIE is seldom studied. We examine the behavior of complex [Au(N9-hypoxanthinate)(PTA)] (1) in an aqueous solution with the aid of variable-temperature NMR, time-resolved UV-Vis and photoluminescence spectroscopy, and PGSE NMR. The studies suggest that partial ligand scrambling in favor of the ionic [Au(PTA)2][Au(N9-hypoxanthinate)2] pair followed by anion oligomerization takes place. The results are rationalized with the aid of computational calculations at the TD-DFT level of theory and IRI analysis of the electron density.

Homodimerization Counteracts the Detrimental Effect of Nitrogenous Heme Ligands on the Enzymatic Activity of Acanthamoeba castellanii CYP51.

Acanthamoeba castellanii is a free-living amoeba that can cause severe eye and brain infections in humans. At present, there is no uniformly effective treatment for any of these infections. However, sterol 14α-demethylases (CYP51s), heme-containing cytochrome P450 enzymes, are known to be validated drug targets in pathogenic fungi and protozoa. The catalytically active P450 form of CYP51 from A. castellanii (AcCYP51) is stabilized against conversion to the inactive P420 form by dimerization. In contrast, Naegleria fowleri CYP51 (NfCYP51) is monomeric in its active P450 and inactive P420 forms. For these two CYP51 enzymes, we have investigated the interplay between the enzyme activity and oligomerization state using steady-state and time-resolved UV-visible absorption spectroscopy. In both enzymes, the P450 → P420 transition is favored under reducing conditions. The transition is accelerated at higher pH, which excludes a protonated thiol as the proximal ligand in P420. Displacement of the proximal thiolate ligand is also promoted by adding exogenous nitrogenous ligands (N-ligands) such as imidazole, isavuconazole, and clotrimazole that bind at the opposite, distal heme side. In AcCYP51, the P450 → P420 transition is faster in the monomer than in the dimer, indicating that the dimeric assembly is critical for stabilizing thiolate coordination to the heme and thus for sustaining AcCYP51 activity. The spectroscopic experiments were complemented with size-exclusion chromatography and X-ray crystallography studies. Collectively, our results indicate that effective inactivation of the AcCYP51 function by azole drugs is due to synergistic interference with AcCYP51 dimerization and promoting irreversible displacement of the proximal heme-thiolate ligand.

Time-Resolved UV-VIS Spectroscopy of Microbial Rhodopsins.

Absorption of light quanta by microbial rhodopsins (or more generally by retinal proteins) leads to conversion of the light energy to the generation of transmembrane anion or cation gradients, optically gated channels, or signal states in photoreception. All these processes are accompanied by series of reaction steps with half-times ranging from femtoseconds to seconds or longer (photocycles). The number of these steps and their kinetic and spectral properties are the essential experimental information required for determination of the mechanism of light energy conversion in these proteins. Here we describe experiments and data analysis providing this information.

Kinetics-based design of a flow platform for highly reproducible on demand synthesis of gold nanoparticles with controlled size between 50 and 150 nm and their application in SERS and PIERS sensing

Seeded-growth synthetic protocols enable precise control of particle size and shape, crucial for many sensing applications. However, scaling-up these syntheses in a reproducible way is challenging, as minimal variation in process parameters such as seed size, concentration or reaction temperature can significantly alter the final product. Flow reactors enable tight control in the process parameters and high reproducibility of the synthesis, representing a potential technology to perform seeded-growth syntheses in large scale. This work reports the design of a flow platform for the controlled synthesis of spherical gold nanoparticles with size up to 150nm through a seeded-growth approach, and their use in Surface Enhanced Raman Scattering (SERS) and Photoinduced Enhanced Raman Spectroscopy (PIERS). The particle growth kinetics were studied via in situ time-resolved UV–Vis spectroscopy. The spectroscopic data were fitted with a kinetic model, which was subsequently used for the design of the reactor. The kinetics-based design approach enabled fast translation of the growth synthesis in flow, eventually allowing the on demand flow synthesis of particles with controllable size, ranging from 50 to 150nm, with high reproducibility and full precursor conversion. The particles were tested for SERS and PIERS for different substrates, including warfare agents and biomolecules, with enhancement factors between 103 and 108 depending on the analyte, demonstrating their potential for detection of various analytes.

Ferrocenium Ions as Catalysts: Decomposition Studies and Counteranion Influence on Catalytic Activity

AbstractCatalyst decomposition has a negative effect on catalytic activity, and knowledge of decomposition pathways can assist with catalyst development. Ferrocenium cations have been employed as catalysts in a number of organic transformations, and we investigated the stability of a number of ferrocenium salts in solution. The observed rate decomposition constants for [Fc]Cl, [Fc]PF6, [Fc]BF4, [Fc]CSA [Fc = ferrocenium, CSA = camphor-10-sulfonate (β)], [AcFc]SbF6, (AcFc = acetylated ferrocene), and [FcB(OH)2]SbF6 [FcB(OH)2 = ferrocenylboronic acid] were determined in CH2Cl2 solution by time-resolved UV-vis spectroscopy. The rate decomposition constants depended on the nature of the counterion, with [Fc]Cl being the most stable complex in solution. The decomposition rate constants dropped by roughly an order of magnitude in most cases when the experiments were performed in nitrogenated solvent, demonstrating that the decomposition is mainly an oxidative process. The cosolvent HFIP (1,1,1,3,3,3-hexafluoropropan-2-ol) slowed the decomposition of the ferrocenium cations as well. Many catalytic or stoichiometric reactions of ferrocenium cations are performed with alcohols; we determined that hexan-1-ol is decomposed over the course of 16 hours, but not oxidized in the presence of a ferrocenium cation. Finally, the different ferrocenium cations were employed in a test reaction to determine catalytic activity. The nucleophilic substitution of hydroxyl groups in a tertiary propargylic alcohol by an alcohol is catalyzed by all complexes, and, again, a counterion dependency of the catalytic activity was observed. Also, HFIP increases the catalytic activity of the ferrocenium cations. The research has importance in the development of ferrocenium-based catalyst systems, because changes in the counterion as well as the architecture of the ferrocenium cation have an influence on stability and catalytic activity.

Particle Size Evolution during the Synthesis of Gold Nanoparticles Using In Situ Time-Resolved UV–Vis Spectroscopy: An Experimental and Theoretical Study Unravelling the Effect of Adsorbed Gold Precursor Species

The quantitative analysis of nanomaterials synthesis kinetics is valuable both for understanding the synthesis and for development of manufacturing processes, and it usually requires the use of synchrotron-based instrumentation, making it challenging to perform experiments in the large parametric space needed to develop quantitative kinetic models. UV–vis spectroscopy represents a convenient technique to circumvent such difficulties, as it is available in most chemistry laboratories and allows fast data acquisition. This technique can in theory be used for the characterization of plasmonic nanomaterials synthesis kinetics. However, linking UV–vis spectra with characteristic features of the produced nanomaterials, such as size and shape, is a challenging task. This work presents a detailed spectroscopic analysis of gold nanoparticles syntheses via in situ time-resolved UV–vis spectroscopy, with emphasis on the role of gold precursor adsorption on the particle surface during nanoparticle growth. We show that the classic Turkevich synthesis and the growth of preformed gold nanoparticles with two different methods exhibit significant commonalities in the spectra evolution, explained in terms of the interaction between gold precursor species and the nanoparticle surface. Such interaction was accounted for in a model based on the Mie theory, describing the growing nanoparticles as core–shell spheres with an outer shell of few Ångströms characterized by reduced conductivity and increased electron damping rate. The proposed model led to the determination of the nanoparticle size and concentration evolution throughout the synthesis with good quantitative agreement against literature SAXS data. Furthermore, the core–shell model enabled the reproduction of the progressive blue-shift in the SPR peak position observed during the synthesis. Thus, this work reconciles the “seed-mediated growth” mechanism for the Turkevich synthesis with the temporal evolution of the spectra within the framework of the Mie theory, showing that the distinctive purple-greyish hue observed during the synthesis can be related to the interaction of gold precursor with the growing gold nanoparticles.

A Detailed Kinetico-Mechanistic Investigation on the Palladium C-H Bond Activation in Azobenzenes and Their Monopalladated Derivatives.

Palladium C-H bond activation in azobenzenes with R1 and R2 at para positions of the phenyl rings (R1 = NMe2, R2 = H (L1); R1 = NMe2, R2 = Cl (L2); R1 = NMe2, R2 = I (L3); R1 = NMe2, R2 = NO2 (L4); R1 = H, R2 = H (L5)) and their monopalladated derivatives, using cis-[PdCl2(DMF)2], has been studied in detail by in situ 1H NMR spectroscopy in N,N-dimethylformamide-d7 (DMF-d7) at room temperature; the same processes have been monitored in parallel via time-resolved UV-vis spectroscopy in DMF at different temperatures and pressures. The final goal was to achieve, from a kinetico-mechanistic perspective, a complete insight into previously reported reactivity results. The results suggest the operation of an electrophilic concerted metalation-deprotonation mechanism for both the mono- and dipalladation reactions, occurring from the coordination compound and the monopalladated intermediates, respectively. The process involves deprotonation of the C-H bond assisted by the presence of a coordinated DMF molecule, which acts as a base. For the first time, NMR monitoring provides a direct evidence of all the intermediate stages: that is, (i) coordination of the azo ligand to the PdII center, (ii) formation of the monopalladated species, and (iii) coordination of the monopalladated species to another PdII unit, which finally result in the (iv) formation of the dipalladated product. All of these species have been identified as intermediates in the dipalladation of azobenzenes, evidenced also by UV-vis spectroscopy time-resolved monitoring. The data also confirm that the cyclopalladation of asymmetrically substituted azobenzenes occurs by two concurrent reaction paths. In order to identify the species observed by NMR and by UV-vis spectroscopy, the final products, intermediates, and the PdII precursor have been prepared and characterized by X-ray diffraction and IR and NMR spectroscopy. DFT calculations have also been used in order to explain the isomerism observed for the isolated complexes, as well to assign their NMR and IR spectra.

Copper Complexes of Benzoylacetone Bis-Thiosemicarbazones: Metal and Ligand Based Redox Reactivity

Bis-thiosemicarbazones derived from the β-diketone benzoylacetone (H3banR, R=Me, Et, Ph) are potentially tetradentate N2S2 ligands whose coordination chemistry with copper is reported. In the absence of oxygen and in the presence of base they form anionic CuII complexes of the fully deprotonated ligands [CuII(banR)]–. Upon exposure to atmospheric oxygen they undergo a complex series of reactions leading to two types of products; one a ligand oxidised ketone complex [CuII(banRO)] and the other an unprecedented dimeric di-CuIII complex [(CuIII(banR))2] depending on the R substituent. Time-resolved UV-vis spectroscopy, cyclic voltammetry, spectroelectrochemistry, and electron paramagnetic resonance (EPR) spectroscopy have been used to identify intermediates on the way to stable products formed under both anaerobic and aerobic conditions. It is found that both ligand-centred and Cu-centred oxidation reactions are occurring in parallel leading to this unusually complicated mixture of products.

The search for panchromatic light-harvesting systems: Ternary and binary antennae based on self-organised materials

This study presents a series of light-harvesting materials, where multiple chromophores are organised into host-guest silica-micelle structures at specific locations by means of self-assembly strategies. Binary and ternary mesoscopic antennae were realized, using organometallic complexes and organic dyes as energy transfer units and varying their content and localization to manipulate transfer rate and efficiency inside the materials. Steady-state and time-resolved UV–vis spectroscopy revealed that the three-dye systems show excitation energy cascade from intramicellar dyes to a silica-grafted acceptor, with transfer efficiencies of 20–24 % per step and overall light emission spanning the whole visible range. The two-dye system reaches analogous panchromatic response, featuring almost-white light emission and 47 % efficient transfer, by exploiting the blue-green dual emission of a metallosurfactant as energy donor inside the micellar template and the red emission of a rhodamine acceptor on the silica framework. Both systems show that control over the donor-acceptor distances can be achieved to a certain extent in complex mesoscopic materials and that a vast potential is available for transfer and colour tuning, and specific use of the materials as solid-state sensitisers.

Following the Formation of Silver Nanoparticles Using In Situ X-ray Absorption Spectroscopy.

The formation of silver and Au@Ag core@shell nanoparticles via reduction of AgNO3 by trisodium citrate was followed using in situ X-ray absorption near-edge structure (XANES) spectroscopy and time-resolved UV–visible (UV–vis) spectroscopy. The XANES data were analyzed through linear combination fitting, and the reaction kinetics were found to be consistent with first-order behavior with respect to silver cations. For the Au@Ag nanoparticles, the UV–vis data of a lab-scale reaction showed a gradual shift in dominance between the gold- and silver-localized surface plasmon absorbance bands. Notably, throughout much of the reaction, distinct gold and silver contributions to the UV–vis spectra were observed; however, in the final product, the contributions were not distinct.

Probing the role of arginine 323 of the D1 protein in photosystem II function.

The Mn4 CaO5 cluster of photosystem II (PSII) advances sequentially through five oxidation states (S0 to S4 ). Under the enzyme cycle, two water molecules are oxidized, O2 is generated and four protons are released into the lumen. Umena et al. (2011) have proposed that, with other charged amino acids, the R323 residue of the D1 protein could contribute to regulate a proton egress pathway from the Mn4 CaO5 cluster and TyrZ via a proton channel identified from the 3D structure. To test this suggestion, a PsbA3/R323E site-directed mutant has been constructed and the properties of its PSII have been compared to those of the PsbA3-PSII by using EPR spectroscopy, polarography, thermoluminescence and time-resolved UV-visible absorption spectroscopy. Neither the oscillations with a period four nor the kinetics and S-state-dependent stoichiometry of the proton release were affected. However, several differences have been found: (1) the P680 + decay in the hundreds of ns time domain was much slower in the mutant, (2) the S2 QA - /DCMU and S3 QA - /DCMU radiative charge recombination occurred at higher temperatures and (3) the S0 TyrZ • , S1 TyrZ • , S2 TyrZ • split EPR signals induced at 4.2 K by visible light from the S0 TyrZ , S1 TyrZ , S2 TyrZ , respectively, and the (S2 TyrZ • )' induced by NIR illumination at 4.2 K of the S3 TyrZ state differed. It is proposed that the R323 residue of the D1 protein interacts with TyrZ likely via the H-bond network previously proposed to be a proton channel. Therefore, rather than participating in the egress of protons to the lumen, this channel could be involved in the relaxations of the H-bonds around TyrZ by interacting with the bulk, thus tuning the driving force required for TyrZ oxidation.

Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers.

Photoinduced intramolecular electron transfer dynamics following metal-to-ligand charge-transfer (MLCT) excitation of [Fe(CN)4(2,2′-bipyridine)]2− (1), [Fe(CN)4(2,3-bis(2-pyridyl)pyrazine)]2− (2) and [Fe(CN)4(2,2′-bipyrimidine)]2− (3) were investigated in various solvents with static and time-resolved UV-Visible absorption spectroscopy and Fe 2p3d resonant inelastic X-ray scattering (RIXS). This series of polypyridyl ligands, combined with the strong solvatochromism of the complexes, enables the 1MLCT vertical energy to be varied from 1.64 eV to 2.64 eV and the 3MLCT lifetime to range from 180 fs to 67 ps. The 3MLCT lifetimes in 1 and 2 decrease exponentially as the MLCT energy increases, consistent with electron transfer to the lowest energy triplet metal-centred (3MC) excited state, as established by the Tanabe–Sugano analysis of the Fe 2p3d RIXS data. In contrast, the 3MLCT lifetime in 3 changes non-monotonically with MLCT energy, exhibiting a maximum. This qualitatively distinct behaviour results from a competing 3MLCT → ground state (GS) electron transfer pathway that exhibits energy gap law behaviour. The 3MLCT → GS pathway involves nuclear tunnelling for the high-frequency polypyridyl breathing mode (hν = 1530 cm−1), which is most displaced for complex 3, making this pathway significantly more efficient. Our study demonstrates that the excited state relaxation mechanism of Fe polypyridyl photosensitizers can be readily tuned by ligand and solvent environment. Furthermore, our study reveals that extending charge transfer lifetimes requires control of the relative energies of the 3MLCT and the 3MC states and suppression of the intramolecular distortion of the acceptor ligand in the 3MLCT excited state.

Effect of Iodination on the Photophysics of the Laser Borane anti-B18H22: Generation of Efficient Photosensitizers of Oxygen

Treatment of the laser borane anti-B18H22 (compound 1) with iodine in ethanol gives the monoiodinated derivative 7-I-anti-B18H21 (compound 2) in 67% yield, or, by reaction with iodine or ICl in the presence of AlCl3 in dichloromethane, the diiodinated derivative 4,4'-I2-anti-B18H20 (compound 3) in 85% yield. On excitation with 360 nm light, both compounds 2 and 3 give strong green phosphorescent emissions (λmax = 525 nm, ΦL = 0.41 and λmax = 545 nm, ΦL = 0.71 respectively) that are quenched by dioxygen to produce O2(1Δg) singlet oxygen with quantum yields of ΦΔ = 0.52 and 0.36 respectively. Similarly strong emissions can be stimulated via the nonlinear process of two-photon absorption when exciting with 720 or 800 nm light. The high quantum yields of singlet-oxygen production, coupled with the option of two-photon excitation, make compounds 2 and 3 promising O2(1Δg) photosensitizers. The molecular structures of compounds 2 and 3 were determined by single-crystal X-ray crystallographic studies as well as multinuclear NMR spectroscopy and mass spectrometry. Time-resolved UV-vis spectroscopy was used to delineate their photophysical properties, and the electronic-structure properties of the emitting species were determined by means of multiconfigurational quantum-chemistry computations.

Effect of Moisture on Dopant Segregation in Solid Hosts

Transition metal-doped semiconductor materials are extensively employed for light harvesting and photocatalytic applications owing to their increased light absorption and charge mobility. In this work, spatial tailoring of the Ni dopant in TiO2 nanostructures is performed by varying the secondary processing parameters to engineer the resulting optoelectronic properties for select applications. Specifically, the aging of the dried Ti sol and the resulting Ni segregation are observed to be moisture-driven phenomena based on the infrared and time-resolved UV–vis spectroscopy measurements. While X-ray diffraction and scanning transmission electron microscopy coupled with electron energy-loss spectroscopy characterizations show a clear difference in the crystal structures between pristine TiO2 powders and phase-segregated NiO–TiO2, the thermogravimetric measurements reveal substitution of the ethoxy group by ambient moisture, resulting in the ejection of hydroxylated Ni clusters. Furthermore, the doped system ...

Photoactivation Mechanism, Timing of Protein Secondary Structure Dynamics and Carotenoid Translocation in the Orange Carotenoid Protein.

The orange carotenoid protein (OCP) is a two-domain photoactive protein that noncovalently binds an echinenone (ECN) carotenoid and mediates photoprotection in cyanobacteria. In the dark, OCP assumes an orange, inactive state known as OCPO; blue light illumination results in the red active state, known as OCPR. The OCPR state is characterized by large-scale structural changes that involve dissociation and separation of C-terminal and N-terminal domains accompanied by carotenoid translocation into the N-terminal domain. The mechanistic and dynamic-structural relations between photon absorption and formation of the OCPR state have remained largely unknown. Here, we employ a combination of time-resolved UV–visible and (polarized) mid-infrared spectroscopy to assess the electronic and structural dynamics of the carotenoid and the protein secondary structure, from femtoseconds to 0.5 ms. We identify a hereto unidentified carotenoid excited state in OCP, the so-called S* state, which we propose to play a key role in breaking conserved hydrogen-bond interactions between carotenoid and aromatic amino acids in the binding pocket. We arrive at a comprehensive reaction model where the hydrogen-bond rupture with conserved aromatic side chains at the carotenoid β1-ring in picoseconds occurs at a low yield of <1%, whereby the β1-ring retains a trans configuration with respect to the conjugated π-electron chain. This event initiates structural changes at the N-terminal domain in 1 μs, which allow the carotenoid to translocate into the N-terminal domain in 10 μs. We identified infrared signatures of helical elements that dock on the C-terminal domain β-sheet in the dark and unfold in the light to allow domain separation. These helical elements do not move within the experimental range of 0.5 ms, indicating that domain separation occurs on longer time scales, lagging carotenoid translocation by at least 2 decades of time.

Mesoscopic FRET Antenna Materials by Self-Assembling Iridium(III) Complexes and BODIPY Dyes.

This study presents a new design of light-harvesting antenna materials using two dyes organised into mesoporous silica: an iridium(III) complex and a BODIPY-derived surfactant that undergo Förster resonance energy transfer (FRET), acting, respectively, as donor and acceptor. The chemical structure of each dye determines the position taken within the micellar templates used for the synthesis of the silica host, which maintains mesopore order as shown by TEM imaging. Steady-state and time-resolved UV-visible spectroscopy revealed that incorporation of the iridium complex into the silica shields it from oxygen-induced quenching and allows a degree of control over the donor-acceptor distance, yielding FRET efficiencies from 24 to 76 % and tuneable emission ranges. Such silica-based antennae show promising properties for the realisation of polychromatic sensitisers for photovoltaics and photocatalysis.

Photocatalytic CO2 Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO2, CO, or Proton.

The cobalt complexes CoIIL1(PF6)2 (1; L1 = 2,6-bis[2-(2,2'-bipyridin-6'-yl)ethyl]pyridine) and CoIIL2(PF6)2 (2; L2 = 2,6-bis[2-(4-methoxy-2,2'-bipyridin-6'-yl)ethyl]pyridine) were synthesized and used for photocatalytic CO2 reduction in acetonitrile. X-ray structures of complexes 1 and 2 reveal distorted trigonal-bipyramidal geometries with all nitrogen atoms of the ligand coordinated to the Co(II) center, in contrast to the common six-coordinate cobalt complexes with pentadentate polypyridine ligands, where a monodentate solvent completes the coordination sphere. Under electrochemical conditions, the catalytic current for CO2 reduction was observed near the Co(I/0) redox couple for both complexes 1 and 2 at E1/2 = -1.77 and -1.85 V versus Ag/AgNO3 (or -1.86 and -1.94 V vs Fc+/0), respectively. Under photochemical conditions with 2 as the catalyst, [Ru(bpy)3]2+ as a photosensitizer, tri- p-tolylamine (TTA) as a reversible quencher, and triethylamine (TEA) as a sacrificial electron donor, CO and H2 were produced under visible-light irradiation, despite the endergonic reduction of Co(I) to Co(0) by the photogenerated [Ru(bpy)3]+. However, bulk electrolysis in a wet CH3CN solution resulted in the generation of formate as the major product, indicating the facile production of Co(0) and [Co-H] n+ ( n = 1 and 0) under electrochemical conditions. The one-electron-reduced complex 2 reacts with CO to produce [Co0L2(CO)] with νCO = 1894 cm-1 together with [CoIIL2]2+ through a disproportionation reaction in acetonitrile, based on the spectroscopic and electrochemical data. Electrochemistry and time-resolved UV-vis spectroscopy indicate a slow CO binding rate with the [CoIL2]+ species, consistent with density functional theory calculations with CoL1 complexes, which predict a large structural change from trigonal-bipyramidal to distorted tetragonal geometry. The reduction of CO2 is much slower than the photochemical formation of [Ru(bpy)3]+ because of the large structural changes, spin flipping in the cobalt catalytic intermediates, and an uphill reaction for the reduction to Co(0) by the photoproduced [Ru(bpy)3]+.

Binding Interactions of Zinc Cationic Porphyrin with Duplex DNA: From B-DNA to Z-DNA.

Recognition of unusual left-handed Z-DNA by specific binding of small molecules is crucial for understanding biological functions in which this particular structure participates. Recent investigations indicate that zinc cationic porphyrin (ZnTMPyP4) is promising as a probe for recognizing Z-DNA due to its characteristic chiroptical properties upon binding with Z-DNA. However, binding mechanisms of the ZnTMPyP4/Z-DNA complex remain unclear. By employing time-resolved UV-visible absorption spectroscopy in conjunction with induced circular dichroism (ICD), UV-vis, and fluorescence measurements, we examined the binding interactions of ZnTMPyP4 towards B-DNA and Z-DNA. For the ZnTMPyP4/Z-DNA complex, two coexisting binding modes were identified as the electrostatic interaction between pyridyl groups and phosphate backbones, and the major groove binding by zinc(II) coordinating with the exposed guanine N7. The respective contribution of each mode is assessed, allowing a complete scenario of binding modes revealed for the ZnTMPyP4/Z-DNA. These interaction modes are quite different from those (intercalation and partial intercalation modes) for the ZnTMPyP4/B-DNA complex, thereby resulting in explicit differentiation between B-DNA and Z-DNA. Additionally, the binding interactions of planar TMPyP4 to DNA were also investigated as a comparison. It is shown that without available virtual orbitals to coordinate, TMPyP4 binds with Z-DNA solely in the intercalation mode, as with B-DNA, and the intercalation results in a structural transition from Z-DNA to B-ZNA. These results provide mechanistic insights for understanding ZnTMPyP4 as a probe of recognizing Z-DNA and afford a possible strategy for designing new porphyrin derivatives with available virtual orbitals for the discrimination of B-DNA and Z-DNA.

The low spin - high spin equilibrium in the S2-state of the water oxidizing enzyme

In Photosystem II (PSII), the Mn4CaO5-cluster of the active site advances through five sequential oxidation states (S0 to S4) before water is oxidized and O2 is generated. Here, we have studied the transition between the low spin (LS) and high spin (HS) configurations of S2 using EPR spectroscopy, quantum chemical calculations using Density Functional Theory (DFT), and time-resolved UV-visible absorption spectroscopy. The EPR experiments show that the equilibrium between S2LS and S2HS is pH dependent, with a pKa ≈ 8.3 (n ≈ 4) for the native Mn4CaO5 and pKa ≈ 7.5 (n ≈ 1) for Mn4SrO5. The DFT results suggest that exchanging Ca with Sr modifies the electronic structure of several titratable groups within the active site, including groups that are not direct ligands to Ca/Sr, e.g., W1/W2, Asp61, His332 and His337. This is consistent with the complex modification of the pKa upon the Ca/Sr exchange. EPR also showed that NH3 addition reversed the effect of high pH, NH3-S2LS being present at all pH values studied. Absorption spectroscopy indicates that NH3 is no longer bound in the S3TyrZ state, consistent with EPR data showing minor or no NH3-induced modification of S3 and S0. In both Ca-PSII and Sr-PSII, S2HS was capable of advancing to S3 at low temperature (198 K). This is an experimental demonstration that the S2LS is formed first and advances to S3via the S2HS state without detectable intermediates. We discuss the nature of the changes occurring in the S2LS to S2HS transition which allow the S2HS to S3 transition to occur below 200 K. This work also provides a protocol for generating S3 in concentrated samples without the need for saturating flashes.

Photophysics of Coumarin and Carbostyril-Sensitized Luminescent Lanthanide Complexes: Implications for Complex Design in Multiplex Detection.

Luminescent lanthanide (Ln(III)) complexes with coumarin or carbostyril antennae were synthesized and their photophysical properties evaluated using steady-state and time-resolved UV-vis spectroscopy. Ligands bearing distant hydroxycoumarin-derived antennae attached through triazole linkers were modest sensitizers for Eu(III) and Tb(III), whereas ligands with 7-amidocarbostyrils directly linked to the coordination site could reach good quantum yields for multiple Ln(III), including the visible emitters Sm(III) and Dy(III), and the near-infrared emitters Nd(III) and Yb(III). The highest lanthanide-centered luminescence quantum yields were 35% (Tb), 7.9% (Eu), 0.67% (Dy), and 0.18% (Sm). Antennae providing similar luminescence intensities with 2-4 Ln-emitters were identified. Photoredox quenching of the carbostyril antenna excited states was observed for all Eu(III)-complexes and should be sensitizing in the case of Yb(III); the scope of the process extends to Ln(III) for which it has not been seen previously, specifically Dy(III) and Sm(III). The proposed process is supported by photophysical and electrochemical data. A FRET-type mechanism was identified in architectures with both distant and close antennae for all of the Lns. This mechanism seems to be the only sensitizing one at long distance and probably contributes to the sensitization at shorter distances along with the triplet pathway. The complexes were nontoxic to either bacterial or mammalian cells. Complexes of an ester-functionalized ligand were taken up by bacteria in a concentration-dependent manner. Our results suggest that the effects of FRET and photoredox quenching should be taken into consideration when designing luminescent Ln complexes. These results also establish these Ln(III)-complexes for multiplex detection beyond the available two-color systems.