Preparation and structural characterization of transition-metal complexes featuring the cymantrenyl(bromo)boryl ligand

In the present paper, we describe the first structural characterization of cymantrenyl(dihalo)borane and report on its use for the synthesis of novel cymantrenylboryl complexes.     

Proteomics study of anthrax lethal toxin-treated murine macrophages

The anthrax lethal toxin (LeTx) is composed of two proteins, protective antigen and lethal factor, which bind and enter the cell through a host receptor termed the anthrax toxin receptor (ATR). In the cell, LeTx targets p38, part of the MAP kinase signaling pathway. The toxin appears to initiate an apoptotic pathway in infected cells, indicating additional downstream targets of the toxin. We have applied a proteomics approach to investigate these downstream targets and the affected processes. In this study we have used an improved strategy for fractionation based on protein pI, off-gel electrophoresis, employed in conjunction with relative quantitation using the mass labeling approach. In our survey, 67 proteins were observed and quantified from the cytosol of RAW 264.7 cells with respect to control versus toxin-treated cells. Many of these proteins are involved in the oxidative stress response, as well as apoptosis, and thus likely to be relevant to the effects of anthrax in infected cells. Our results indicate that the tumor necrosis factor-alpha-mediated pathway is compromised in intoxicated cells. The knowledge of such changes and the pathways leading to the changes should be of great value in understanding and combating this disease.     

Improved efficiency of extraction of polycyclic aromatic hydrocarbons (PAHs) from the National Institute of Standards and Technology (NIST) Standard Reference Material Diesel Particulate Matter (SRM 2975) using accelerated solvent extraction

The efficiency of extraction of polycyclic aromatic hydrocarbons (PAHs) with molecular masses of 252, 276, 278, 300, and 302 Da from standard reference material diesel particulate matter (SRM 2975) has been investigated using accelerated solvent extraction (ASE) with dichloromethane, toluene, methanol, and mixtures of toluene and methanol. Extraction of SRM 2975 using toluene/methanol (9:1, v/v) at maximum instrumental settings (200 °C, 20.7 MPa, and five extraction cycles) with 30-min extraction times resulted in the following elevations of the measured concentration when compared with the certified and reference concentrations reported by the National Institute of Standards and Technology (NIST): benzo[b]fluoranthene, 46%; benzo[k]fluoranthene, 137%; benzo[e]pyrene, 103%; benzo[a]pyrene, 1,570%; perylene, 37%; indeno[1,2,3-cd]pyrene, 41%; benzo[ghi]perylene, 163%; and coronene, 361%. The concentrations of the following PAHs were comparable to the reference values assigned by NIST: indeno[1,2,3-cd]fluoranthene, dibenz[a,h]anthracene, and picene. The measured concentration of dibenzo[a,e]-pyrene was lower than the information value reported by the NIST. The measured concentrations of other highly carcinogenic PAHs (dibenzo[a,l]pyrene, dibenzo[a,i]pyrene, and dibenzo[a,h]pyrene) in SRM 2975 are also reported. Comparison of measurements using the optimized ASE method and using similar conditions to those applied by the NIST for the assignment of PAH concentrations in SRM 2975 indicated that the higher values obtained in the present study were associated with more complete extraction of PAHs from the diesel particulate material. Re-extraction of the particulate samples demonstrated that the deuterated internal standards were more readily recovered than the native PAHs, which may explain the lower values reported by the NIST. The analytical results obtained in the study demonstrated that the efficient extraction of PAHs from SRM 2975 is a critical requirement for the accurate determination of PAHs with high molecular masses in this standard reference material and that the optimization of extraction conditions is essential to avoid underestimation of the PAH concentrations. The requirement is especially relevant to the human carcinogen benzo[a]pyrene, which is commonly used as an indicator of the carcinogenic risk presented by PAH mixtures.     

Exploiting clock transitions for the chemical design of resilient molecular spin qubits

Molecular spin qubits are chemical nanoobjects with promising applications that are so far hampered by the rapid loss of quantum information, a process known as decoherence. A strategy to improve this situation involves employing so-called Clock Transitions (CTs), which arise at anticrossings between spin energy levels. At CTs, the spin states are protected from magnetic noise and present an enhanced quantum coherence. Unfortunately, these optimal points are intrinsically hard to control since their transition energy cannot be tuned by an external magnetic field; moreover, their resilience towards geometric distortions has not yet been analyzed. Here we employ a python-based computational tool for the systematic theoretical analysis and chemical optimization of CTs. We compare three relevant case studies with increasingly complex ground states. First, we start with vanadium(iv)-based spin qubits, where the avoided crossings are controlled by hyperfine interaction and find that these S = 1/2 systems are very promising, in particular in the case of vanadyl complexes in an L-band pulsed EPR setup. Second, we proceed with a study of the effect of symmetry distortions in a holmium polyoxotungstate of formula [Ho(W₅O₁₈)₂]⁹⁻ where CTs had already been experimentally demonstrated. Here we determine the relative importance of the different structural distortions that causes the anticrossings. Third, we study the most complicated case, a polyoxopalladate cube [HoPd₁₂(AsPh)₈O₃₂]⁵⁻ which presents an unusually rich ground spin multiplet. This system allows us to find uniquely favorable CTs that could nevertheless be accessible with standard pulsed EPR equipment (X-band or Q-band) after a suitable chemical distortion to break the perfect cubic symmetry. Since anticrossings and CTs constitute a rich source of physical phenomena in very different kinds of quantum systems, the generalization of this study is expected to have impact not only in molecular spin science but also in other related fields such as molecular photophysics and photochemistry.

We employ a python computational tool to compare 3 relevant case studies with increasingly complex ground states: vanadyl complexes, Ho(iii) square antiprisms and Ho(iii) cubic structures.     

Development of a multi-residue method for the determination of organic micropollutants in water, sediment and mussels using gas chromatography-tandem mass spectrometry

This study describes the development of a multiresidue method based on gas chromatography-electron ionization-tandem mass spectrometry (GC-EI-MS/MS) for the detection of sixteen polycyclic aromatic hydrocarbons (PAHs), five phthalate esters (PEs), seven polychlorinated biphenyls (PCBs), six polybrominated diphenyl ethers (PBDEs), six alkylphenols (APs), three organochlorined pesticides and their isomers or degradation products (OCPs) and bisphenol A in seawater, river water, wastewater treatment plant (WWTP) effluents, sediments and mussels. Solid phase extraction (SPE) was used for the extraction of target analytes in aqueous samples, and ultrasound assisted extraction for solid samples. GC-EI-MS/MS acquisition conditions in selected reaction monitoring (SRM) using two transitions per compound were optimized. In this way, quantification and unequivocal identification of organic micropollutants were performed in compliance with the Decision 2002/657/EC. Good linearity responses with coefficients of determination higher than 0.99 were obtained. Methodological detection limits (MDLs) in seawater ranged from 0.1 to 6 ng L(-1); in river water from 0.1 to 4.8 ng L(-1); in WWTP effluents from 1 to 75 ng L(-1); in sediments from 1 to 150 ng g(-1) and in mussels from 1 to 125 ng g(-1). MDLs and recovery yields were compared with other published methods and similarities or even improvements were achieved. The optimized method was applied to analyze five samples from each matrix collected in coastal areas, showing its potential use for marine pollution monitoring.     

Rapid determination of minority organic acids in honey by high-performance liquid chromatography

A rapid high-performance liquid chromatographic method for the determination of organic acids in honey is reported. Malic, maleic, citric, succinic and fumaric acids were identified and quantified in 15 min. First time repeatibility, reproducibility and recoveries were determined out for these acids in honey samples. Maleic acid was also quantified for first time by a chromatographic method. The organic acids were removed from honey by using a solid-phase extraction procedure with anion-exchange cartridges. Previously, the solution of honey was adjusted to pH 10.50 with 0.1 M NaOH and stirred for 15 min at room temperature. Then, this solution was adjusted to pH 5.00 with 0.1 M H2SO4. This procedure was carried out to avoid interferences in the baseline. The chromatographic separation was achieved with only one Spherisorb ODS-2 S5 column thermostated at 25 degrees C. Metaphosphoric acid (pH 2.20) was used as mobile phase at a flow-rate of 0.7 ml/min. Organic acids were detected with a UV-vis detector (215 nm). The precision results showed that the relative standard deviations of the repeatability and reproducibility were < or =3.20% and < or =4.86%, respectively. The recoveries of the organic acids ranged from 62.9 to 99.4%. Under optimum conditions the detection limits ranged from 0.0064 to 7.57 mg/kg and the quantification limits ranged from 0.025 to 10.93 mg/kg.     

Spectroelectrochemical identity of Prussian blue films in various electrolytes: comparison of time-derivative voltabsorptometric responses with conventional cyclic voltammetry

Using Prussian blue (PB) electrodeposited on gold-covered foil as a model system, we have demonstrated the usefulness of the time-derivative measurements of absorbance versus potential (linear potential-scan voltabsorptometry) for spectroelectrochemical characterization of thin electrochromic films. The time-derivative signals were monitored for PB at 680 and 420 nm in potassium, sodium and lithium electrolytes. Information obtained from cyclic voltabsorptometry is equivalent or complementary to that from conventional cyclic voltammetry. In the case of PB films investigated in lithium electrolyte, the voltabsorptometric time-derivative peaks are better defined than the respective voltammetric peaks. The combination of voltabsorptometry with voltammetry enables molar absorptivity and/or film loading to be determined. Also, concentration changes of differently colored mixed-valence redox centers can be monitored as a function of applied potential. Received: 16 January 1997 / Accepted: 11 March 1997     

Design, synthesis, and photophysical studies of a porphyrin-fullerene dyad with parachute topology; charge recombination in the marcus inverted region

As part of a continuing investigation of the topological control of intramolecular electron transfer (ET) in donor-acceptor systems, a symmetrical parachute-shaped octaethylporphyrin-fullerene dyad has been synthesized. A symmetrical strap, attached to ortho positions of phenyl groups at opposing meso positions of the porphyrin, was linked to [60]-fullerene in the final step of the synthesis. The dyad structures were confirmed by (1)H, (13)C, and (3)He NMR, and MALDI-TOF mass spectra. The free-base and Zn-containing dyads were subjected to extensive spectroscopic, electrochemical and photophysical studies. UV-vis spectra of the dyads are superimposable on the sum of the spectra of appropriate model systems, indicating that there is no significant ground-state electronic interaction between the component chromophores. Molecular modeling studies reveal that the lowest energy conformation of the dyad is not the C(2)(v)() symmetrical structure, but rather one in which the porphyrin moves over to the side of the fullerene sphere, bringing the two pi-systems into close proximity, which enhances van der Waals attractive forces. To account for the NMR data, it is proposed that the dyad is conformationally mobile at room temperature, with the porphyrin swinging back and forth from one side of the fullerene to the other. The extensive fluorescence quenching in both the free base and Zn dyads is associated with an extremely rapid photoinduced electron-transfer process, k(ET) approximately 10(11) s(-)(1), generating porphyrin radical cations and C(60) radical anions, detected by transient absorption spectroscopy. Back electron transfer (BET) is slower than charge separation by up to 2 orders of magnitude in these systems. The BET rate is slower in nonpolar than in polar solvents, indicating that BET occurs in the Marcus inverted region, where the rate decreases as the thermodynamic driving force for BET increases. Transient absorption and singlet molecular oxygen sensitization data show that fullerene triplets are formed only with the free base dyad in toluene, where triplet formation from the charge-separated state is competitive with decay to the ground state. The photophysical properties of the P-C(60) dyads with parachute topology are very similar to those of structurally related rigid pi-stacked P-C(60) dyads, with the exception that there is no detectable charge-transfer absorption in the parachute systems, attributed to their conformational flexibility. It is concluded that charge separation in these hybrid systems occurs through space in unsymmetrical conformations, where the center-to-center distance between the component pi-systems is minimized. Analysis of the BET data using Marcus theory gives reorganization energies for these systems between 0.6 and 0.8 eV and electronic coupling matrix elements between 4.8 and 5.6 cm(-)(1).