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Articles 1 - 24 of 24
Full-Text Articles in Physical Sciences and Mathematics
Synthesis And Reactivity Of Acyclic (Pentadienyl)Iron(1+) Cations: Model Studies For The Preparation Of The 8E,10Z,16E,18E-Tetraene Segment Of Macrolactin A, Abdel-Aziz S. El-Ahl, Young Yun, William Donaldson
Synthesis And Reactivity Of Acyclic (Pentadienyl)Iron(1+) Cations: Model Studies For The Preparation Of The 8E,10Z,16E,18E-Tetraene Segment Of Macrolactin A, Abdel-Aziz S. El-Ahl, Young Yun, William Donaldson
Chemistry Faculty Research and Publications
The dicarbonyl(1,2-dimethylpentadienyl)triphenylphosphineiron(1+) cation (11) has been prepared from methyl 4-methyl-2E,4E-hexadienoate in four steps. The cation (11) reacts with hydride and carbon nucleophiles in a regiospecific fashion to afford (3-methyl-2E,4Z-diene)iron complexes. Dicarbonyl(3-methyl-7-nitro-2E,4Z-heptadiene)triphenylphosphineiron (15), the product from the reaction of 11 with nitromethane anion, has been utilized as a precursor for nitrile oxide–olefin cyclocondensations.
A Hierarchy Of Maps Between Compacta, Paul Bankston
A Hierarchy Of Maps Between Compacta, Paul Bankston
Mathematics, Statistics and Computer Science Faculty Research and Publications
Let CH be the class of compacta (i.e., compact Hausdorff spaces), with BS the subclass of Boolean spaces. For each ordinal α and pair $\langle K,L\rangle$ of subclasses of CH, we define Lev≥α K,L), the class of maps of level at least α from spaces in K to spaces in L, in such a way that, for finite α, Lev≥α (BS,BS) consists of the Stone duals of Boolean lattice embeddings that preserve all prenex first-order formulas of quantifier rank α. Maps of level ≥ 0 are just the continuous surjections, and the maps of level ≥ 1 are …
Thermal Degradation Of Blends Of Polystyrene And Poly(Sodium 4-Styrenesulfonate) And The Copolymer, Poly(Styrene-Co-Sodium 4-Styrenesulfonate), Qiang Yao, Charles Wilkie
Thermal Degradation Of Blends Of Polystyrene And Poly(Sodium 4-Styrenesulfonate) And The Copolymer, Poly(Styrene-Co-Sodium 4-Styrenesulfonate), Qiang Yao, Charles Wilkie
Chemistry Faculty Research and Publications
The thermal degradation of blends and copolymers of styrene with styrenesulfonic acid has been studied using thermogravimetric analysis, TGA/FTIR, and cone calorimetry. The blends have enhanced thermal stability relative to virgin polystyrene but there is no enhancement in thermal stability for the copolymers. Apparently, it is necessary to have adjacent sulfonic acid groups in order to permit the formation of a graphite-like char which can provide thermal protection to the polymer. It is necessary to have a good match in degradation temperatures of the two components if one is to have significantly enhanced thermal stability.
Tga/Ftir: An Extremely Useful Technique For Studying Polymer Degradation, Charles A. Wilkie
Tga/Ftir: An Extremely Useful Technique For Studying Polymer Degradation, Charles A. Wilkie
Chemistry Faculty Research and Publications
Thermogravimetric analysis coupled to Fourier transform infrared spectroscopy, TGA/FTIR, has been used to probe the degradation of several polymeric systems. These include poly(methyl methacrylate) in the presence of various additives, graft copolymers of acrylonitrile-butadiene-styrene and styrene-butadiene with sodium methacrylate and styrene with acrylonitrile, blends of styrene-butadiene block copolymers with poly(vinylphosphonic acid) and poly(vinylsulfonic acid), and cross-linked polystyrenes. Additives may interact with poly(methyl methacrylate) by coordination to the carbonyl oxygen to a Lewis acid and the subsequent transfer of an electron from the polymer chain to the metal atom or by the formation of a radical which can trap the degrading …
Further Studies On Fire Retardant Polystyrene By Friedel–Crafts Chemistry, Zhitao Wang, David D. Jiang, Charles Wilkie, Jeffery W. Gilman
Further Studies On Fire Retardant Polystyrene By Friedel–Crafts Chemistry, Zhitao Wang, David D. Jiang, Charles Wilkie, Jeffery W. Gilman
Chemistry Faculty Research and Publications
The combination of a copolymer of 4-vinylbenzyl alcohol and styrene with 2-ethylhexyldiphenylphosphate (DPP) and with metal chlorides has been studied by TGA, radiative gasification, Cone Calorimetry, and oxygen index measurements. Evidence is presented in support of a cross-linking reaction with the additives and the copolymer, which proceeds through a Friedel–Crafts mechanism. This approach reduces the peak heat release rate (HRR) by 60% as measured in the Cone Calorimeter. There is a significant reduction in the mass loss rate during the thermal degradation, and evidence of char formation is observed in the radiative gasification experiments.
Evaluating Maximum Likelihood Estimation Methods To Determine The Hurst Coefficient, Christina Marie Kendziorski, J. B. Bassingthwaighte, Peter J. Tonellato
Evaluating Maximum Likelihood Estimation Methods To Determine The Hurst Coefficient, Christina Marie Kendziorski, J. B. Bassingthwaighte, Peter J. Tonellato
Mathematics, Statistics and Computer Science Faculty Research and Publications
A maximum likelihood estimation method implemented in S-PLUS (S-MLE) to estimate the Hurst coefficient (H) is evaluated. The Hurst coefficient, with 0.5<HS-MLE was developed to estimate H for fractionally differenced (fd) processes. However, in practice it is difficult to distinguish between fd processes and fractional Gaussian noise (fGn) processes. Thus, the method is evaluated for estimating H for both fd and fGn processes. S-MLE gave biased results of H for fGn processes of any length and for fd processes of lengths less than 210. A modified method is proposed to correct for …
Slow-Binding Inhibition Of The Aminopeptidase From Aeromonas Proteolytica By Peptide Thiols: Synthesis And Spectroscopic Characterization, Kristi M. Huntington, David L. Bienvenue, Yaoming Wei, Brian Bennett, Richard C. Holz, Dehua Pei
Slow-Binding Inhibition Of The Aminopeptidase From Aeromonas Proteolytica By Peptide Thiols: Synthesis And Spectroscopic Characterization, Kristi M. Huntington, David L. Bienvenue, Yaoming Wei, Brian Bennett, Richard C. Holz, Dehua Pei
Physics Faculty Research and Publications
Peptide-derived thiols of the general structure N-mercaptoacyl-leucyl-p-nitroanilide (1a−c) were synthesized and found to be potent, slow-binding inhibitors of the aminopeptidase from Aeromonas proteolytica (AAP). The overall potencies (KI*) of these inhibitors against AAP range from 2.5 to 57 nM exceeding that of the natural product bestatin and approaching that of amastatin. The corresponding alcohols (2a−b) are simple competitive inhibitors of much lower potencies (KI = 23 and 360 μM). These data suggest that the free thiols are involved in the formation of the E·I …
Stabilization Of Polystyrene By Friedel-Crafts Chemistry: Effect Of Position Of Alcohol And The Catalyst, Jin Zhu, Michael A. Mckinney, Charles Wilkie
Stabilization Of Polystyrene By Friedel-Crafts Chemistry: Effect Of Position Of Alcohol And The Catalyst, Jin Zhu, Michael A. Mckinney, Charles Wilkie
Chemistry Faculty Research and Publications
Polystyrene has been copolymerized with 4-vinylbenzyl alcohol, 4-(2-hydroxyethyl)styrene, and 4-(3-hydroxypropyl)styrene and it has been shown that thermal cross-linking of these copolymers is dependent upon the alcohol content. When the alcohol content is low, no thermal cross-linking is observed. When various phosphate esters are present as catalysts with these low alcohol content copolymers, cross-linking is observed at temperatures of about 250°C but not at lower temperatures. Cross-linking enhances the thermal stability of the copolymers. Studies of the thermal stability of the copolymers and their blends with the catalysts have been performed using thermogravimetric analysis and thermogravimetric analysis coupled to Fourier transform …
An Xps Study Of The Radiation-Induced Effect On The Thermal Degradation And Charring Of Butadiene And Its Copolymers, Jianwei Hao, Shaoli Wu, Charles Wilkie, Jianqi Wang
An Xps Study Of The Radiation-Induced Effect On The Thermal Degradation And Charring Of Butadiene And Its Copolymers, Jianwei Hao, Shaoli Wu, Charles Wilkie, Jianqi Wang
Chemistry Faculty Research and Publications
A pseudo-in-situ XPS approach shows that cross-linking induced by irradiation may lead to char formation even though it shows only a small or no effect on the onset temperature of degradation.
Thermal Decomposition Of Cross-Linked Polybutadiene And Its Copolymers, David D. Jiang, Galina F. Levchik, Sergei V. Levchik, Charles Wilkie
Thermal Decomposition Of Cross-Linked Polybutadiene And Its Copolymers, David D. Jiang, Galina F. Levchik, Sergei V. Levchik, Charles Wilkie
Chemistry Faculty Research and Publications
Polybutadiene and two copolymers containing butadiene and styrene have been cross-linked by thermal processes, by the use of initiators in solution, and by intimately mixing an initiator with the polymer and then heating this blend. The most efficient cross-linking process is the use of an intimate blend of the initiator and polymer. Most cross-linking processes lower the onset temperature of degradation, presumably because chain scission reactions occur simultaneously with cross-linking, while also increasing the fraction of non-volatile residue which is produced. It is believed that the density of cross-links is responsible for the increased yield of non-volatile residue.
1H Nmr Studies On The Cua Center Of Nitrous Oxide Reductase From Pseudomonas Stutzeri, Richard C. Holz, Marcela L. Alvarez, Walter G. Zumft, David M. Dooley
1H Nmr Studies On The Cua Center Of Nitrous Oxide Reductase From Pseudomonas Stutzeri, Richard C. Holz, Marcela L. Alvarez, Walter G. Zumft, David M. Dooley
Chemistry Faculty Research and Publications
1H NMR spectra of the CuA center of N2OR from Pseudomonas stutzeri, and a mutant enzyme that contains only CuA, were recorded in both H2O- and D2O-buffered solution at pH 7.5. Several sharp, well-resolved hyperfine-shifted 1H NMR signals were observed in the 60 to −10 ppm chemical shift range. Comparison of the native and mutant N2OR spectra recorded in H2O-buffered solutions indicated that several additional signals are present in the native protein spectrum. These signals are attributed to a dinuclear copper(II) center. At least …
The Methionyl Aminopeptidase From Escherichia Coli Can Function As An Iron(Ii) Enzyme, Ventris M. D'Souza, Richard C. Holz
The Methionyl Aminopeptidase From Escherichia Coli Can Function As An Iron(Ii) Enzyme, Ventris M. D'Souza, Richard C. Holz
Chemistry Faculty Research and Publications
The identity of the physiologically relevant metal ions for the methionyl aminopeptidase (MetAP) from Escherichia coli was investigated and is suggested to be Fe(II). The metal content of whole cells in the absence and presence of expression of the type I MetAP from E. coli was determined by inductively coupled plasma (ICP) emission analysis. The observed change in whole cell concentrations of cobalt, cadmium, copper, nickel, strontium, titanium, and vanadium upon expression of MetAP was negligible. On the other hand, significant increases in the cellular metal ion concentrations of chromium, zinc, manganese, and iron were observed with the increase in …
Inhibition Of The Aminopeptidase From Aeromonas Proteolytica By Aliphatic Alcohols. Characterization Of The Hydrophobic Substrate Recognition Site, Leila Ustynyuk, Brian Bennett, Tanya Edwards, Richard C. Holz
Inhibition Of The Aminopeptidase From Aeromonas Proteolytica By Aliphatic Alcohols. Characterization Of The Hydrophobic Substrate Recognition Site, Leila Ustynyuk, Brian Bennett, Tanya Edwards, Richard C. Holz
Physics Faculty Research and Publications
Seven aliphatic and two aromatic alcohols were tested as reporters of the substrate selectivity of the aminopeptidase from Aeromonas proteolytica (AAP). This series of alcohols was chosen to systematically probe the effect of carbon chain length, steric bulk, and inhibitor shape on the inhibition of AAP. Initially, however, the question of whether AAP is denatured in the presence of aliphatic alcohols was addressed. On the basis of circular dichroism (CD), electronic absorption, and fluorescence spectra, the secondary structure of AAP, with and without added aliphatic alcohols, was unchanged. These data clearly indicate that AAP is not denatured in aliphatic alcohols, …
1-Butaneboronic Acid Binding To Aeromonas Proteolytica Aminopeptidase: A Case Of Arrested Development, Carin C. De Paola, Brian Bennett, Richard C. Holz, Dagmar Ringe, Gregory A. Petsko
1-Butaneboronic Acid Binding To Aeromonas Proteolytica Aminopeptidase: A Case Of Arrested Development, Carin C. De Paola, Brian Bennett, Richard C. Holz, Dagmar Ringe, Gregory A. Petsko
Physics Faculty Research and Publications
Hydrolases containing two metal ions connected by a bridging ligand catalyze reactions important in carcinogensis, tissue repair, post-translational modification, control and regulation of biochemical pathways, and protein degradation. The aminopeptidase from Aeromonas proteolytica serves as a paradigm for the study of such bridged bimetallic proteases since its three-dimensional structure is known to very high resolution and its catalytic reaction is amenable to spectroscopic examination. Herein, we report the X-ray crystal structure at 1.9 Å resolution of AAP complexed with 1-butaneboronic acid (BuBA). This structure suggests that this complex represents a snapshot of the proteolytic reaction in an arrested form between …
Models For Molybdenum Coordination During The Catalytic Cycle Of Periplasmic Nitrate Reductase From Paracoccus Denitrificans Derived From Epr And Exafs Spectroscopy, Clive S. Butler, John M. Charnock, Brian Bennett, Heather J. Sears, Ann J. Reilly, Stuart J. Ferguson, C. David Garner, David J. Lowe, Andrew J. Thomson, Ben C. Berks, David J. Richardson
Models For Molybdenum Coordination During The Catalytic Cycle Of Periplasmic Nitrate Reductase From Paracoccus Denitrificans Derived From Epr And Exafs Spectroscopy, Clive S. Butler, John M. Charnock, Brian Bennett, Heather J. Sears, Ann J. Reilly, Stuart J. Ferguson, C. David Garner, David J. Lowe, Andrew J. Thomson, Ben C. Berks, David J. Richardson
Physics Faculty Research and Publications
The periplasmic nitrate reductase from Paracoccus denitrificans is a soluble two-subunit enzyme which binds two hemes (c-type), a [4Fe-4S] center, and a bis molybdopterin guanine dinucleotide cofactor (bis-MGD). A catalytic cycle for this enzyme is presented based on a study of these redox centers using electron paramagnetic resonance (EPR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The Mo(V) EPR signal of resting NAP (High g [resting]) has gav = 1.9898 is rhombic, exhibits low anisotropy, and is split by two weakly interacting protons which are not solvent-exchangeable. Addition of exogenous ligands to this resting …
Cross-Linking Of Polystyrene By Friedel–Crafts Chemistry To Improve Thermal Stability, Zhitao Wang, David D. Jiang, Michael A. Mckinney, Charles Wilkie
Cross-Linking Of Polystyrene By Friedel–Crafts Chemistry To Improve Thermal Stability, Zhitao Wang, David D. Jiang, Michael A. Mckinney, Charles Wilkie
Chemistry Faculty Research and Publications
Copolymers which contain either alcohol or chloride functionalized polystyrene units have been prepared and they participate in Friedel–Crafts chemistry to give cross-linked polymers by the evolution of either hydrogen chloride or water. Proof of cross-linking comes from the identification of the evolved gas, the insolubility of the product, and the thermal resistance of the newly formed polymer. The onset temperature for the degradation is raised by about 100°C relative to that of polystyrene and the fraction which is not volatile at 800°C ranges from 10% for the alcohol copolymers to 20% for the chloride copolymers.
Thermal Decomposition And Combustion Of Γ-Irradiated Polyamide 6 Containing Phosphorus Oxynitride Or Phospham, A. I. Balabanovich, Sergei V. Levchik, Galina F. Levchik, W. Schnabel, Charles Wilkie
Thermal Decomposition And Combustion Of Γ-Irradiated Polyamide 6 Containing Phosphorus Oxynitride Or Phospham, A. I. Balabanovich, Sergei V. Levchik, Galina F. Levchik, W. Schnabel, Charles Wilkie
Chemistry Faculty Research and Publications
Polyamide 6 (PA-6) containing the fire retardants phosphorus oxynitride ((PON)m) or phospham ((PN2H)n) was exposed to 60Co-γ-rays (absorbed dose: 1.0–4.0 MGy). The irradiation led to crosslinking of the polymer which caused an increase in the char yield and a decrease in the flammability of the polymer. The combustion behavior was strongly affected by irradiation: dripping was totally prevented in the case of system PA-6/(PN2H)n and strongly retarded in the case of system PA-6/(PON)m. The thermal stability of the system PA-6/(PN2H)n decreased with increasing absorbed dose …
Use Of Evolutionary Factor Analysis In The Spectroelectrochemistry Of Escherichia Coli Sulfite Reductase Hemoprotein And A Mo/Fe/S Cluster, Robert L. Keesey, Michael D. Ryan
Use Of Evolutionary Factor Analysis In The Spectroelectrochemistry Of Escherichia Coli Sulfite Reductase Hemoprotein And A Mo/Fe/S Cluster, Robert L. Keesey, Michael D. Ryan
Chemistry Faculty Research and Publications
The deconvolution of spectroelectrochemical data is often quite difficult if the spectra of intermediates are not known. Factor analysis, however, has been shown to be a powerful technique which can make it possible to deconvolute overlapping spectra. In this work, evolving factor analysis will be used to determine the number of intermediates and the spectra of those species for two typical spectroelectrochemical experiments: linear scan voltammetry and chronoabsorptometry in a thin-layer cell. The first system was the reduction of E. coli sulfite reductase hemoprotein (SiR-HP). Principal factor analysis indicated that three species were present. By using evolving factor analysis, …
Thermal Degradation Of Polystyrene, Poly(1,4-Butadiene) And Copolymers Of Styrene And 1,4-Butadiene Irradiated Under Air Or Argon With 60co-Γ-Rays, W. Schnabel, Galina F. Levchik, Charles Wilkie, David D. Jiang, Sergei V. Levchik
Thermal Degradation Of Polystyrene, Poly(1,4-Butadiene) And Copolymers Of Styrene And 1,4-Butadiene Irradiated Under Air Or Argon With 60co-Γ-Rays, W. Schnabel, Galina F. Levchik, Charles Wilkie, David D. Jiang, Sergei V. Levchik
Chemistry Faculty Research and Publications
60Co-γ-irradiated samples of polystyrene (PSt), poly(1,4-butadiene), (PBD), and two poly(styrene-co-butadiene)s containing 25 and 75% BD were subjected to thermogravimetric analysis (TGA), in the presence and absence of O2. In the case of PSt the irradiation caused a significant shift in Tonset, the onset temperature for mass loss, to higher temperatures, whereas in the cases of the BD-containing polymers irradiation caused a decrease in Tonset (oxic irradiation) or had little or no effect on Tonset (anoxic irradiation). The amount of non-volatile residue formed in the cases of BD-containing polymers was augmented by …
Visible And Resonance Raman Spectra Of Low Valent Iron Porphyrins, Chathra De Silva, Kazimierz Czarnecki, Michael D. Ryan
Visible And Resonance Raman Spectra Of Low Valent Iron Porphyrins, Chathra De Silva, Kazimierz Czarnecki, Michael D. Ryan
Chemistry Faculty Research and Publications
The resonance Raman spectrum of Fe(TPP)2− was obtained after the three-electron electrochemical reduction of Fe(TPP)(Cl). The coulometric reduction was carried out in the presence of bis(triphenylphosphoanylidine)ammonium chloride in DMF in order to avoid the formation of iron–σ-alkyl complexes. The resonance Raman spectrum of the intermediate oxidation states (Fe(TPP) and Fe(TPP)−) were consistent with previous work. The spectrum of the three-electron product, Fe(TPP)2−, obtained at 442 nm, was qualitatively similar to the two-electron reduced product, Fe(TPP)−, with an intense ν2 band at 1537 cm−1. The high frequency bands generally decreased in …
Tga/Ftir Studies On The Thermal Degradation Of Some Polymeric Sulfonic And Phosphonic Acids And Their Sodium Salts, David D. Jiang, Qiang Yao, Michael A. Mckinney, Charles Wilkie
Tga/Ftir Studies On The Thermal Degradation Of Some Polymeric Sulfonic And Phosphonic Acids And Their Sodium Salts, David D. Jiang, Qiang Yao, Michael A. Mckinney, Charles Wilkie
Chemistry Faculty Research and Publications
The thermal degradation of poly(vinyl sulfonic acid) and its sodium salt, poly(4-styrenesulfonic acid) and its sodium salt, and poly(vinylphosphonic acid) was studied by a combination of techniques, including TGA/FTIR, to identify the volatile products which were evolved during the degradation as well as analysis of the residues which were obtained in order to propose a mechanism for the degradation. The motivation for the work was to attempt to identify new monomers which could be graft copolymerized onto a polymer in order to improve the thermal stability of that polymer.
Structure-Function Relationships In The Pulmonary Arterial Tree, Christopher A. Dawson, Gary S. Krenz, Kelly Lynn Karau, Steven Thomas Haworth, Christopher C. Hanger, John H. Linehan
Structure-Function Relationships In The Pulmonary Arterial Tree, Christopher A. Dawson, Gary S. Krenz, Kelly Lynn Karau, Steven Thomas Haworth, Christopher C. Hanger, John H. Linehan
Mathematics, Statistics and Computer Science Faculty Research and Publications
Knowledge of the relationship between structure and function of the normal pulmonary arterial tree is necessary for understanding normal pulmonary hemodynamics and the functional consequences of the vascular remodeling that accompanies pulmonary vascular diseases. In an effort to provide a means for relating the measurable vascular geometry and vessel mechanics data to the mean pressure-flow relationship and longitudinal pressure profile, we present a mathematical model of the pulmonary arterial tree. The model is based on the observation that the normal pulmonary arterial tree is a bifurcating tree in which the parent-to-daughter diameter ratios at a bifurcation and vessel distensibility are …
Adventures In Fire Retardancy, Charles A. Wilkie
Adventures In Fire Retardancy, Charles A. Wilkie
Chemistry Faculty Research and Publications
The work of the author's research group on fire retardancy over the past twenty years is reviewed.
X-Ray Crystal Structures And The Facile Oxidative (Au−C) Cleavage Of The Dimethylaurate(I) And Tetramethylaurate(Iii) Homologues, Dunming Zhu, Sergey V. Lindeman, Jay K. Kochi
X-Ray Crystal Structures And The Facile Oxidative (Au−C) Cleavage Of The Dimethylaurate(I) And Tetramethylaurate(Iii) Homologues, Dunming Zhu, Sergey V. Lindeman, Jay K. Kochi
Chemistry Faculty Research and Publications
Dimethylaurate(I) has been prepared as the crystalline tetrabutylammonium salt for comparison with the known tetramethylaurate(III) analogue. The linear structure of dimethylaurate(I) and the square-planar structure of tetramethylaurate(III) have both been confirmed by X-ray crystallography. One-electron oxidation of dimethylaurate(I) by either ferrocenium or arenediazonium cations produces the metastable dimethylgold(II) intermediate, which can be trapped as the paramagnetic 9,10-phenanthrenequinone (PQ) adduct. Otherwise, dimethylgold(II) is subject to rapid reductive elimination of ethane and affords metallic gold (mirror). The analogous oxidation of tetramethylaurate(III) by ferrocenium, arenediazonium, or nitrosonium cations also proceeds via electron transfer to generate the putative tetramethylgold(IV) intermediate. The highly unstable (CH …