Theoretical Insights into the Substitution Effect of Phenanthroline Derivatives on Am(III)/Eu(III) Separation.

Separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) poses a huge challenge in the reprocessing of spent nuclear fuel due to their similar chemical properties. N,N'-Diethyl-N,N'-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen) is a potential ligand for the extraction of An(III) from Ln(III), while there are still few reports on the effect of its substituent including electron-withdrawing and electron-donating groups on An(III)/Ln(III) separation. Herein, the interaction of Et-Tol-DAPhen ligands modified by the electron-withdrawing groups (CF3, Br) and electron-donating groups (OH) with Am(III)/Eu(III) ions was investigated using scalar relativistic density functional theory (DFT). The analyses of bond order, quantum theory of atoms in molecules (QTAIM), and molecular orbital (MO) indicate that the substitution groups have a slight effect on the electronic structures of the [M(L-X)(NO3)3] (X = CF3, Br, OH) complexes. However, the thermodynamic results suggest that a ligand with the electron-donating group (L-OH) improves the extraction ability of metal ions, and the ligand modified by the electron-withdrawing group (L-Br) has the best Am(III)/Eu(III) selectivity. This work could render new insights into understanding the effect of electron-withdrawing and electron-donating groups in tuning the selectivity of Et-Tol-DAPhen derivatives and pave the way for designing new ligands modified by substituted groups with better extraction ability and An(III)/Ln(III) selectivity.

Theoretical insights into the separation of Am(III)/Eu(III): designing ligands based on a preorganization strategy.

Separation of trivalent actinide (An(III)) and lanthanide (Ln(III)) is a worldwide challenge of nuclear waste treatment. Designing ligands with efficient An(III)/Ln(III) separation performance is still one of the key issues for the disposal of accumulated radioactive waste and the recovery of minor actinides. Recently, N-heterocyclic ligands modified with amide groups have shown excellent An(III)/Ln(III) separation performance. The preorganized structure of the ligands has a great impact on the An(III)/Ln(III) separation performance. We theoretically investigated the extraction behaviors of Am(III) and Eu(III) using phenanthroline (L1 and L2) and bipyridine (L3 and L4) based ligands with a completely or partially preorganized structure. The properties of these ligands and their coordination structures, bonding nature and thermodynamic behaviors with the Am(III) and Eu(III) complexes have been systematically studied in a theoretical fashion. The analyses of the bonding nature suggest that the Am-N bonds possess more covalence than the Eu-N bonds. The thermodynamic results indicate that L2 with a completely preorganized structure has the strongest extraction ability and the best Am(III)/Eu(III) selectivity, while L3 with the most flexible skeleton appears to have the weakest extraction ability and the lowest Am(III)/Eu(III) selectivity. And L1 and L4 have similar performances with regard to Am(III)/Eu(III) selectivity. The results suggest that a certain degree of preorganization of the ligand structure can enhance the extraction ability and Am(III)/Eu(III) selectivity. This work provides valuable information for designing efficient ligands for An(III)/Ln(III) separation by the preorganization strategy.

Construction of the Largest Metal-Centered Double-Ring Tubular Boron Clusters Based on Actinide Metal Doping.

Metal doping has been considered to be an effective approach to stabilize various boron clusters. In this work, we constructed a series of largest metal-centered double-ring tubular boron clusters An@B24 (An = Th, Pa, Pu, and Am). Extensive global minimum structural searches combined with density functional theory predicted that the global minima of An@B24 (An = Th, Pu, and Am) are double-ring tubular structures. Formation energy analysis indicates that these boron clusters are highly stable, especially for Th@B24 and Pa@B24. Detailed bonding analysis shows that the significant stability of An@B24 is determined by the covalent character of the An-B bonding, which stems from the interactions of An 5f and 6d orbitals and B 2p orbitals. These results show that actinide metal doping is a feasible route to construct stable large metal-centered double-ring tubular boron clusters, offering the possibility to design boron nanomaterials with special physiochemical properties.

Theoretical probing of twenty-coordinate actinide-centered boron molecular drums.

The exploration of metal-doped boron clusters has a great significance in the design of high coordination number (CN) compounds. Actinide-doped boron clusters are probable candidates for achieving high CNs. In this work, we systematically explored a series of actinide metal atom (U, Np, and Pu) doped B20 boron clusters An@B20 (An = U, Np, and Pu) by global minimum structural searches and density functional theory (DFT). Each An@B20 cluster is confirmed to be a twenty-coordinate complex, which is the highest CN obtained in the chemistry of actinide-doped boron clusters so far. The predicted global minima of An@B20 are tubular structures with actinide atoms as centers, which can be considered as boron molecular drums. In An@B20, U@B20 has a relatively high symmetry of C2, while both Np@B20 and Pu@B20 exhibit C1 symmetry. Extensive bonding analysis demonstrates that An@B20 has σ and π delocalized bonding, and the U-B bonds possess a relatively higher covalency than the Np-B and Pu-B bonds, resulting in the higher formation energy of U@B20. Therefore, the covalent character of An-B bonding may be crucial for the formation of these high CN actinide-centered boron clusters. These results deepen our understanding of actinide metal doped boron clusters and provide new clues for developing stable high CN boron-based nanomaterials.

Double-Layer Nitrogen-Rich Two-Dimensional Anionic Uranyl-Organic Framework for Cation Dye Capture and Catalytic Fixation of Carbon Dioxide.

A novel two-dimensional double-layer anionic uranyl-organic framework, U-TBPCA {[NH2(CH3)2][(UO2)(TBPCA)], where H3TBPCA = 4,4',4″-s-triazine-1,3,5-triyltripamino-methylene-cyclohexane-carboxylate}, with abundant active sites and stability was obtained by assembling UO2(NO3)2·6H2O and a triazine tricarboxylate linker, TBPCA3-. Due to the flexibility of the ligand and diverse coordination modes between carboxyl groups and uranyl ions, U-TBPCA exhibits an intriguing topological structure and steric configuration. This double-layer anionic uranyl-organic framework is highly porous and can be used for selective adsorption of cationic dyes. Due to the presence of high-density metal ions and basic -NH- groups, U-TBPCA acts as an effective heterogeneous catalyst for the cycloaddition reaction of carbon dioxide with epoxy compounds. Moreover, the various modes of coordination between the tricarboxylic ligand and uranyl ion were studied by density functional theory calculations, and several simplified models were established to probe the influence of hydrogen bonding between carbon dioxide and U-TBPCA on the ability of U-TBPCA to bind carbon dioxide. This work should aid in improving our understanding of the coordination behavior of uranyl ion as well as the development and utilization of new actinide materials.

Theoretical insights into selective separation of trivalent actinide and lanthanide by ester and amide ligands based on phenanthroline skeleton.

Phenanthroline based ligands have shown potential performance for partitioning trivalent actinides from lanthanides. In this work, we have explored four ester and amide ligands based on the phenanthroline skeleton and elucidated the separation mechanism between Am(iii) and Eu(iii) ions. The molecular geometries and extraction reactions of the metal-ligand complexes were modeled by using scalar-relativistic density functional theory. The results show that the amide based ligands have stronger coordination ability with the metal ions than the corresponding ester based ligands. According to the thermodynamic results, ligands N,N'-diethyl-N,N'-ditolyl-2,9-diamide-1,10-phenanthroline (L2) and N,N'-(1,10-phenanthroline-2,9-diyl)bis(N-ethyl-P-methyl-N-(p-tolyl)phosphinic amide) (L4) appear to have the strongest complexing ability, which is supported by the result of electrostatic potential (ESP) and the M-OL bond orders. Moreover, ligand L2 has excellent selectivity for Am(iii)/Eu(iii) among the four ligands. Additionally, the bonding properties between the metal ions and the ligands reveal that the Am(iii)/Eu(iii) selectivity stems from the Am-N bonds with more covalent character, which is supported by the analysis of the hardness of the ligands and the bond orders. This work provides useful information for understanding the Am(iii)/Eu(iii) selectivity of phenanthroline derived ligands bearing ester and amide groups.

Theoretical Insights into Modification of Nitrogen-Donor Ligands to Improve Performance on Am(III)/Eu(III) Separation.

Nitrogen-donor ligands have been considered to be promising agents for separating trivalent actinides (An(III)) from lanthanides (Ln(III)). Thereinto, how to decorate these ligands for better extraction performance is urgent to design "perfect" separating extractants. In this work, we systematically explored a series of heterocyclic N-donor ligands (L1 = dipyridazino[4,3-c:3',4'-h]acridine, L2 = dipyridazino[3,4-a:4',3'-j]phenazine, L3 = 2,6-di(cinnolin-3-yl)pyridine)), as well as their substituted derivatives, and compared their extraction and complexation ability toward An(III) and Ln(III) ions by using quasi-relativistic density functional theory (DFT). We found that the pyridazine N atoms probably play a notable role in electron donation to metal cations by molecular orbital (MO) and bond order analyses. Besides, the calculated results clearly verified that these N-donor ligands possess higher coordination affinity toward Am(III) over Eu(III). The rigid ligands (L1 and L2) exhibit higher selective abilities for the Am(III)/Eu(III) separation compared with that of the flexible ligand (L3). For each ligand, the 1:2 (metal/ligand) extraction reaction is predicted to be most probable in the separation process. The introduction of an alkyl group on the lateral chain or an electron-donating group on the main chain gives rise to a better extraction performance of the ligands, and the CyMe4 or MeO substituted ligands show higher extraction and separation ability. Simultaneous introduction of CyMe4 and MeO groups can enhance the extraction ability of the ligand to metal ions, but the separating ability depends on the differences of the extraction capacity of An(III) and Ln(III). This work can help to gain a more in-depth understanding the selectivity differences of similar N-donor ligands and provide more theoretical insights into the design of novel extractants for An(III)/Ln(III) separation.

Theoretical Insights into the Selective Extraction of Americium(III) over Europium(III) with Dithioamide-Based Ligands.

Separation of trivalent actinides An(III) from lanthanides Ln(III) is a worldwide challenge owing to their very similar chemical behaviors. It is highly desirable to understand the nature of selectivity for the An(III)/Ln(III) separation with various ligands through theoretical calculations because of their radiotoxicity and experimental difficulties. In this work, we have investigated three dithioamide-based ligands and their extraction behaviors with Am(III) and Eu(III) ions using the scalar-relativistic density functional theory. The results show that the dithioamide-based ligands have stronger electron donating ability than do the corresponding diamide-based ones. All analyses including geometry, Mulliken population, QTAIM (quantum theory of atoms in molecules), and NBO (natural bond orbital) suggest that the Am-S/N bonds possess more covalency compared to the Eu-S/N bonds, and the M-S bonds have more covalent character than the M-N bonds. Thermodynamic results reveal that N2,N9-diethyl-N2,N9-di-p-tolyl-1,10-phenanthroline-2,9-bis(carbothioamide) (L1) has a stronger complexing ability with metal ions owing to its rigid structure and that N6,N6'-diethyl-N6,N6'-di-p-tolyl-[2,2'-bipyridine]-6,6'-bis(carbothioamide) (L2) shows a higher selectivity for the Am(III)/Eu(III) separation. In addition, these dithioamide-based ligands possess Am(III)/Eu(III) selectivity higher than those of the corresponding diamide-based ones, although the former have weaker complexing ability with metal ions, probably due to the greater covalency of the M-S bonds. This theoretical evaluation provides valuable insights into the nature of the selectivity for the Am(III)/Eu(III) separation and information on designing of efficient An(III)/Ln(III) separation with dithioamide-based ligands.

Metal-Carboxyl Helical Chain Secondary Units Supported Ion-Exchangeable Anionic Uranyl-Organic Framework.

As a less explored avenue, actinide-based metal-organic frameworks (MOFs) are worth studying for the particularity of actinide nodes in coordination behaviour and assembly modes. In this work, an azobenzenetetracarboxylate-based anionic MOF supported by uranyl-carboxyl helical chain units was synthesized, incorporating linear uranyl as the metal centre. This kind of helical chain-type building unit is reported for the first time in uranyl-based MOFs. Structural analysis reveals that the formation of helical chain secondary units can be attributed to restricted equatorial coordination of rigid flat azobenzene ligand to uranyl centres. Meanwhile, this newly-synthesized anionic material has been used to remove Eu3+ ions, as a non-radioactive surrogate of Am3+ ion, through an ion-exchange process with [(CH3 )2 NH2 ]+ ions in its open channels, as evidenced by a combination of 1 H NMR spectroscopy, EDS and PXRD.

Theoretical Insights into Preorganized Pyridylpyrazole-Based Ligands toward the Separation of Am(III)/Eu(III).

Pyridylpyrazole ligands have shown excellent competence for partitioning actinides from lanthanides. As far as we know, the preorganization structure of the ligand has a great impact on the extraction separation ability. However, the mechanism that works well for some ligands but fails for others needs to be clearly elucidated. In this work, we designed three various pyridylpyrazole ligands, BPP, BPBP, and BPPhen, and further preorganized one or both side pyrazole rings of these ligands. The properties of these ligands and the coordination structures, bonding nature and thermodynamic behaviors of the related Am(III) and Eu(III) complexes have been systematically studied in a theoretical fashion. All analyses of geometries, charge transfer, QTAIM (quantum theory of atoms in molecules) and NBO (natural bond orbital) suggest that the Am-N bonds possess more covalence compared to that of Eu-N bonds. According to the thermodynamic results, increasing the rigidity of the bridging skeleton rather than the side chain can enhance the extraction ability and Am(III)/Eu(III) selectivity of the ligand. This work may identify the reasonability of a useful approach on achieving highly efficient Am(III)/Eu(III) separation through tuning the preorganization level of the ligand and further provide meaningful theoretical basis on the input of preorganization toward ligand design and screening.

Uranyl-Organic Coordination Compounds Incorporating Photoactive Vinylpyridine Moieties: Synthesis, Structural Characterization, and Light-Induced Fluorescence Attenuation.

The fluorescence of uranyl originated from electronic transitions (S11-S00 and S10-S0v, v = 0-4) of the ligand-to-metal charge transfer (LMCT) process is an intrinsic property of many uranyl coordination compounds. However, light-induced regulation on fluorescence features of uranyl hybrid materials through photoactive functional groups is less investigated. In this work, the photoactive vinyl group-containing ligands, ( E)-methyl 3-(pyridin-4-yl)acrylate and ( E)-methyl 3-(pyridin-3-yl)acrylate, have been used in the construction of uranyl coordination polymers in the presence of 1,10-phenanthroline (phen). Five compounds (UO2)3(µ3-O)(µ2-OH)2(L1)2( phen)2(1), (UO2)3(µ3-O)(µ2-OH)3(L1)( phen)2 (2), (UO2)3(µ3-O)(µ2-OH)3(L2)( phen)2 (3), [(UO2)2(µ2-OH)2(L2)2( phen)2]·2H2O (4), and (UO2)Zn(SO4)(phen)(H2O)(OH)2(5) were obtained under hydrothermal conditions. Compounds 1-4 are polynuclear uranyl structures with abundant π-π interactions and hydrogen bonds contributed to the 3D crystal packing of them. As model compounds, 1 and 3 are selected for exploring photoresponsive behaviors. The emission intensities of these two compounds are found to decrease gradually over the exposure time of UV irradiation. X-ray single crystal structural analysis suggests that the fluorescence attenuation can be explained by the slight rotation of pyridinyl groups around the carbon-carbon double bond during UV irradiation, which is accompanied by the change of weak interactions, i.e., π-π interactions and hydrogen bonds in strength and density. This feature of light-induced fluorescence attenuation may enable these two compounds to act as potential photoresponsive sensor materials.

Insight into the Extraction Mechanism of Americium(III) over Europium(III) with Pyridylpyrazole: A Relativistic Quantum Chemistry Study.

Separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) is one of the most important steps in spent nuclear fuel reprocessing. However, it is very difficult and challenging to separate them due to their similar chemical properties. Recently the pyridylpyrazole ligand (PypzH) has been identified to show good separation ability toward Am(III) over Eu(III). In this work, to explore the Am(III)/Eu(III) separation mechanism of PypzH at the molecular level, the geometrical structures, bonding nature, and thermodynamic behaviors of the Am(III) and Eu(III) complexes with PypzH ligands modified by alkyl chains (Cn-PypzH, n = 2, 4, 8) have been systematically investigated using scalar relativistic density functional theory (DFT). According to the NBO (natural bonding orbital) and QTAIM (quantum theory of atoms in molecules) analyses, the M-N bonds exhibit a certain degree of covalent character, and more covalency appears in Am-N bonds compared to Eu-N bonds. Thermodynamic analyses suggest that the 1:1 extraction reaction, [M(NO3)(H2O)6]2+ + PypzH + 2NO3- → M(PypzH)(NO3)3(H2O) + 5H2O, is the most suitable for Am(III)/Eu(III) separation. Furthermore, the extraction ability and the Am(III)/Eu(III) selectivity of the ligand PypzH is indeed enhanced by adding alkyl-substituted chains in agreement with experimental observations. Besides this, the nitrogen atom of pyrazole ring plays a more significant role in the extraction reactions related to Am(III)/Eu(III) separation compared to that of pyridine ring. This work could identify the mechanism of the Am(III)/Eu(III) selectivity of the ligand PypzH and provide valuable theoretical information for achieving an efficient Am(III)/Eu(III) separation process for spent nuclear fuel reprocessing.

Regulating the nitrite reductase activity of myoglobin by redesigning the heme active center.

Heme proteins perform diverse functions in living systems, of which nitrite reductase (NIR) activity receives much attention recently. In this study, to better understand the structural elements responsible for the NIR activity, we used myoglobin (Mb) as a model heme protein and redesigned the heme active center, by introducing one or two distal histidines, and by creating a channel to the heme center with removal of the native distal His64 gate (His to Ala mutation). UV-Vis kinetic studies, combined with EPR studies, showed that a single distal histidine with a suitable position to the heme iron, i.e., His43, is crucial for nitrite (NO2(-)) to nitric oxide (NO) reduction. Moreover, creation of a water channel to the heme center significantly enhanced the NIR activity compared to the corresponding mutant without the channel. In addition, X-ray crystallographic studies of F43H/H64A Mb and its complexes with NO2(-) or NO revealed a unique hydrogen-bonding network in the heme active center, as well as unique substrate and product binding models, providing valuable structural information for the enhanced NIR activity. These findings enriched our understanding of the structure and NIR activity relationship of heme proteins. The approach of creating a channel in this study is also useful for rational design of other functional heme proteins.

An intramolecular disulfide bond designed in myoglobin fine-tunes both protein structure and peroxidase activity.

Disulfide bond plays crucial roles in stabilization of protein structure and in fine-tuning protein functions. To explore an approach for rational heme protein design, we herein rationally introduced a pair of cysteines (F46C/M55C) into the scaffold of myoglobin (Mb), mimicking those in native neuroglobin. Molecular modeling suggested that it is possible for Cys46 and Cys55 to form an intramolecular disulfide bond, which was confirmed experimentally by ESI-MS analysis, DTNB reaction and CD spectrum. Moreover, it was shown that the spontaneously formed disulfide bond of Cys46-Cys55 fine-tunes not only the heme active site structure, but also the protein functions. The substitution of Phe46 with Ser46 in F46S Mb destabilizes the protein while facilitates H2O2 activation. Remarkably, the formation of an intramolecular disulfide bond of Cys46-Cys55 in F46C/M55C Mb improves the protein stability and regulates the heme site to be more favorable for substrate binding, resulting in enhanced peroxidase activity. This study provides valuable information of structure-function relationship for heme proteins regulated by an intramolecular disulfide bond, and also suggests that construction of such a covalent bond is useful for design of functional heme proteins.

How a novel tyrosine-heme cross-link fine-tunes the structure and functions of heme proteins: a direct comparitive study of L29H/F43Y myoglobin.

A heme-protein cross-link is a key post-translational modification (PTM) of heme proteins. Meanwhile, the structural and functional consequences of heme-protein cross-links are not fully understood, due to limited studies on a direct comparison of the same protein with and without the cross-link. A Tyr-heme cross-link with a C-O bond is a newly discovered PTM of heme proteins, and is spontaneously formed in F43Y myoglobin (Mb) between the Tyr hydroxyl group and the heme 4-vinyl group in vivo. In this study, we found that with an additional distal His29 introduced in the heme pocket, the double mutant L29H/F43Y Mb can form two distinct forms under different protein purification conditions, with and without a novel Tyr-heme cross-link. By solving the X-ray structures of both forms of L29H/F43Y Mb and performing spectroscopic studies, we made a direct structural and functional comparison in the same protein scaffold. It revealed that the Tyr-heme cross-link regulates the heme distal hydrogen-bonding network, and fine-tunes not only the spectroscopic and ligand binding properties, but also the protein reactivity. Moreover, the formation of the Tyr-heme cross-link in the double mutant L29H/F43Y Mb was investigated in vitro. This study addressed the key issue of how Tyr-heme cross-link fine-tunes the structure and functions of the heme protein, and provided a plausible mechanism for the formation of the newly discovered Tyr-heme cross-link.

Theoretical studies on the AnO2(n+) (An = U, Np; n = 1, 2) complexes with di-(2-ethylhexyl)phosphoric acid.

Actinide separation in spent nuclear fuel reprocessing is essential for the closed nuclear fuel cycle. Organophosphorus reagents have been found to exhibit strong affinities for actinides in experiments. In this work, the extraction complexes of AnO2(n+) (An = U, Np; n = 1, 2) with the traditional organophosphorus ligand HDEHP (di-(2-ethylhexyl)phosphoric acid) have been investigated using density functional theory together with scalar-relativistic effective core potentials (ECPs) for actinide elements. According to our calculations, the HDEHP dimer prefers to act as a bidentate ligand in most of the studied complexes. HDEHP ligands show a higher extraction ability for An(vi) over An(v), and the formation of Np(vi) complexes is slightly more favorable than those of U(vi) analogues, which is mainly attributed to the stronger donor-acceptor interaction in Np(vi) complexes. The intramolecular hydrogen bonds play a significant role in the stability of the 1 : 1 type complexes AnO2(HL)2(NO3)2 (L = DEHP(-)). Moreover, AnO2(HL)2(NO3)2 are the most stable species in nitrate-rich acid solutions, while at low nitric acid concentrations, the complexing reaction of AnO2(H2O)5(2+) + 2(HL)2 → AnO2(HL2)2 + 2H(+) + 5H2O is probably the dominant reaction in the extraction process. Our results can help to understand the speciation of actinyl complexes in real solvent extraction of actinides with HDEHP at the molecular level.

Computational insight into nitration of human myoglobin.

Protein nitration is an important post-translational modification regulating protein structure and function, especially for heme proteins. Myoglobin (Mb) is an ideal protein model for investigating the structure and function relationship of heme proteins. With limited structural information available for nitrated heme proteins from experiments, we herein performed a molecular dynamics study of human Mb with successive nitration of Tyr103, Tyr146, Trp7 and Trp14. We made a detailed comparison of protein motions, intramolecular contacts and internal cavities of nitrated Mbs with that of native Mb. It showed that although nitration of both Tyr103 and Tyr146 slightly alters the local conformation of heme active site, further nitration of both Trp7 and Trp14 shifts helix A apart from the rest of protein, which results in altered internal cavities and forms a water channel, representing an initial stage of Mb unfolding. The computational study provides an insight into the nitration of heme proteins at an atomic level, which is valuable for understanding the structure and function relationship of heme proteins in non-native states by nitration.

A spectroscopic study of uranyl-cytochrome b5/cytochrome c interactions.

Uranium is harmful to human health due to its radiation damage and the ability of uranyl ion (UO2(2+)) to interact with various proteins and disturb their biological functions. Cytochrome b5 (cyt b5) is a highly negatively charged heme protein and plays a key role in mediating cytochrome c (cyt c) signaling in apoptosis by forming a dynamic cyt b5-cyt c complex. In previous molecular modeling study in combination with UV-Vis studies, we found that UO2(2+) is capable of binding to cyt b5 at surface residues, Glu37 and Glu43. In this study, we further investigated the structural consequences of cyt b5 and cyt c, as well as cyt b5-cyt c complex, upon uranyl binding, by fluorescence spectroscopic and circular dichroism techniques. Moreover, we proposed a uranyl binding site for cyt c at surface residues, Glu66 and Glu69, by performing a molecular modeling study. It was shown that uranyl binds to cyt b5 (KD=10 µM), cyt c (KD=87 µM), and cyt b5-cyt c complex (KD=30 µM) with a different affinity, which slightly alters the protein conformation and disturbs the interaction of cyt b5-cyt c complex. Additionally, we investigated the functional consequences of uranyl binding to the protein surface, which decreases the inherent peroxidase activity of cyt c. The information of uranyl-cyt b5/cyt c interactions gained in this study likely provides a clue for the mechanism of uranyl toxicity.

Determination of chloramphenicol residues in aquatic products using immunoaffinity column cleanup and high performance liquid chromatography with ultraviolet detection.

A method based on HPLC with UV detection was developed for the quantitative determination of chloramphenicol (CAP) residues in aquatic products. The samples were extracted with ethyl acetate-ammonium hydroxide (98 + 2, v/v), followed by a cleanup step using an immunoaffinity column. The analytes were determined by HPLC-UV. Optimal conditions for the extraction and cleanup procedures are described. The linear regression equation was y = 91.47x - 8.60 with R = 0.9998 (y = peak area and x = CAP concentration) and showed a good reproducibility. The LOQ was 0.25 microg/kg for determining CAP spiked in the aquatic products. The mean recoveries of CAP from fish and shrimp samples fortified at 0.25-1.0 microg/kg were 88.7-93.1 and 92.0-97.3%, respectively; the repeatability RSDs were less than 8.1%. It was concluded that the method is simple, highly sensitive, and low cost for quantitatively measuring CAP residues in aquatic products. Analyte identification was confirmed by HPLC/MSIMS analysis.

Rational design of a nitrite reductase based on myoglobin: a molecular modeling and dynamics simulation study.

Myoglobin (Mb) is an ideal scaffold protein for rational protein design mimicking native enzymes. We recently designed a nitrite reductase (NiR) based on sperm whale Mb by introducing an additional distal histidine (Leu29 to His29 mutation) and generating a distal tyrosine (Phe43 to Tyr43 mutation) in the heme pocket, namely L29H/F43Y Mb, to mimic the active site of cytochrome cd (1) NiR from Ps. aeruginosa that contains two distal histidines and one distal tyrosine. The molecular modeling and dynamics simulation study herein revealed that L29H/F43Y Mb has the necessary structural features of native cytochrome cd (1) NiR and can provide comparable interactions with nitrite as in native NiRs, which provides rationality for the protein design and guides the protein engineering. Additionally, the present study provides an insight into the relatively low NiR activity of Mb in biological systems.