Steric control in the metal-ligand electron transfer of iminopyridine-ytterbocene complexes

International audienceA systematic study of reactions between Cp*Yb-2(THF) (Cp* = eta(5)-C5Me5, 1) and iminopyridine ligands (IPy = 2,6-(Pr2C6H3N)-Pr-i=CH(C5H3N-R), R = H (2a), 6-C4H3O (2b), 6-C4H3S (2c), 6-C6H5 (2d)) featuring similar electron accepting properties but variable denticity and steric demand, has provided a new example of steric control on the redox chemistry of ytterbocenes. The reaction of the unsubstituted IPy 2a with 1, either in THF or toluene, gives rise to the paramagnetic species Cp*Yb-2(III)(IPy)(center dot-) (3a) as a result of a formal one-electron oxidation of the Yb-II ion along with IPy reduction to a radical-anionic state. The reactions of 1 with substituted iminopyridines 2b-d, bearing aryl or hetero-aryl dangling arms on the 6 position of the pyridine ring occur in a non-coordinating solvent (toluene) only and afford coordination compounds of a formally divalent ytterbium ion, coordinated by neutral IPy ligands Cp*Yb-2(II)(IPy)(0) (3b-d). The X-ray diffraction studies revealed that 2a-c act as bidentate ligands; while the radical-anionic IPy in 3a chelates the Yb-III ion with both nitrogens, neutral IPy ligands in 3b and 3c participate in the metal coordination sphere through the pyridine nitrogen and O or S atoms from the furan or thiophene moieties, respectively. Finally, in complex 3d the neutral IPy ligand formally adopts a monodentate coordination mode. However, an agostic interaction between the Yb-II ion and an ortho C-H bond of the phenyl ring has been detected. Imino-nitrogens in 3b-d are not involved in the metal coordination. Variable temperature magnetic measurements on 3a are consistent with a multiconfigurational ground state of the Yb ion and suggest that the largest contribution arises from the 4f(13)-radical configuration. For complexes 3b and 3c the data of magnetic measurements are indicative of a Yb-II-closed shell ligand electronic distribution. Complex 3d is characterized by a complex magnetic behavior which does not allow for an unambiguous estimation of its electronic structure. The results are rationalized using DFT and CSSCF calculations. Unlike diazabutadiene analogues, 3a does not undergo a solvent mediated metalligand electron transfer and remains paramagnetic in THF solution. On the other hand, complexes 3b-d readily react with THF to afford 1 and free IPy 2b-d

In Situ SERS Sensing by a Laser-Induced Aggregation of Silver Nanoparticles Templated on a Thermoresponsive Polymer

A stimuli-responsive (pH- and thermoresponsive) micelle-forming diblock copolymer, poly(1,2-butadiene) 290 - block -poly( N , N -dimethylaminoethyl methacrylate) 240 (PB- b -PDMAEMA), was used as a polymer template for the in situ synthesis of silver nanoparticles (AgNPs) through Ag + complexation with PDMAEMA blocks, followed by the reduction of the bound Ag + with sodium borohydride. A successful synthesis of the AgNPs on a PB- b -PDMAEMA micellar template was confirmed by means of UV–Vis spectroscopy and transmission electron microscopy, wherein the shape and size of the AgNPs were determined. A phase transition of the polymer matrix in the AgNPs/PB- b -PDMAEMA metallopolymer hybrids, which results from a collapse and aggregation of PDMAEMA blocks, was manifested by changes in the transmittance of their aqueous solutions as a function of temperature. A SERS reporting probe, 4-mercaptophenylboronic acid (4-MPBA), was used to demonstrate a laser-induced enhancement of the SERS signal observed under constant laser irradiation. The local heating of the AgNPs/PB- b -PDMAEMA sample in the laser spot is thought to be responsible for the triggered SERS effect, which is caused by the approaching of AgNPs and the generation of “hot spots” under a thermo-induced collapse and the aggregation of the PDMAEMA blocks of the polymer matrix. The triggered SERS effect depends on the time of a laser exposure and on the concentration of 4-MPBA. Possible mechanisms of the laser-induced heating for the AgNPs/PB- b -PDMAEMA metallopolymer hybrids are discussed

The complete genome sequence of Pantoea ananatis AJ13355, an organism with great biotechnological potential

Pantoea ananatis AJ13355 is a newly identified member of the Enterobacteriaceae family with promising biotechnological applications. This bacterium is able to grow at an acidic pH and is resistant to saturating concentrations of L-glutamic acid, making this organism a suitable host for the production of L-glutamate. In the current study, the complete genomic sequence of P. ananatis AJ13355 was determined. The genome was found to consist of a single circular chromosome consisting of 4,555,536 bp [DDBJ: AP012032] and a circular plasmid, pEA320, of 321,744 bp [DDBJ: AP012033]. After automated annotation, 4,071 protein-coding sequences were identified in the P. ananatis AJ13355 genome. For 4,025 of these genes, functions were assigned based on homologies to known proteins. A high level of nucleotide sequence identity (99%) was revealed between the genome of P. ananatis AJ13355 and the previously published genome of P. ananatis LMG 20103. Short colinear regions, which are identical to DNA sequences in the Escherichia coli MG1655 chromosome, were found to be widely dispersed along the P. ananatis AJ13355 genome. Conjugal gene transfer from E. coli to P. ananatis, mediated by homologous recombination between short identical sequences, was also experimentally demonstrated. The determination of the genome sequence has paved the way for the directed metabolic engineering of P. ananatis to produce biotechnologically relevant compounds

Selective Intermolecular C–H Bond Activation: A Straightforward Synthetic Approach to Heteroalkyl Yttrium Complexes Containing a Bis(pyrazolyl)methyl Ligand

The reactions of bis­(pyrazolyl)­methanes CH₂(C₃HN₂R₂-3,5)₂ (R = Me, <i>t</i>Bu) with Y­(CH₂SiMe₃)₃(THF)₂ and LY­(CH₂SiMe₃)₂­(THF)_<i>n</i> (L = amidopyridinate (Ap′), amidinate (Amd), tridentate amidinate bearing 2-methoxyphenyl pendant in a side arm (Amd^OMe) and pentamethylcyclopentadienyl (Cp*); <i>n</i> = 0, 1) were investigated. CH₂(C₃HN₂<i>t</i>Bu₂-3,5)₂ turned out to be inert in these reactions, while less bulky CH₂(C₃HN₂Me₂-3,5)₂ easily undergoes metalation by yttrium alkyls. The reaction of Y­(CH₂SiMe₃)₃(THF)₂ with CH₂(C₃HN₂Me₂-3,5)₂ regardless of the molar ratio of the reagents affords a homoleptic tris­(alkyl) species, Y­[CH­(C₃HN₂Me₂-3,5)₂]₃ (<b>1</b>). However, the reactions of equimolar amounts of LY­(CH₂SiMe₃)₂(THF)_<i>n</i> and CH₂(C₃HN₂Me₂-3,5)₂ occur selectively with replacement of a sole CH₂SiMe₃ fragment and afford the related heteroalkyl complexes LY­(CH₂SiMe₃)­[CH­(C₃HN₂Me₂-3,5)₂]­(THF)_<i>n</i> (L = Ap′, <i>n</i> = 1 (<b>6</b>); Amd, <i>n</i> = 0 (<b>7</b>); Amd^OMe, <i>n</i> = 1 (<b>8</b>); Cp*, <i>n</i> = 1 (<b>9</b>)) in good yields. The second equivalent of CH₂(C₃HN₂Me₂-3,5)₂ does not react with heteroalkyl yttrium complexes. The X-ray studies revealed that in complexes <b>1</b> and <b>6</b>–<b>9</b> the bis­(pyrazolyl)­methyl ligands are bound to the yttrium centers in a similar fashion via one covalent Y–C and two coordination Y–N bonds. Thermal decomposition of complexes <b>6</b>–<b>9</b> (C₆D₆, 80 °C) as evidenced by ¹H NMR spectroscopy resulted in SiMe₄ elimination, while no activation of the C–H bonds of bis­(pyrazolyl)­methyl ligands was detected. When <b>6</b> was treated with an equimolar amount of PhSiH₃, only the YCH₂SiMe₃ bond selectively underwent σ-bond metathesis and a dimeric yttrium alkyl-hydrido complex, {Ap′Y­[CH­(C₃HN₂Me₂-3,5)₂]­(μ²-H)}₂ (<b>10</b>), was formed. The reaction of <b>6</b> with 2,6-diisopropylaniline also resulted in the selective protonation of the YCH₂SiMe₃ bond and cleanly afforded alkyl-anilido complex Ap′Y­(NHC₆H₃<i>i</i>Pr₂-2,6)­[CH­(C₃HN₂Me₂-3,5)₂]­(THF) (<b>11</b>). The ternary catalytic systems <b>6</b>–<b>9</b>/borate/Al<i>i</i>Bu₃ (borate = [HNMe₂Ph]­[B­(C₆F₅)₄], [Ph₃C]­[B­(C₆F₅)₄]; [Ln]:[borate]:[Al<i>i</i>Bu₃] = 1:1:10) demonstrated moderate catalytic activity in isoprene polymerization; they allow quantitative conversion into polymer of up to 1000 equiv of monomer in 2–4 h. The best activity and 1,4-cis selectivity (83.5%) were demonstrated by amidinato complex <b>8</b>

Reactivity of Ytterbium(II) Hydride. Redox Reactions: Ytterbium(II) vs Hydrido Ligand. Metathesis of the Yb–H Bond

Oxidation reactions of the Yb­(II) hydride [{<i>t</i>BuC­(NC₆H₃-2,6-<i>i</i>Pr₂)₂}­Yb­(μ-H)]₂ (<b>1</b>) with CuCl (1:2 molar ratio) and (PhCH₂S)₂ (1:1 molar ratio) revealed that the hydrido anion in <b>1</b> is a stronger reductant than the Yb­(II) cation. Both reactions occur with evolution of H₂ and afford the dimeric Yb­(II) species [{<i>t</i>BuC­(NC₆H₃-2,6-<i>i</i>Pr₂)₂}­Yb­(μ-X)]₂ (X = Cl (<b>2</b>), SCH₂Ph (<b>3</b>)) in which a κ¹-amido,η⁶-arene type of coordination of amidinate ligand is retained. Reaction of <b>1</b> with 2 equiv of (PhCH₂S)₂ results in oxidation of both Yb­(II) and hydrido centers and leads to the formation of the Yb­(III) complex [{<i>t</i>BuC­(NC₆H₃-2,6-<i>i</i>Pr₂)₂}­Yb­(μ-SCH₂Ph)₂]₂ (<b>4</b>). Complex <b>4</b> can be also synthesized by oxidation of <b>3</b> with an equimolar amount of (PhCH₂S)₂. It was demonstrated that oxidation of the ytterbium center to the trivalent state leads to switching of the coordination mode of amidinate ligand from κ¹-amido, η⁶-arene to “classical” κ¹,κ¹-N,N-chelating. Unlike Yb­(III) bis­(alkyl) species supported by bulky amidopyridinate ligands, the reaction of [{<i>t</i>BuC­(NC₆H₃-2,6-<i>i</i>Pr₂)₂}­Yb­(CH₂SiMe₃)₂(THF)] (<b>6</b>) with PhSiH₃ (1:2 molar ratio) occurs with reduction of ytterbium to a divalent state and affords <b>1</b>. Thus, reduction of Yb­(III) to Yb­(II) leads to a change of coordination mode from κ¹,κ¹-N,N to κ¹-N, η⁶-arene. Oxidation of <b>1</b> by 2,6-<i>i</i>Pr₂C₆H₃NC­(H)­C­(H)NC₆H₃-2,6-<i>i</i>Pr₂ was found to result in oxidation of the hydrido ligand and ytterbium ion and formation of the mixed-valent ion-pair complex [{<i>t</i>BuC­(NC₆H₃-2,6-<i>i</i>Pr₂)₂}­Yb­(DME)₂]⁺[{2,6-<i>i</i>Pr₂C₆H₃NC­(H)C­(H)­NC₆H₃-2,6-<i>i</i>Pr₂}₂Yb]⁻ (<b>5</b>). The σ-bond metathesis reaction of <b>1</b> with Ph₂PH allowed for the synthesis of the first mixed-ligand hydrido–phosphido Yb­(II) species [{<i>t</i>BuC­(NC₆H₃-2,6-<i>i</i>Pr₂)₂}­Yb­(μ-H)­(μ-PPh₂)­Yb­{<i>t</i>BuC­(NC₆H₃-2,6-<i>i</i>Pr₂)₂}] (<b>7</b>). The second hydrido ligand cannot be replaced by a phosphido ligand

Metallacyclic yttrium alkyl and hydrido complexes: synthesis, structures and catalytic activity in intermolecular olefin hydrophosphination and hydroamination

International audienceMetallacyclic neutral and ionic yttrium alkyl complexes coordinated by a dianionic ene-diamido ligand ([2,6-iPr2C6H3NC(Me)[double bond, length as m-dash]C(Me)NC6H3iPr2-2,6] = L1) [L1]Y(CH2SiMe3)(THF)2 (2), {[L1]Y(CH2SiMe3)2}−{Li(THF)4}+ (3), [L1]Y(OEt2)(μ-Me)2Li(TMEDA) (4) were synthesized using a salt-metathesis approach starting from the related chloro complex [L1]Y(THF)2(μ-Cl)2Li(THF)2 (1) in 70, 85 and 72% yields respectively. The reactions of 2 with H2 or PhSiH3 afford the dimeric hydride {[L1]Y(THF)(μ-H)}2(μ-THF) (5) containing two μ-bridging hydrido and one μ-bridging THF ligands (91 and 85% yields). The X-ray studies of complexes 2, 3 and 5 revealed η2-coordination of the C[double bond, length as m-dash]C fragment of an ene-diamido ligand to a Y cation. DFT calculations were carried out to give an insight into the metal–ligand bonding and especially the interaction between the metal and the ene-diamido ligand. The observed bonding of the ene-diamido fragment is found to reflect the acidity of the metal center in the complex that is partially overcome by a better donation from the double bond (better overlap with an empty d orbital at the yttrium center). The treatment of complex 4 with DME resulted in the C–O bond cleavage of DME and afforded a three nuclear methoxide oxide complex [{[L1]Y}3(μ2-OMe)3(μ3-O)]2−[Li(DME)3]+2 (6). Complexes 2, 3, 5 and 7 proved to be efficient precatalysts for the intermolecular hydrophosphination of styrene, 4-vinylpyridine, and 1-nonene with PhPH2 and Ph2PH as well as hydroamination of styrene and pyrrolidine

Adhesion and Proliferation of Mesenchymal Stem Cells on Plasma-Coated Biodegradable Nanofibers

Various biomedical applications of biodegradable nanofibers are a hot topic, as evidenced by the ever-increasing number of publications in this field. However, as-prepared nanofibers suffer from poor cell adhesion, so their surface is often modified. In this work, active polymeric surface layers with different densities of COOH groups from 5.1 to 14.4% were successfully prepared by Ar/CO2/C2H4 plasma polymerization. It has been shown that adhesion and proliferation of mesenchymal stem cells (MSCs) seeded onto plasma-modified PCL nanofibers are controlled by the CO2:C2H4 ratio. At a high CO2:C2H4 ratio, a well-defined network of actin microfilaments is observed in the MSCs. Nanofibers produced at a low CO2:C2H4 ratio showed poor cell adhesion and very poor survival. There were significantly fewer cells on the surface, they had a small spreading area, a poorly developed network of actin filaments, and there were almost no stress fibrils. The maximum percentage of proliferating cells was recorded at a CO2:C2H4 ratio of 35:15 compared with gaseous environments of 25:20 and 20:25 (24.1 ± 1.5; 8.4 ± 0.9, and 4.1 ± 0.4%, respectively). Interestingly, no differences were observed between the number of cells on the untreated surface and the plasma-polymerized surface at CO2:C2H4 = 20:25 (4.9 ± 0.6 and 4.1 ± 0.4, respectively). Thus, Ar/CO2/C2H4 plasma polymerization can be an excellent tool for regulating the viability of MSCs by simply adjusting the CO2:C2H4 ratio

Recoverin Is a Zinc-binding Protein

Recoverin is an N-myristoylated 23 kDa calcium-binding protein from retina, which modulates the Ca2+-sensitive deactivation of rhodopsin via Ca2+-dependent inhibition of rhodopsin kinase. It was shown by intrinsic and bis-ANS probe fluorescence, circular dichroism, and differential scanning calorimetry that myristoylated recombinant recoverin interacts specifically with zinc ions. Similar to the calcium binding, the binding of zinc to Ca2+-loaded recoverin additionally increases its α-helical content, hydrophobic surface area, and environmental mobility/polarity of its tryptophan residues. In contrast to the calcium binding, the binding of zinc decreases thermal stability of the Ca2+-loaded protein. Zn2+-titration of recoverin, traced by bis-ANS fluorescence, reveals binding of a single Zn2+ ion per protein molecule. It was shown that the double-mutant E85Q/E121Q with inactivated Ca2+-binding EF-hands 2 and 3 (Alekseev, A. M.; Shulga-Morskoy, S. V.; Zinchenko, D. V.; Shulga-Morskaya, S. A.; Suchkov, D. V.; Vaganova, S. A.; Senin, I. I.; Zargarov, A. A.; Lipkin, V. M.; Akhtar, M.; Philippov, P. P. FEBS Lett.1998, 440, 116−118), which can be considered as an analogue of the apo-protein, binds Zn2+ ion as well. Apparent zinc equilibrium binding constants evaluated from spectrofluorimetric Zn2+-titrations of the protein are 1.4 × 105 M-1 (dissociation constant 7.1 μM) for Ca2+-loaded wild-type recoverin and 3.3 × 104 M-1 (dissociation constant 30 μM) for the E85Q/E121Q mutant (analogue of apo-recoverin). Study of the binding of wild-type recoverin to ROS membranes showed a zinc-dependent increase of its affinity for the membranes, without regard to calcium content, suggesting further solvation of a protein myristoyl group upon Zn2+ binding. Possible implications of these findings to the functioning of recoverin are discussed

Self-consistency test reveals systematic bias in programs for prediction change of stability upon mutation

Motivation: Computational prediction of the effect of mutations on protein stability is used by researchers in many fields. The utility of the prediction methods is affected by their accuracy and bias. Bias, a systematic shift of the predicted change of stability, has been noted as an issue for several methods, but has not been investigated systematically. Presence of the bias may lead to misleading results especially when exploring the effects of combination of different mutations. Results: Here we use a protocol to measure the bias as a function of the number of introduced mutations. It is based on a self-consistency test of the reciprocity the effect of a mutation. An advantage of the used approach is that it relies solely on crystal structures without experimentally measured stability values. We applied the protocol to four popular algorithms predicting change of protein stability upon mutation, FoldX, Eris, Rosetta and I-Mutant, and found an inherent bias. For one program, FoldX, we manage to substantially reduce the bias using additional relaxation by Modeller. Authors using algorithms for predicting effects of mutations should be aware of the bias described here. Availability and implementation: All calculations were implemented by in-house PERL scripts. Supplementary information: Supplementary data are available at Bioinformatics online.This work was supported by the HHMI International Early Career Scientist Program [55007424], the MINECO [BFU2015-68723-P], Spanish Ministry of Economy and Competitiveness Centro de Excelencia Severo Ochoa 2013-2017 [grant SEV-2012-0208], Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement de la Generalitat’s AGAUR [program 2014 SGR 0974], the European Research Council under the European Union's Seventh Framework Programme [FP7/2007-2013, ERC grant agreement 335980_EinME] and Russian Scientific Foundation (RSF #14-24-00157, the part about I-Mutant calculations). The work was started at the School of Molecular and Theoretical Biology supported by the Dynasty Foundation