M4 Model Research Articles

NMR crystallography of α-poly(l-lactide)

A complementary approach that combines NMR measurements, analysis of X-ray and neutron powder diffraction data and advanced quantum mechanical calculations was employed to study the α-polymorph of L-polylactide. Such a strategy, which is known as NMR crystallography, to the best of our knowledge, is used here for the first time for the fine refinement of the crystal structure of a synthetic polymer. The GIPAW method was used to compute the NMR shielding parameters for the different models, which included the α-PLLA structure obtained by 2-dimensional wide-angle X-ray diffraction (WAXD) at -150 °C (model M1) and at 25 °C (model M2), neutron diffraction (WAND) measurements (model M3) and the fully optimized geometry of the PLLA chains in the unit cell with defined size (model M4). The influence of changes in the chain conformation on the (13)C σ(ii) NMR shielding parameters is shown. The correlation between the σ(ii) and δ(ii) values for the M1-M4 models revealed that the M4 model provided the best fit. Moreover, a comparison of the experimental (13)C NMR spectra with the spectra calculated using the M1-M4 models strongly supports the data for the M4 model. The GIPAW method, via verification using NMR measurements, was shown to be capable of the fine refinement of the crystal structures of polymers when coarse X-ray diffraction data for powdered samples are available.

Sparse moving maxima models for tail dependence in multivariate financial time series

The multivariate maxima of moving maxima (M4) model has the potential to model both the cross-sectional and temporal tail-dependence for a rich class of multivariate time series. The main difficulty of applying M4 model to real data is due to the estimation of a large number of parameters in the model and the intractability of its joint likelihood. In this paper, we consider a sparse M4 random coefficient model (SM4R), which has a parsimonious number of parameters and it can potentially capture the major stylized facts exhibited by devolatized asset returns found in empirical studies. We study the probabilistic properties of the newly proposed model. Statistical inference can be made based on the Generalized Method of Moments (GMM) approach. We also demonstrate through real data analysis that the SM4R model can be effectively used to improve the estimates of the Value-at-Risk (VaR) for portfolios consisting of multivariate financial returns while ignoring either temporal or cross-sectional tail dependence could potentially result in serious underestimate of market risk.

Improved SCS-CN–Inspired Model

The present study enhances the Soil Conservation Service curve number (SCS-CN) predictions by improving the model structure, considering the following issues of concern: implementation of antecedent moisture condition procedure, fixation of initial abstraction ratio (λ) at 0.2, usage of the potential maximum retention parameter, and incorporation of storm intensity or duration in runoff estimation. A five-parameter M3 model is proposed, with storm duration and a new parameter Sabs (potential maximum retention), to overcome most of the above limitations prevailing in the SCS-CN model. For simplicity and practical applications obviating storm-duration data, a four-parameter M4 model is also proposed. The performance of the suggested and the available models has been evaluated using the data of 82 small watersheds in the United States of America. As demonstrated, the M3 model performs the best, whereas the conventional SCS-CN model performs the poorest among all the models studied.

Does a prediction model for pregnancy of unknown location developed in the UK validate on a US population?

A logistic regression model (M4) was developed in the UK to predict the outcome for women with a pregnancy of unknown location (PUL) based on the initial two human chorionic gonadotrophin (hCG) values, 48 h apart. The purpose of this paper was to assess the utility of this model to predict the outcome for a woman (PUL) in a US population. Diagnostic variables included log-transformed serum hCG average of two measurements, and linear and quadratic hCG ratios. Outcomes modeled were failing PUL, intrauterine pregnancy (IUP) and ectopic pregnancy (EP). This model was applied to a US cohort of 604 women presenting with symptomatic first-trimester pregnancies, who were followed until a definitive diagnosis was made. The model was applied before and after correcting for differences in terminology and diagnostic criteria. When retrospectively applied to the adjusted US population, the M4 model demonstrated lower areas under the curve compared with the UK population, 0.898 versus 0.988 for failing PUL/spontaneous miscarriage, 0.915 versus 0.981 for IUP and 0.831 versus 0.904 for EP. Whereas the model had 80% sensitivity for EP using UK data, this decreased to 49% for the US data, with similar specificities. Performance only improved slightly (55% sensitivity) when the US population was adjusted to better match the UK diagnostic criteria. A logistic regression model based on two hCG values performed with modest decreases in predictive ability in a US cohort for women at risk for EP compared with the original UK population. However, the sensitivity for EP was too low for the model to be used in clinical practice in its present form. Our data illustrate the difficulties of applying algorithms from one center to another, where the definitions of pathology may differ.

Applying a British mathematical model to predict the outcome of pregnancy of unknown location in a United States population

OBJECTIVE: To assess utility of a logistic regression model in a US population of womenDESIGN: Validation of prediction of outcome of a pregnancy of unknown location (PUL)MATERIALS AND METHODS: The “M4 model” was developed on cases of PUL from St. George's Hospital, London; prognostic variables included log transformed serum hCG average of 2 measurements, linear and quadratic hCG ratio, with the outcomes of failing PUL, intrauterine pregnancy (IUP) and ectopic pregnancy (EP). (Condous et al. Ultrasound Obstet Gynecol 2007; 29:680-87) The US cohort included patients who presented to the University of Pennsylvania with symptomatic first trimester pregnancy, and were followed until a definitive diagnosis was made: spontaneous abortion (SAB), IUP or EP. The model was applied to the US cohort and its performance was evaluated.TableModel performanceCohortTypeSens(%)Spec(%)AUCUKEP80.088.60.900IUP85.996.30.974Failing PUL87.297.50.978USEP49.087.40.821IUP84.192.80.961SAB81.483.00.933 Open table in a new tab In an attempt to improve performance in the US population by including only PULs and reclassifying US patients based on the British hCG values and ultrasound criteria (data not shown), we were unable to demonstrate an improvement in its performance.CONCLUSIONS: The M4 model is not equivalent in its predictive ability in a US cohort. Specifically, it had a lower AUC for SABs in the US cohort, and although the AUCs for EP were not significantly different, it had poor sensitivity for EP in the US. OBJECTIVE: To assess utility of a logistic regression model in a US population of women DESIGN: Validation of prediction of outcome of a pregnancy of unknown location (PUL) MATERIALS AND METHODS: The “M4 model” was developed on cases of PUL from St. George's Hospital, London; prognostic variables included log transformed serum hCG average of 2 measurements, linear and quadratic hCG ratio, with the outcomes of failing PUL, intrauterine pregnancy (IUP) and ectopic pregnancy (EP). (Condous et al. Ultrasound Obstet Gynecol 2007; 29:680-87) The US cohort included patients who presented to the University of Pennsylvania with symptomatic first trimester pregnancy, and were followed until a definitive diagnosis was made: spontaneous abortion (SAB), IUP or EP. The model was applied to the US cohort and its performance was evaluated. In an attempt to improve performance in the US population by including only PULs and reclassifying US patients based on the British hCG values and ultrasound criteria (data not shown), we were unable to demonstrate an improvement in its performance. CONCLUSIONS: The M4 model is not equivalent in its predictive ability in a US cohort. Specifically, it had a lower AUC for SABs in the US cohort, and although the AUCs for EP were not significantly different, it had poor sensitivity for EP in the US.

Theoretical and Experimental Study of Tip Electronic Structure in Scanning Tunneling Microscope

AbstractThe atomic and electronic structures of pyramidal model STM tips of transition metals (W, Rh, Pd, Ir and Pt) were investigated using density functional theory (DFT) method. The calculated density of states show that d electrons of the apex atoms in the M4 (M = W, Rh, Pd, Ir, Pt) model tips behave differently near the Fermi level, with the dz2 state being dominant only for W tip. The electronic structures of pyramid structures of W and Pd single-atom tips with larger sizes are studied and compared. The density of states of Pd apex atom and W apex atom show different occupation of d-bands leading to asymmetric density of states for Pd tip. The asymmetric tunneling currents measured by W and Pt-Ir STM tips are explained by the calculated electronic structures of W and Pd model tips.

Asymptotically (in)dependent multivariate maxima of moving maxima processes

Smith and Weissman introduced a M4 class of processes which are very flexible models for temporally dependent multivariate extreme value processes. However all variables in these M4 models are asymptotically dependent and what this paper does is to extend this M4 class in a number of ways to produce classes of models which are also asymptotically independent. We shall study properties of the proposed models. In particular, asymptotic dependence indexes, coefficients of tail dependence, and extremal indexes are derived for each case.

Limit analysis of multi-layered plates. Part II: Shear effects

In the first part of this work [Dallot, J., Sab, K., 2007. Limit analysis of multi-layered plates. Part I: the homogenized Love–Kirchhoff model. J. Mech. Phys. Solids, in press, doi: 10.1016/j.jmps.2007.05.005], the limit analysis of a multi-layered plastic plate submitted to out-of-plane loads was studied. The authors have shown that a homogeneous equivalent Love–Kirchhoff plate can be substituted for the heterogeneous multi-layered plate, as the slenderness (length-to-thickness) ratio goes to infinity. In fact, the out-of-plane shear stresses are shown to become asymptotically negligible when compared to in-plane stresses, as the slenderness ratio goes to infinity. Actually, failure of thick multi-layered structures often occurs by shearing in the core layers and sliding at the interfaces between the layers. Both shearing and sliding are caused by the out-of-plane shear stresses. The purpose of the present paper is to build an enhanced Multi-particular Model for Multi-layered Material (M4) taking into account shear stress effects. In this model, each layer is seen as a Reissner–Mindlin plate interacting with its neighboring layers through interfaces. The proposed model is asymptotically consistent with the homogenized Love–Kirchhoff model described in the first part of the work, as the slenderness ratio goes to infinity. Kinematic and static methods for the determination of the limit load of a thick multi-layered plate which is submitted to out-of-plane distributed forces are described. The special case of multi-layered plates under cylindrical bending conditions is studied. These conditions lead to simplifications which often allow for the analytical resolution of the Love–Kirchhoff and the M4 limit analysis problems. The benefit of the proposed M4 model is demonstrated on an example. A comparison between the heterogeneous 3D model, the Love–Kirchhoff model and the M4 model is performed on a three-layer sandwich plate under cylindrical bending conditions. Finite element calculations are used to solve the 3D problem, while both the Love–Kirchhoff and the M4 problems are analytically solved. It is shown that, when the contrast between the core and the skins strengths is high, the Love–Kirchhoff model fails to capture the plastic collapse modes that cause the ruin of the sandwich plate. These modes are well captured by the M4 model which predicts limit loads that are very consistent with the limit loads predicted by the heterogeneous 3D model (the relative error is found to be smaller than 1%).

Highly Chiral Muscarinic Ligands: The Discovery of (2S,2‘R,3‘S,5‘R)-1-Methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine 3-sulfoxide Methyl Iodide, a Potent, Functionally Selective, M2 Partial Agonist

By further steric complication of previously studied highly chiral muscarinic agonists, we have obtained a small chiral library of enantiomeric and diasteromeric 1-methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine 3-sulfoxides. Binding studies on cloned human muscarinic receptors expressed in CHO cells show that the introduction of a fourth stereogenic center gives undetectable affinity for hm1, hm3, hm4 and hm5 subtypes while leaving a quite modest affinity only for hm2 subtypes. However, functional studies on model M1-M4 muscarinic tissues have shown that three compounds of the series [(-)-5, (-)-7, (+)-8] are endowed with functional activity and behave as M2 selective partial agonists. Among them, compound (2S,2'R,3'S,5'R)-1-methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine 3-sulfoxide methyl iodide [(+)-8] is particularly interesting, as it is a potent partial agonist on guinea pig atrium (force) (M2; pD2=7.65, alpha=0.41) while being a poor antagonist on M1, M3, and M4 model tissues (pKb<5).

Modelling triaxial compression using the Microplane formulation for low confinement

A numerical model is developed to examine the effect of low confinement on the behaviour of high strength concrete columns. A 3-D model using the Microplane-M4 formulation with second order effects included using a total Lagrangian formulation is developed. A cylinder under uniaxial and triaxial stress states is modelled. Adjustments in the M4 model are proposed for the particular case of concrete columns under low confinement. Comparisons with empirical formulas for uniaxial and triaxially confined concrete show excellent agreement.

Kinetics of added organic matter decomposition in a Mediterranean sandy soil

Carbon mineralization kinetics of 17 organic materials were studied in a Mediterranean sandy soil. These added organic matters (AOM) used in the organic fertilizer industry differed in their origin and composition: plant residues from the agri-food industry, animal wastes, manures (plant and animal origin), composts at different composting times and organic fertilizers. The mixtures AOM-soils were incubated under aerobic conditions at 28°C during 6 months. Soil moisture was maintained at 75% water holding capacity and respired-CO2 was regularly trapped into alkali media in closed chambers, then checked by HCl titration. Analyses of CO2 were performed in triplicate at 17 sampling occasions. The mineralized AOM fraction (MAOMF) varied according to the AOM origin: from 12–33% of added C for composts, to 65–90% for animal-originated AOM, with many intermediate patterns for plant-originated AOM.Seven decomposition models from the literature were fitted to actual MAOMF: (a) three consecutive models with two 1st-order-kinetic compartments and three parameters (m1, humification; m2, exchange; m3, decomposition), (b) three parallel models (m4, with two compartments and three parameters; m8, a 1st-order plus 0-order model with three parameters; m5, a three-compartment model with four parameters), and (c) m7, a model with one 2nd-order-kinetic compartment and two parameters. Additionally, m6, a simplified version of m5 was proposed. Models m2 and m7 did not match with actual data or gave a poor fit. By the correlation parameters, the most simple model m4 was chosen instead of the consecutive models m1 and m3. Residual sums of squares were always greater—but not significantly—in m8 than in m4, which confirmed the superiority of the models with two 1st-order compartments against 1st-order plus 0-order models for incubation times higher than 100 days. Model m5 (most of its parameters being not correlated) gave the best predictions of our data. The proposed m6 version gave predictions with similar precision as m4 and appeared powerful with only two parameters (very labile and stable fractions of the AOM). A compromise between the precision of the predictions and the simplicity of the formulae allowed the recommendation of the well-known m4 model, and above all the simpler m6 model.

Stress analysis in adhesive joints with a multiparticle model of multilayered materials (M4)

We propose a new model to calculate the stresses in adhesive joints. It is well known that the classical models only deal with adhesive joints with constant layer thickness. The model we propose is up to calculate the stresses in adhesive joints with any geometry and with anisotropic adherends. If we consider an adhesive joint as a multilayered medium, our model describes it as a surface with as many particles in each point of the surface as the number of layers in the medium (three layers for a simple lap joint and six layers for a double lap joint). For this reason, it is named multiparticle modelisation of multilayered materials (M4 in the following of the text). The M4 models are developed at ENPC Chabot B. Thèse de l’école nationale des fonts et chaussées, 1997; Ehrlacher A, Caron JF, Chabot A, Douçot E, Naciri T. Modélisation multiphasique des plaques composites en flexion, 1er Congrès de Mécanique, 13–16 Avril 1993, E.N.I.M Rabat; Naciri T, Ehrlacher A, Chabot A. Compos. Sci. Technol. 1998; 58:337 for the calculation of interlaminar stresses in composite materials. The model we propose is an application of a M4 model to adhesive joints. It deals with every geometry which we can find in the literature. It only suffices to know the layer thickness in each point on the overlap to calculate the stresses in the joint. In this paper, we firstly present the classical steps to build the M4. This construction is based on the Hellinger–Reissner (Reissner E. JMath Phys, 1950; 29: 90–5) formulation. Secondly, through some joint shapes, we show that we can have a good prediction of stresses in the adhesive layer and that the M4 is a pertinent tool to study adhesive joints.

A tri-particle model of sandwich panels

A multiparticle model (referred to as M4; multi-particle model of multilayered materials) (Naciri T, Ehrlacher A, Chabot A. Interlaminar stress analysis with a new multiparticle model. Composites Science and Technology 1998;58:337–343) has been used to describe the behavior of sandwich panels in monotonic bending. The main purpose of this study is the analysis of global response and shear stresses in sandwich panels. The equilibrium equations, boundary conditions and generalized constitutive law of the M4 model are given and applied to sandwich panels, which are assumed to be made up of three layers (two skins and a core). This sandwich multi-particle model is called a tri-particle model. The accuracy of this model is then assessed for bending problems the solutions of which can be derived analytically or computed. The example of sandwich panels in cylindrical bending is first investigated by taking Pagano’s solutions as reference. A second example is proposed for the estimation of the accuracy of the tri-particle model with classical finite elements used in sandwich design. A refined SAMCEF 3-D mesh served as the calculation reference. The tri-particle model was found to be more accurate than the classical sandwich theory based on the Reissner cinematic hypotheses in the first example and more precise than the SAMCEF finite-element plate models in the second example.

Interlaminar stress analysis with a new Multiparticle modelization of Multilayered Materials (M4)

We propose a new model for calculating the interlaminar stresses in a multilayered medium. It is well known that the classical analysis methods for multilayered materials do not provide sufficient information about the interlaminar stresses that cause damage. The model we propose describes the multilayered medium as a surface with as many particles in each point of the surface as the number of layers in the medium. For this reason, it is referred to as a Multiparticle Model of Multilayered Materials (M4 in the following text). It permits the direct calculation of the stresses as interactions between particles at the same point of the surface. In a certain sense, the M4 model belongs to the same family as the well known shear-lag model (Garett and Bailey, J. Mater. Sci., 12, 1977; 1 Macquire et al., Composites Polymers, 5, 1992 2) and Pagano's global-local model (Pagano and Soni, Int.J. Solids Structures, 19, 207–228, 1983 3). It is more general than the shear-lag approach and simpler than the Pagano global-local model. In this paper we present the equations of the model and establish an analytical solution for a cylindrical bending multilayered plate problem. Through this example, we show that we may obtain good predictions of displacements and shear stresses and that the M4 model is an appropriate tool for the study of interlaminar stresses.

Kinetics of a dimer-dimer catalytic surface reaction type AB+C2to 1/2 A2+C2B

Dimer-dimer catalytic surface reaction of the type AB+C2 to 1/2 A2+C2B is studied by Monte Carlo simulation both on square and hexagonal lattices. Various models are proposed and studied. For the case of a square lattice the three models in which we (i) ignore both the diffusion and desorption of various reactants (M1 model), (ii) consider diffusion of C species only (M2 model), and (iii) consider the diffusion of C atoms as well as their recombination and desorption (M3 Model) all give a final poisoned state for all feed concentrations. For model M1 there is a continuous crossover from one poisoned state to another, while for M2 and M3 an irreversible phase transition separates one poisoned state from the other. The diffusion of A atoms (M4 model) is found to be very crucial to the evolution of the system towards a final steady reactive state. The slightest movement of the A atoms releases the trapped vacancies and other reactants which are available for further reaction. An irreversible phase transition now separates a poisoned state from a steady reactive state. For the hexagonal lattice the M3 model already leads the system towards a steady reactive state. The role of increasing the number of nearest neighbouring sites is seen to be similar to that of the diffusion of A atoms.

Voltage-gated transient currents in bovine adrenal fasciculata cells. I. T-type Ca2+ current.

The whole cell version of the patch clamp technique was used to identify and characterize voltage-gated Ca2+ channels in enzymatically dissociated bovine adrenal zona fasciculata (AZF) cells. The great majority of cells (84 of 86) expressed only low voltage-activated, rapidly inactivating Ca2+ current with properties of T-type Ca2+ current described in other cells. Voltage-dependent activation of this current was fit by a Boltzmann function raised to an integer power of 4 with a midpoint at -17 mV. Independent estimates of the single channel gating charge obtained from the activation curve and using the "limiting logarithmic potential sensitivity" were 8.1 and 6.8 elementary charges, respectively. Inactivation was a steep function of voltage with a v1/2 of -49.9 mV and a slope factor K of 3.73 mV. The expression of a single Ca2+ channel subtype by AZF cells allowed the voltage-dependent gating and kinetic properties of T current to be studied over a wide range of potentials. Analysis of the gating kinetics of this Ca2+ current indicate that T channel activation, inactivation, deactivation (closing), and reactivation (recovery from inactivation) each include voltage-independent transitions that become rate limiting at extreme voltages. Ca2+ current activated with voltage-dependent sigmoidal kinetics that were described by an m4 model. The activation time constant varied exponentially at test potentials between -30 and +10 mV, approaching a voltage-independent minimum of 1.6 ms. The inactivation time constant (tau i) also decreased exponentially to a minimum of 18.3 ms at potentials positive to 0 mV. T channel closing (deactivation) was faster at more negative voltages; the deactivation time constant (tau d) decreased from 8.14 +/- 0.7 to 0.48 +/- 0.1 ms at potentials between -40 and -150 mV. T channels inactivated by depolarization returned to the closed state along pathways that included two voltage-dependent time constants. tau rec-s ranged from 8.11 to 4.80 s when the recovery potential was varied from -50 to -90 mV, while tau rec-f decreased from 1.01 to 0.372 s. At potentials negative to -70 mV, both time constants approached minimum values. The low voltage-activated Ca2+ current in AZF cells was blocked by the T channel selective antagonist Ni2+ with an IC50 of 20 microM. At similar concentrations, Ni2+ also blocked cortisol secretion stimulated by adrenocorticotropic hormone. Our results indicate that bovine AZF cells are distinctive among secretory cells in expressing primarily or exclusively T-type Ca2+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)