Marco Flores

High-quality manganese-doped zinc sulfide quantum rods with tunable dual-color and multiphoton emissions.

We report a simple, fast and green phosphine-free colloidal chemistry to synthesize high-quality wurtzite-type Mn-doped ZnS quantum rods (QRs) with tunable diameters (1.6-5.6 nm), high aspect ratios (up to 50), variable Mn doping levels (0.18-1.60%), and high quantum yields (up to 45%). The electron paramagnetic resonance spectra with modeling reveal the successful doping of paramagnetic Mn(2+) ions in the host ZnS QRs. The Mn-doped ZnS QRs demonstrate tunable dual-color (orange and blue) emissions by tuning the doping levels and UV excitation wavelengths. The orange emission with long decay lifetime (3.3 ms) originates from the doped Mn(2+) states, while the blue emission with fast decay lifetime (0.31 ns) is attributed to the QR surface states. The bright two- and three-photon excitation upconversion luminescence from the Mn-doped ZnS QRs have been observed using tunable near-infrared femtosecond laser. Our strategy provides a versatile route to programmably control the optical properties of anisotropic semiconductor nanomaterials, which may create new opportunities for photonic devices and bioimaging applications.

A Highly Active Manganese Precatalyst for the Hydrosilylation of Ketones and Esters

The reduction of ((Ph2PPr)PDI)MnCl2 allowed the preparation of the formally zerovalent complex, ((Ph2PPr)PDI)Mn, which features a pentadentate bis(imino)pyridine chelate. This complex is a highly active precatalyst for the hydrosilylation of ketones, exhibiting TOFs of up to 76,800 h(-1) in the absence of solvent. Loadings as low as 0.01 mol % were employed, and ((Ph2PPr)PDI)Mn was found to mediate the atom-efficient utilization of Si-H bonds to form quaternary silane products. ((Ph2PPr)PDI)Mn was also shown to catalyze the dihydrosilylation of esters following cleavage of the substrate acyl C-O bond. Electronic structure investigation of ((Ph2PPr)PDI)Mn revealed that this complex possesses an unpaired electron on the metal center, rendering it likely that catalysis takes place following electron transfer to the incoming carbonyl substituent.

Inhibition of the [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F by carbon monoxide: An FTIR and EPR spectroscopic study

X-ray crystallographic studies [Ogata et al., J. Am. Chem. Soc. 124 (2002) 11628-11635] have shown that carbon monoxide binds to the nickel ion at the active site of the [NiFe] hydrogenase from Desulfovibriovulgaris Miyazaki F and inhibits its catalytic function. In the present work spectroscopic aspects of the CO inhibition for this bacterial organism are reported for the first time and enable a direct comparison with the existing crystallographic data. The binding affinity of each specific redox state for CO is probed by FTIR spectro-electrochemistry. It is shown that only the physiological state Ni-SI(a) reacts with CO. The CO-inhibited product state is EPR-silent (Ni2+) and exists in two forms, Ni-SCO and Ni-SCO(red). At very negative potentials, the exogenous CO is electrochemically detached from the active site and the active Ni-R states are obtained. At temperatures below 100 K, photodissociation of the extrinsic CO from the Ni-SCO state results in Ni-SI(a) that is identified to be the only light-induced state. In the dark, rebinding of CO takes place; the recombination rate constants are of biexponential character and the activation barrier is determined to be approximately 9 kJ mol(-1). In addition, formation of a paramagnetic CO-inhibited state (Ni-CO) was observed that results from the interaction of carbon monoxide with the Ni-L state. It is proposed that the nickel in Ni-CO is in a formal monovalent state (Ni1+).

Probing hydrogen bonding to quinone anion radicals by 1H and 2H ENDOR spectroscopy at 35 GHz

Abstract ENDOR spectroscopy at 35 GHz and 80 K was used to study the radical anion of 1,4-benzoquinone (BQ) in water and various alcohols. BQ -d 4 in H 2 O and BQ -h 4 in D 2 O were investigated with the aim to obtain information on the hydrogen bonds between the quinone oxygen and the respective solvent. The observed spectra were analyzed using the GENDOR program for the simulation of orientationally selected powder ENDOR spectra. From the spectral simulations the hyperfine coupling and nuclear quadrupolar coupling tensor components and the angles with respect to the quinone axes were obtained. Hydrogen bond lengths and geometries were deduced from the hyperfine couplings using the point-dipole model and from the nuclear quadrupolar couplings using empirical relationships. The experimental results were compared, and found to be in fair agreement, with those derived from Density Functional Theory (DFT) calculations performed on geometry optimized structures of solute and solvent.

Protein–cofactor interactions in bacterial reaction centers from Rhodobacter sphaeroides R-26: Effect of hydrogen bonding on the electronic and geometric structure of the primary quinone. A density functional theory study

The effect of hydrogen bonding to the primary quinone (Q(A) and Q(*)(-)(A)) in bacterial reaction centers was studied using density functional theory (DFT) calculations. The charge neutral state Q(A) was investigated by optimizing the hydrogen atom positions of model systems extracted from 15 different X-ray structures. From this analysis, mean values of the H-bond lengths and directions were derived. It was found that the N(delta)-H of His M219 forms a shorter H-bond to Q(A) than the N-H of Ala M260. The H-bond of His M219 is linear and more twisted out of the quinone plane. The radical anion Q(*)(-)(A) in the protein environment was investigated by using a mixed quantum mechanics/molecular mechanics (QM/MM) approach. Two geometry optimizations with a different number of flexible atoms were performed. H-bond lengths were obtained and spectroscopic parameters calculated, i.e. the hyperfine and nuclear quadrupole couplings of magnetic nuclei coupled to the radical. Good agreement was found with the results provided by EPR/ENDOR spectroscopy. This implies that the calculated lengths and directions of the H-bonds to Q(*)(-)(A) are reliable values. From a comparison of the neutral and reduced state of Q(A) it was concluded that the H-bond distances are shortened by approximately 0.17 Angstroms (His M219) and approximately 0.13 Angstroms (Ala M260) upon single reduction of the quinone. It is shown that the point-dipole approximation can not be used for an estimation of H-bond lengths from measured hyperfine couplings in a system with out-of-plane H-bonding. In contrast, the evaluation of the nuclear quadrupole couplings of (2)H nuclei substituted in the hydrogen bonds yields H-bond lengths close to the values that were deduced from DFT geometry optimizations. The significance of hydrogen bonding to the quinone cofactors in biological systems is discussed.

A Pentacoordinate Mn(II) Precatalyst That Exhibits Notable Aldehyde and Ketone Hydrosilylation Turnover Frequencies

Heating (THF)2MnCl2 in the presence of the pyridine-substituted bis(imino)pyridine ligand, (PyEt)PDI, allowed preparation of the respective dihalide complex, ((PyEt)PDI)MnCl2. Reduction of this precursor using excess Na/Hg resulted in deprotonation of the chelate methyl groups to yield the bis(enamide)tris(pyridine)-supported product, (κ(5)-N,N,N,N,N-(PyEt)PDEA)Mn. This complex was characterized by single-crystal X-ray diffraction and found to possess an intermediate-spin (S = (3)/2) Mn(II) center by the Evans method and electron paramagnetic resonance spectroscopy. Furthermore, (κ(5)-N,N,N,N,N-(PyEt)PDEA)Mn was determined to be an effective precatalyst for the hydrosilylation of aldehydes and ketones, exhibiting turnover frequencies of up to 2475 min(-1) when employed under solvent-free conditions. This optimization allowed for isolation of the respective alcohols and, in two cases, the partially reacted silyl ethers, PhSiH(OR)2 [R = Cy and CH(Me)((n)Bu)]. The aldehyde hydrosilylation activity observed for (κ(5)-N,N,N,N,N-(PyEt)PDEA)Mn renders it one of the most efficient first-row transition metal catalysts for this transformation reported to date.

Electron−Nuclear and Electron−Electron Double Resonance Spectroscopies Show that the Primary Quinone Acceptor QA in Reaction Centers from Photosynthetic Bacteria Rhodobacter sphaeroides Remains in the Same Orientation Upon Light-Induced Reduction

Reaction centers (RCs) from the photosynthetic bacterium Rhodobacter (Rb.) sphaeroides R-26 exhibit changes in the recombination kinetics of the charge-separated radical-pair state, P(·+) Q(A)(·-), composed of the dimeric bacteriochlorophyll donor P and the ubiquinone-10 acceptor Q(A), depending on whether the RCs are cooled to cryogenic temperatures in the dark or under continuous illumination (Kleinfeld et al. Biochemistry 1984, 23, 5780-5786). Structural changes near redox-active cofactors have been postulated to be responsible for these changes in kinetics and to occur in the course of light-induced oxidation and reduction of the cofactors thereby assuring a high quantum yield. Here we investigated such potential light-induced structural changes, associated with the formation of P(·+) Q(A)(·-), via pulsed electron-nuclear double resonance (ENDOR) at Q-band (34 GHz) and pulsed electron-electron double resonance (PELDOR) at W-band (95 GHz). Two types of light excitation have been employed for which identical RC samples were prepared: (a) one sample was frozen in the dark and then illuminated to generate transient P(·+) Q(A)(·-), and (b) one was frozen under illumination which resulted in both trapped and transient P(·+) Q(A)(·-) at 80 K. The hyperfine interactions between Q(A)(·-) and the protein were found to be the same in RCs frozen in the dark as in RCs frozen under illumination. Furthermore, these interactions are completely consistent with those observed in RC crystals frozen in the dark. Thus, QA remains in its binding site with the same position and orientation upon reduction. This conclusion is consistent with the result of our orientation-resolving PELDOR experiments on transient P(·+) Q(A)(·-) radical pairs. However, these findings are incompatible with the recently proposed ~60° reorientation of Q(A) upon its photoreduction, as deduced from an analysis of Q-band quantum-beat oscillations (Heinen et al. J. Am. Chem. Soc. 2007, 129, 15935-15946). Such a large reorientation appears improbable, and our objections against this proposition are substantiated here in detail. Our results show that Q(A) is initially in an orientation that is favorable for its light-driven reduction. This diminishes the reorganization requirements for fast electron reduction and high quantum efficiency.

Electron-electron double resonance-detected NMR to measure metal hyperfine interactions: 61Ni in the Ni-B state of the [NiFe] hydrogenase of Desulfovibrio vulgaris Miyazaki F.

Electron double resonance-detected NMR (EDNMR) is introduced as a powerful technique to directly measure metal hyperfine interactions in the active sites of metalloproteins. Measurement of these quantities by electron nuclear double resonance is usually difficult because of fast relaxation times and large anisotropic (dipolar) hyperfine couplings. In EDNMR, electron paramagnetic resonance (EPR) “forbidden” transitions are excited, which become EPR allowed to some extent because of the presence of a large dipolar hyperfine and possibly quadrupole interaction. The usefulness of EDNMR is demonstrated with measurements on 61Ni-enriched hydrogenase of D. vulgaris Miyazaki F in the Ni−B state.

Temperature dependence of Q-band electron paramagnetic resonance spectra of nitrosyl heme proteins

The Q-band (35 GHz) electron paramagnetic resonance (EPR) spectra of nitrosyl hemoglobin (HbNO) and nitrosyl myoglobin (MbNO) were studied as a function of temperature between 19 K and 200 K. The spectra of both heme proteins show two classes of variations as a function of temperature. The first one has previously been associated with the existence of two paramagnetic species, one with rhombic and the other with axial symmetry. The second one manifests itself in changes in the g-factors and linewidths of each species. These changes are correlated with the conformational substates model and associate the variations of g-values with changes in the angle of the N(his)-Fe-N(NO) bond in the rhombic species and with changes in the distance between Fe and N of the proximal (F8) histidine in the axial species.

Electronic Structure of a Binuclear Nickel Complex of Relevance to (NiFe) Hydrogenase

The binuclear complex [Ni(2)(L)(MeCN)(2)](3+) (L(2-) = compartmental macrocycle incorporating imine N and thiolate S donors) has a Ni(III) center bridged via two thiolate S-donors to a diamagnetic Ni(II) center. The ground-state has dominant 3d(z)(1)(2) character similar to that observed for [NiFe] hydrogenases in which Ni(III) is bridged via two thiolate donors to a diamagnetic center (Fe(II)). The system has been studied by X-ray crystallography and pulse EPR, ESEEM, and ENDOR spectroscopy in order to determine the extent of spin-delocalization onto the macrocycle L(2-). The hyperfine coupling constants of six nitrogen atoms have been identified and divided into three sets of two equivalent nitrogens. The most strongly coupled nitrogen atoms (a(iso) approximately 53 MHz) stem from axially bound solvent acetonitrile molecules. The two macrocycle nitrogens on the Ni(III) side have a coupling of a(iso) approximately 11 MHz, and those on the Ni(II) side have a coupling of a(iso) approximately 1-2 MHz. Density functional theory (DFT) calculations confirm this assignment, while comparison of the calculated and experimental (14)N hyperfine coupling constants yields a complete picture of the electron-spin density distribution. In total, 91% spin density is found at the Ni(III) of which 72% is in the 3d(z)(2) orbital and 16% in the 3d(xy) orbital. The Ni(II) contains -3.5% spin density, and 7.5% spin density is found at the axial MeCN ligands. In analogy to hydrogenases, it becomes apparent that binding of a substrate to Ni at the axial positions causes a redistribution of the electron charge and spin density, and this redistribution polarizes the chemical bonds of the axial ligand. For [NiFe] hydrogenases this implies that the H(2) bond becomes polarized upon binding of the substrate, which may facilitate its heterolytic splitting.