Equivalent Monomer Research Articles

Influences of solvents and monomer concentrations on the electrochemical performance and structural properties of electrodeposited PEDOT films: a comparative study in water and acetonitrile.

Poly(3,4-ethylenedioxythiophene) (PEDOT) has emerged as a promising coating for neural electrodes especially through convenient electrodeposition methods. To investigate the influences of solvents and EDOT monomer concentrations on the electrochemical performance and structural characteristics of PEDOT, both aqueous and acetonitrile solutions were employed with varying monomer concentrations during deposition. The prepared PEDOT films were examined for the surface morphology, electrochemical performance, and chemical structures. The results showed that an increase in EDOT concentration in either solvent led to PEDOT films with improved charge storage capacity and reduced impedance magnitude. At equivalent monomer concentrations, PEDOT films generated in acetonitrile exhibited a rougher surface texture and better electrochemical performance. Notably, the growth rate of charge storage capacity of PEDOT prepared in acetonitrile relative to the deposited charge density was 2.5 times that of PEDOT prepared in water. These findings could help to the optimization of PEDOT coating preparation to enhance electrode performance.

Ring-Opening Polymerization of rac-β-Butyrolactone Promoted by New Tetradentate Thioether-Amide Ligand-Type Zinc Complexes.

In this work, thioether-amide ligands featuring a combination of hard amide groups with soft donor groups have been employed to develop new zinc catalysts for the ring-opening polymerization of cyclic esters. All complexes were prepared in high yields through alkane elimination reactions with diethyl zinc and characterized using nuclear magnetic resonance (NMR) spectroscopy. Density functional theory (DFT) characterization provided insight into the parameters that influence catalytic activity, such as steric hindrance at the metal center, Lewis acidity and electronic density of thioether-amide ligands. In the presence of one equivalent of isopropanol, all complexes were active in the ring-opening polymerization of rac-β-butyrolactone. Quantitative conversion of 100 monomer equivalents was achieved within 1 h at 80 °C in a toluene solution. Number-average molecular weights increased linearly with monomer conversion; the values were in optimal agreement with those expected, and polydispersity index values were narrow and relatively constant throughout the course of polymerization. The most active complex was also effective in the ring-opening polymerization of ε-caprolactone and L-lactide. To propose a reliable reaction path, DFT calculations were undertaken. In the first step of the reaction, the acidic proton of the alcohol is transferred to the basic nitrogen atom of the amide ligand coordinated to the zinc ion. This leads to the alcoholysis of the Zn-N bond and the formation of an alcoholate derivative that starts the polymerization. In subsequent steps, the reaction follows the classical coordination-insertion mechanism.

Exploring pulse mitigation in heterogeneous granular metamaterials

Recognizing the need for effective control of vibration and sound propagation in various industries, this study investigates the potential of designing heterogeneous granular networks for vibroacoustic transmission mitigation. It introduces new models of granular systems: decorated, stepped, and tapered 2-level branching structures. The research assesses changes in particle size (5–10 mm radii) and material properties (density and Young's Modulus) to create finely-tuned composites that significantly modulate pulse waves. The discrete element method predicts wave propagation in these granular metamaterials, comparing monodispersed chains, conventional chain networks, and the proposed heterogeneous structures. Their pulse diffusion capacity is evaluated, showing how collective responses can be adjusted by altering physical parameters like particle size and composition. Preliminary findings underscore the utility of these configurations in advancing the development of elastic and acoustic metamaterials, demonstrating a peak amplitude reduction more than five times greater than an equivalent monomer system. With versatility across a wide frequency range, these metamaterials could pioneer a new direction in impact mitigation.

Rare‐Earth Amido and Borohydrido Complexes Supported by Tetradentate Amidinate Ligands: Synthesis, Structure, and Catalytic Activity in Polymerization of Cyclic Esters

Amido [ButC(NC6H4‐2‐OMe)2]2LnN(SiMe3)2 (Ln = Y (3), Nd (4)) and borohydrido complexes [ButC(NC6H4‐2‐OMe)2]2LnBH4 (Ln = Y (5), Nd (6)) coordinated by new amidinate ligand bearing two pendant anisolyl groups are synthesized. According to X‐ray analysis in complexes 3 and 4 one amidinate ligand is coordinated to the Ln3+ cation in a bidentate fashion (κ2‐NN), while the second one is tetradentate (κ4‐NNOO). At the same time in borohydrido neodymium complex 6 the amidinate ligands demonstrate κ4‐NNOO and κ3‐NNO coordination modes. Complexes 3–6 proved to be efficient initiators for ring‐opening polymerization of rac‐lactide and allow for achieving quantitative conversion of 500 equivalents of monomer into polymer in 0.3–4.5 h at 25 °C. The polymerization of ε‐caprolactone initiated by 3–6 proceeds much faster under similar conditions.

Carbohydrate-Based Polymeric Surfactants for Chiral Micellar Electrokinetic Chromatography (CMEKC) Coupled to Mass Spectrometry.

Polymeric surfactants (molecular micelles, MoMs) with a variety of chiral head groups and chain lengths may be the most promising chiral selectors when used for sensitive detection of chiral compounds in micellar electrokinetic chromatography-mass spectrometry (MEKC-MS). Various carbohydrate-based MS compatible surfactants with phosphate and sulfate head groups have been recently synthesized in our laboratory for its application in CMEKC-MS. In this chapter, we illustrate that the synthesized glucopyranoside-based MoMs are fully compatible with electrospray ionization MS and can be successfully used as a chiral selector for high-throughput screening of multiple chiral compounds using MRM mode in CMEKC-MS/MS experiments. This chapter describes in detail synthesis and utility of α- and β-glucopyranoside-based polymeric surfactant with two different chain lengths and head groups. The presented examples optimize the effect of appropriate millimolar concentration of monomer sugar surfactants required for polymerization as it affects the separations of acidic and basic compounds. Under the optimized concentration of the monomer needed for polymerization (i.e., an equivalent monomer concentration of MoMs), the superiority of MEKC-MS over MEKC-UV is evident. Structurally similar basic drugs with the difference in hydrophobicity are first tested in MEKC-MS to find the optimum head group and optimum chain length with the aim for developing a widely applicable polymeric glucopyranoside-based surfactant. The partial enantioresolution of several structurally similar basic compounds is significantly improved when switching from one head group to another head group of the glucopyranoside MoMs. Thus, complementary separations using poly-N-β-D-SUGP versus poly-N-β-D-SUGS were seen. This phenomenon also exists when comparing the MoMs, which differ in an anomeric configuration such as poly-N-α-D-SUGP and poly-N-β-D-SUGP.

New quaternized poly(4-vinylpyridine-co-divinylbenzene) material containing nickel(II) Schiff base complex: synthesis, thermogravimetry, and application for heterogeneous electrooxidation of ethanol

Poly(4-vinylpyridine) crosslinked by 2 % divinylbenzene was partially quaternized (20 % monomer equivalents) by a bidentate Schiff base nickel complex containing C–Br function in its molecular structure using dioxane as solvent. The resulting poly(4-vinylpyridine-co-divinylbenzene)-Ni(II) Schiff base complex (PVP-co-DVB-Ni(II)-L2) was characterized using various techniques such as elemental analysis and Fourier-transform infrared (FT-IR) spectroscopy. Thermogravimetric (TG)/differential thermogravimetry (DTG) analysis of the nonmodified copolymer, free complex, and the copolymer modified with the nickel complex was carried out to elucidate the thermal behavior. The synthesized copolymer covalently grafted with the nickel(II) complex was mixed with carbon black paste to prepare a new modified electrode material. This electrode was characterized by wide-angle X-ray diffraction analysis, scanning electron microscopy/energy-dispersive X-ray spectroscopy, and cyclic voltammetry. The electrochemical properties of the resulting modified electrode, recorded in alkaline solution, were investigated by cyclic voltammetry, revealing a well-defined redox process corresponding to the NiIII/NiII redox couple. The effects of parameters such as the scan rate and ethanol concentration were explored for electrooxidation of ethanol using this modified electrode. A mechanism for the electrooxidation of ethanol is suggested based on the cyclic voltammetry results.

Establishing the Transient Mass Balance of Thrombosis: From Tissue Factor to Thrombin to Fibrin Under Venous Flow.

We investigated the coregulation of thrombin and fibrin as blood flows over a procoagulant surface. Using microfluidic perfusion of factor XIIa-inhibited human whole blood (200 s-1 wall shear rate) over a 250-μm long patch of collagen/TF (tissue factor; ≈1 molecule per μm2) and immunoassays of the effluent for F1.2 (prothrombin fragment 1.2), TAT (thrombin-antithrombin complex), and D-dimer (post-end point plasmin digest), we sought to establish the transient mass balance for clotting under venous flow. F1.2 (but almost no free thrombin detected via TAT assay) continually eluted from clots when fibrin was allowed to form. Low-dose fluorescein-Phe-Pro-Arg-chloromethylketone stained fibrin-bound thrombin-a staining ablated by anti-γ'-fibrinogen or the fibrin inhibitor glypro-arg-pro but highly resistant to 7-minute buffer rinse, demonstrating tight binding of thrombin to γ'-fibrin. With fibrin polymerizing for 500 seconds, 92 000 thrombin molecules and 203 000 clot-associated fibrin monomer equivalents were generated per TF molecule (or per μm2). Fibrin reached 15 mg/mL in the pore space (porosity ≈0.5) of a 15-μm-thick thrombus core by 500 seconds and 30 mg/mL by 800 seconds. For a known rate of ≈60 FPA (fibrinopeptide-A) per thrombin per second, each thrombin molecule generated only 3 fibrin monomer equivalents during 500 seconds, indicating an intraclot thrombin half-life of ≈70 ms, much shorter than its diffusional escape time (≈10 seconds). By 800 seconds, gly-pro-arg-pro allowed 4-fold more F1.2 generation, consistent with gly-pro-arg-pro ablating fibrin's antithrombin-I activity and facilitating thrombin-mediated FXIa activation. Under flow, fibrinogen continually penetrates the clot, and γ'-fibrin regulates thrombin.

Paper-based electrochemiluminescence sensor for highly sensitive detection of amyloid-β oligomerization: Toward potential diagnosis of Alzheimer's disease.

Development of a rapid and sensitive method for Aβ(1-42) aggregation detection is of great importance to overcome the limitations of conventional techniques. In this study, we developed a label-free paper-based electrochemiluminescence sensor for amyloid-β aggregation detection toward potential diagnosis of Alzheimer's disease (AD). The paper-based chip used in the system serves as a low-cost and disposable detection method. In this detection platform, the bonding of [Ru(phen)2dppz]2+ to Aβ(1-42) aggregates results in enhanced electrochemiluminescence due to the change in the polarity of the microenvironment when [Ru(phen)2dppz]2+ intercalated into the β-sheets during oligomerization. The oligomerization process of Aβ(1-42) can be monitored in real time by the novel method, and as low as 100 pM equivalent monomer concentration of Aβ(1-42) could be detected simultaneously. In addition, the cerebrospinal fluid of transgenic AD model mice was tested by this method, which is highly consistent with genetic identification. In addition, we demonstrated that this detection platform could be a potential new method for the screening of Aβ(1-42) aggregation inhibitors, highlighting the practical application capacity of this platform. The platform is label free, low cost and sensitive. Therefore, the proposed platform holds great promise for the diagnosis of AD.

Recent Advances in Ring-Opening Polymerization of O-Carboxyanhydrides

Poly(α-hydroxy acids) are important biodegradable polymers with wide applications. Recently O-carboxyanhydrides (OCAs) have emerged as promising monomer equivalents of lactides to synthesize poly(α-hydroxy acids). We will highlight recent advances in controlled ring-opening polymerization of OCAs catalyzed by organocatalysts, enzymes, or organometallic complexes.1 Introduction2 Organocatalysts for O-Carboxyanhydride Polymerization2.1 Synthesis of O-Carboxyanhydride Monomers2.2 Ring-Opening Polymerization of O-Carboxyanhydrides Catalyzed by 4-Dimethylaminopyridine2.3 Epimerization in the Ring-Opening Polymerization of O-Carboxyanhydrides Catalyzed by the Pyridine Base2.4 N-Heterocyclic Carbenes for Ring-Opening Polymerization of O-Carboxyanhydrides3 Enzymes for O-Carboxyanhydride Polymerization4 Organometallic Catalysts for O-Carboxyanhydride Polymerization4.1 Early Efforts4.2 Ring-Opening Polymerization Catalyzed by Zn Complexes4.3 Photoredox Ring-Opening Polymerization Catalyzed by Zn/Ni Complexes5 Conclusion and Perspective

Living and immortal polymerization of seven and six membered lactones to high molecular weights with aluminum salen and salan catalysts

The ring-opening polymerization (ROP) of seven membered aliphatic cyclic esters with aluminum salen (MeAl[salen]) and salan (MeAl[salan]) catalyst is reported. While the controlled polymerization of lactide and β-butyrolactone is known for these systems, the living polymerization of ɛ-caprolactone (ɛ-CL) was poorly controlled. We now report excellent levels of control upon optimization of reaction concentration, time and temperature. High molecular weight polycaprolactone (PCL), up to 175kDa, was also prepared with exceptional dispersity control. Immortal polymerization was also studied, with up to 100 equivalents of chain transfer alcohol and 10,000 monomer equivalents tolerated without any sacrifice to polymer control. 6-Methyl-ɛ-caprolactone (6-Me-ɛ-CL) and 2,6-dimethyl-ɛ-caprolactone (2,6-Me-ɛ-CL) were subjected to ROP conditions and low rates of polymerization were observed. Polymerization of 4-(4-benzyloxybutyl)-ɛ-caprolactone (4-BOB-ɛ-CL) was achieved with excellent control over dispersity and molecular weight, allowing introduction of functional groups into the polymer backbone. A block copolymer with 4-BOB-ɛ-CL and ɛ-CL was also prepared to produce a family of polymers with predictable composition.

Electrochemical detection of amyloid-β oligomer with the signal amplification of alkaline phosphatase plus electrochemical–chemical–chemical redox cycling

Soluble amyloid-β oligomer (AβO) is believed to be a reliable molecular biomarker for the diagnosis of Alzheimer's disease (AD) because of its high toxicity for neuronal synapse and higher concentration level in cerebrospinal fluid sample from AD patient than from control individual. At present, it is critical to develop a simple method for AβO detection with low cost as well as high sensitivity and selectivity. In this work, we reported an antibody-free electrochemical method for the detection of AβO based on the specific interaction between AβO and PrP(95–110) peptide, a segment of cellular prion protein. Specifically, cysteine-containing PrP(95–110) peptide was first immobilized on a gold electrode for the capture of AβO. Then, alkaline phosphatase-conjugated PrP(95–110) was used for the recognition of the captured AβO and the generation of electroactive species. Furthermore, an “outer-sphere to inner-sphere” electrochemical–chemical–chemical (ECC) redox cycling using ferrocene methanol as the redox mediator was employed to enhance the detection sensitivity. As a result, a detection limit of 3pM for equivalent monomer was achieved. The amenability of this method to AβO analysis in a biological matrix was demonstrated by assays of AβO in serum samples.

Bis(alkyl) rare-earth complexes supported by a new tridentate amidinate ligand with a pendant diphenylphosphine oxide group. Synthesis, structures and catalytic activity in isoprene polymerization.

A new tridentate amidine 2-[Ph2P(O)]C6H4NHC(tBu)[double bond, length as m-dash]N(2,6-Me2C6H3) (1) bearing a side chain pendant Ph2P[double bond, length as m-dash]O group was synthesized and proved to be a suitable ligand for coordination to rare-earths ions. Bis(alkyl) complexes {2-[Ph2P(O)]C6H4NC(tBu)N(2,6-Me2C6H3)}Ln(CH2SiMe3)2(THF)n (Ln = Y, n = 1 (3), Ln = Er, n = 1 (4), Ln = Lu, n = 0 (5)) were prepared using alkane elimination reactions of and Ln(CH2SiMe3)3(THF)2 (Ln = Y, Er, Lu) in hexane and were isolated in 50, 70 and 75% yields respectively. The X-ray studies revealed that complexes 2-5 feature intramolecular coordination of P[double bond, length as m-dash]O groups to metal ions. The lutetium complex 5 proved to be rather stable: at 20 °C its half-life time is 1155 h, while for the yttrium analogue the half-life time was found to be 63 h. Complexes 3-5 were evaluated as precatalysts for isoprene polymerization. The systems Ln/borate/AliBu3 (Ln = 3-5, borate = [PhNHMe2][B(C6F5)4], [Ph3C][B(C6F5)4]) turned out to be highly efficient in isoprene polymerization and enable complete conversion of 1000-10,000 equivalents of monomer into polymer at 20 °C within 0.5-2.5 h affording polyisoprenes with a very high content of 1,4-cis units (up to 96.6%) and from narrow (1.49) to moderate (3.54) polydispersities. A comparative study of catalytic performance of the related bis(alkyl) yttrium complexes supported by amidinate ligands of different denticities and structures [tBuC(N-2,6iPr2C6H4)2](-), [tBuC(N-2,6-iPr2C6H4)(N-2-MeOC6H4)](-) and {2-[Ph2P(O)]C6H4NC(tBu)N(2,6-Me2C6H3)}(-) demonstrated that the introduction of a pendant donor group (2-MeOC6H4 or Ph2P(O)) into a side chain of amidinate scaffolds results in a significant increase in catalytic activity. The amidinate ligand bearing a Ph2P(O)-group provides a high isoprene polymerization rate along with excellent control over regio- and stereoselectivities and allows us to obtain polyisoprenes with a reasonable molecular weight distribution.

Nucleation is second order: an apparent kinetically effective nucleus of two for Ir(0)n nanoparticle formation from [(1,5-COD)Ir(I)·P2W15Nb3O62]8- plus hydrogen.

Nucleation initiates phase changes across nature. A fundamentally important, presently unanswered question is if nucleation begins as classical nucleation theory (CNT) postulates, with n equivalents of monomer A forming a "critical nucleus", A(n), in a thermodynamic (equilibrium) process. Alternatively, is a smaller nucleus formed at a kinetically limited rate? Herein, nucleation kinetics are studied starting with the nanoparticle catalyst precursor, [A] = [(Bu4N)5Na3(1,5-COD)Ir(I)·P2W15Nb3O62], forming soluble/dispersible, B = Ir(0)(∼300) nanoparticles stabilized by the P2W15Nb3O62(9-) polyoxoanion. The resulting sigmoidal kinetic curves are analyzed using the 1997 Finke-Watzky (hereafter FW) two-step mechanism of (i) slow continuous nucleation (A → B, rate constant k(1obs)), then (ii) fast autocatalytic surface growth (A + B → 2B, rate constant k(2obs)). Relatively precise homogeneous nucleation rate constants, k(1obs), examined as a function of the amount of precatalyst, A, reveal that k(1obs) has an added dependence on the concentration of the precursor, k(1obs) = k(1obs(bimolecular))[A]. This in turn implies that the nucleation step of the FW two-step mechanism actually consists of a second-order homogeneous nucleation step, A + A → 2B (rate constant, k(1obs(bimol))). The results are significant and of broad interest as an experimental disproof of the applicability of the "critical nucleus" of CNT to nanocluster formation systems such as the Ir(0)n one studied herein. The results suggest, instead, the experimentally-based concepts of (i) a kinetically effective nucleus and (ii) the concept of a first-observable cluster, that is, the first particle size detectable by whatever physical methods one is currently employing. The 17 most important findings, associated concepts, and conclusions from this work are provided as a summary.

A Footnote on Allostery

A manuscript on allostery signed by Francis Crick and Jeffries Wyman was sent by Crick to Jacques Monod in 1965. Monod transmitted a copy of the manuscript, upon which he had written several comments, to Jean-Pierre Changeux, then a post-doctoral fellow at the University of California Berkeley in the laboratory of Howard Schachman. Changeux provided a copy to Stuart Edelstein, a graduate student in the same laboratory. The manuscript was never submitted for publication, but Edelstein retained his copy since that time and has edited it for publication in the special issue on allostery. The text emphasized the interpretation of the properties of an allosteric oligomer by characterizing its equivalent monomer. The text also developed the concept of the allosteric range and included a simple equation for calculation of the Hill coefficient from the parameters of the Monod-Wyman-Changeux model.

The effect of degree of polymerization on intra- and interchain micellization of a tail-type cationic polysoap

We have used Reversible Addition Fragmentation chain Transfer (RAFT) to polymerize the T-type surfactant monomer α,ω-methacryloylundecyltrimethylammonium bromide (MUTAB) to various degrees of polymerization, and thereby investigate how its self-assembly is affected. Small-angle neutron scattering (SANS) shows that the interchain aggregation into micelles with an approximately constant number of MUTAB monomer equivalents occurs at low degrees of polymerization, but that micelle elongation occurs when the degree of polymerization exceeds a critical value. In this regime interchain aggregation gives way to intrachain assembly into unimolecular or “unimer” micelles. As with conventional cationic surfactant solutions, addition of salicylate produces long, worm-like micelles containing many amphiphilic polymer chains at all degrees of polymerization. Oscillatory rheology reveals a transition from scission- to reptation-dominated relaxation as increasing polymer chain length also increases the distance between potential scission points. The measured relaxation times lie in the range of hundredths to a few seconds – thus demonstrating the rapidly equilibrating nature of these micellar systems even at the highest degrees of polymerization achieved.

Oligopeptides Represent a Preferred Source of Organic N Uptake: A Global Phenomenon?

Over the past 20 years, our understanding of soil nitrogen (N) cycling has changed with evidence that amino acids are major substrates for both soil microorganisms and plants. However, the recent discovery that plants and microorganisms can directly utilize small peptides in soil needs to be evaluated for its ecological significance, because peptides are released earlier in protein decomposition and thus would provide significant competitive advantage to any organism that can use them directly. We tested whether soil microorganisms took up peptides faster than amino acids across a broad range of ecosystems. We show that l-enantiomeric-peptidic-N is taken up significantly faster than the equivalent monomer, and that this is universal across soils from different ecosystems, with distinct microbial communities. Peptides may have an unrecognized, global, importance in the terrestrial N cycle, providing N to soil microorganisms at an earlier stage of decomposition than previously acknowledged.

Five-Coordinate Rearrangements of Metallacyclobutane Intermediates during Ring-Opening Metathesis Polymerization of 2,3-Dicarboalkoxynorbornenes by Molybdenum and Tungsten Monoalkoxide Pyrrolide Initiators

Addition of rac-DCENBE (2,3-dicarboethoxynorbornene) or rac-DCBNBE (2,3-dicarbo-tert-butoxynorbornene) to Mo(NAd)(CHCMe2Ph)(Pyr)(OHMT) (1a) (Ad = 1-adamantyl, OHMT = 2,6-dimesitylphenoxide, Pyr– = NC4H4–) led to the formation of polymers that have a cis,syndiotactic,alt structure analogous to the structure observed for the polymer obtained from rac-DCMNBE (2,3-dicarbomethoxynorbornene). The PDI of cis,syndio,alt-poly(DCBNBE) is low and decreases as the polymer length increases, and there is a linear relationship between the number of equivalents of monomer employed and the molecular weight of the polymers measured in THF versus polystyrene standards. In contrast, polymerization of (+)-DCMNBE by 1a at 25, 0, −25, and −40 °C yields a polymer that contains ∼25% trans,isotactic dyads and 75% cis,syndiotactic dyads. A similar polymerization by Mo(NAd)(CHCMe2Ph)(Pyr)(OHIPT) (1b) (OHIPT = 2,6-(2,4,6-i-Pr3)2C6H3) gives a polymer that contains cis,syndiotactic and trans,isotactic dyads in a ratio of ∼8:92, respect...

Heteroleptic Silylamido Phenolate Complexes of Calcium and the Larger Alkaline Earth Metals: β‐Agostic Ae⋅⋅⋅SiH Stabilization and Activity in the Ring‐Opening Polymerization of L‐Lactide

The factors governing the stability and the reactivity towards cyclic esters of heteroleptic complexes of the large alkaline earth metals (Ae) have been probed. The synthesis and stability of a family of heteroleptic silylamido and alkoxide complexes of calcium [{LO(i)}Ca-Nu(thf)(n)] supported by mono-anionic amino ether phenolate ligands (i = 1, {LO(1)}(-) = 4-(tert-butyl)-2,6-bis(morpholinomethyl)phenolate, Nu(-) = N(SiMe(2)H)(2)(-), n = 0, 4; i = 2, {LO(2)}(-) = 2,4-di-tert-butyl-6-{[2-(methoxymethyl)pyrrolidin-1-yl]methyl}phenolate, Nu(-) = N(SiMe(2)H)(2)(-), n = 0, 5; i = 4, {LO(4)}(-) = 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenolate, Nu(-) = N(SiMe(2)H)(2)(-), n = 1, 6; Nu(-) = HC≡CCH(2)O(-), n = 0, 7) and those of the related [{LO(3)}Ae-N(SiMe(2)H)(2)] ({LO(3)}(-) = 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenolate Ae = Ca, 1; Sr, 2; Ba, 3) have been investigated. The molecular structures of 1, 2, [(4)(2)], 6, and [(7)(2)] have been determined by X-ray diffraction. These highlight Ae⋅⋅⋅H-Si internal β-agostic interactions, which play a key role in the stabilization of [{LO(i)}Ae-N(SiMe(2)H)(2)] complexes against ligand redistribution reactions, in contrast to regular [{LO(i)}Ae-N(SiMe(3))(2)]. Pulse-gradient spin-echo (PGSE) NMR measurements showed that 1, 4, 6, and 7 are monomeric in solution. Complexes 1-7 mediate the ring-opening polymerization (ROP) of L-lactide highly efficiently, converting up to 5000 equivalents of monomer at 25 °C in a controlled fashion. In the immortal ROP performed with up to 100 equivalents of exogenous 9-anthracenylmethanol or benzyl or propargyl alcohols as a transfer agent, the activity of the catalyst increased with the size of the metal (1<2<3). For Ca-based complexes, the enhanced electron-donating ability of the ancillary ligand favored catalyst activity (1>6>4≈5). The nature of the alcohol had little effect over the activity of the binary catalyst system 1/ROH; in all cases, both the control and end-group fidelity were excellent. In the living ROP of L-LA, the HC≡CCH(2)O(-) initiating group (as in 7) proved superior to N(SiMe(2)H)(2)(-) or N(SiMe(3))(2)(-) (as in 6 or [{LO(4)}Ca-N(SiMe(3))(2)] (B), respectively).

Ligand Depletion in vivo Modulates the Dynamic Range of Cooperative Signal Transduction

A common procedure in systems biology is to build models of physiological networks based on simple experimental measurements made under controlled conditions. One of the frequently performed quantitative measurements is the dose-response relationship. Its results are assumed to be characteristic of the target molecule and independent of contingent controlled variables. However, ligand concentrations in vivo are often in the same range as the dissociation constant of their receptors, leading to the phenomenon of ligand depletion. Moreover, biological signaling often involves cooperative interactions for the binding of ligands to their receptors. We show that ligand depletion diminishes cooperativity and broadens the dynamic range of sensitivity to the signaling ligand. The effects are illustrated with the highly cooperative flagella motor of bacteria and with the ubiquitous intracellular calcium-binding molecule, calmodulin. As a result of ligand depletion, the same signal transducer responds to different ranges of signal with various degrees of cooperativity according to its effective cellular concentration. Therefore, results from in vitro dose-response properties cannot be directly applied to understand signaling in vivo. Moreover, receptor concentration is a key element in controlling signal transduction and its modulation constitutes a way of controlling sensitivity to signals. For quantitative measures of cooperative effects of signaling, the commonly used Hill coefficient assigns misleading values under many conditions. To correct this situation, we define a new index, the Greek letter “nu”, based on the dose-response cooperativity of any oligomeric receptor with respect to the hypothetical dose-response properties of an “equivalent monomer”. The index nu provides a robust measure of cooperativity under diverse conditions and reveals that within the context of the Monod-Wyman-Changeux two-state model of cooperative transitions, the true intrinsic cooperativity is equal to the number of ligand-binding subunits in the oligomeric receptor.

Amorphization of Si using cluster ions

Amorphization process associated with cluster implants has been studied by cross-sectional transmission electron microscopy for P4 cluster implants (5 keV equivalent monomer energy) as a function of dose and compared to that of monomer P implant at the same equivalent energy. Amorphous pockets are formed by the cluster implant at doses of as low as 6×1013/cm2, and a continuous amorphous layer is formed at dose of 2×1014/cm2. The reduction by about 2.5 times in amorphization threshold by clusters with respect to the monomer implant is a consequence of the simultaneous deposition of energy carried by the atoms in the clusters, which enhances the damage yield in the overlapped collision cascades.