Coenzyme models. 19. Oxidation of furoin by a flavin immobilized in cationic polyelectrolytes: A flavoenzyme model

The rate constants for the oxidation of furoin by a flavin (vitamin B₂ analog) immobilized in cationic polymers were evaluated under anaerobic conditions, and the influence of reaction environments was discussed in connection with the enzymatic reactivity. It was found that (1) proton abstraction from furoin which forms the carbanionic intermediate is rate determining in the initial stage of the reaction and is facilitated by the environment of cationic polymers, (2) proton abstraction is efficiently accelerated by added cationic surfactant (CTAB), and (3) N-dodecylbenzohydroxamic acid in aqueous CTAB acts as an excellent proton-abstracting reagent. The polymers with high dodecyl group content served, in all cases, as better oxidizing reagents. As a summary of these conclusions, the importance of hydrophobic environments in flavin-mediated oxidation was emphasized.     

Mechanistic model for bond scission in a polymeric system by radiation

A mechanistic model for the kinetics of polymeric bond scission by X-rays has been developed, with certain assumptions and approximations. The equation governing the number of bonds broken by radiation is found to be Y=1−ae^−cD−be^−dD where a–d are constants and D the dose. The model has been tested for the case of CR-39 plastic irradiated with X-rays for doses from 50 to 600 kGy. The concentration of CO₂ molecules produced inside the plastic, which gave a direct measure of the number of bonds broken by irradiation, was measured using the FTIR absorption bands at 670 and 2350 cm⁻¹. The fraction of the broken bonds was plotted against the radiation dose and the experimental data were found to fit well with the mechanistic model. The values of the various breaking constants for the CR-39 plastic under the mechanistic model were also determined.     

A novel polymeric stabilizing system for modified lyocell solutions

A novel polymeric stabilizer consisting of iminodiacetic acid sodium salt (ISDB) and benzyl amine (BSDB) covalently bound to a styrene/divinyl benzene copolymer were studied. Calorimetric, spectroscopic, rheological, and high performance liquid chromatography analysis revealed distinctive improvements of the thermal stability of modified Lyocell solutions compared to the unstabilized solution and to that with conventional NaOH/propyl gallate stabilizer. Segregation processes of the system cellulose/N‐methylmorpholine‐N‐oxide and autocatalytic reactions caused by carboxyl group‐containing additives are suppressed by ISDB/BSDB. Concerning to surface‐active additives, enhanced thermal stability is only received for a weakly reactive charcoal. In the case of nanoscaled carbon black modifier, autocatalytic reactions indicated by isoperibolic measures are prevented by the new polymeric stabilizer system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1702–1713, 2006     

Enantioselective reactions catalyzed by synthetic enzymes. A model for chemical evolution

Polyleucines of various lengths act as enantioselective catalysts in the aldol condensation between cyclohexanone and various aromatic aldehydes. Polyleucine and other polyamino acids behave as synthetic enzymes in the epoxidation of chalcone and other electron-deficient alkenes. Both reactions are of considerable prebiotic significance.     

A sensitive radiochemical assay for Coenzyme A

Summary A simple and sensitive assay for Coenzyme A (CoA) is described. The method is based on coupling the enzymatic reactions of acetyl-CoA synthetase and acetyl-CoA carboxylase. CoA is converted into acetyl-CoA with acetyl-CoA synthetase in the presence of excess ATP and acetate. Acetyl-CoA is subsequently converted into malonyl-CoA with acetyl-CoA carboxylase in the presence of excess ATP and KH¹⁴CO₃. The formation of labelled acid-stable material (i.e. malonyl-CoA) is determined. Under conditions of the assay CoA is quantitatively converted into malonyl-CoA. This procedure permits the detection of as little as 15 pmoles of CoA in biological samples. Elimination of acetyl-CoA synthetase from the reaction mixture allows for the determination of acetyl-CoA.

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Empfindlicher radiochemischer Assay für Coenzym A

Zusammenfassung Ein einfacher und empfindlicher Assay für Coenzym A (CoA) wird beschrieben, der auf einer Kopplung der enzymatischen Reaktionen von Acetyl-CoA-synthetase und Acetyl-CoA-carboxylase beruht. CoA wird mit Hilfe von Acetyl-CoA-synthetase in Gegenwart von überschüssigem ATP und Acetat zu Acetyl-CoA umgesetzt. Dieses wird anschließend mit Acetyl-CoA-carboxylase in Gegenwart von überschüssigem ATP und KH¹⁴CO₃ zu Malonyl-CoA umgewandelt. Die Bildung von markierter säurestabiler Substanz (d.h. Malonyl-CoA) wird bestimmt. Unter den Versuchsbedingungen ist die Umsetzung von CoA zu Malonyl-CoA quantitativ. Noch 15 pMol CoA können in biologischem Material erfaßt werden. Durch Entfernung von Acetyl-CoA-synthetase aus dem Reaktionsgemisch kann auch Acetyl-CoA bestimmt werden.

     

Coenzyme B12 model compounds: A quantitative electrochemical comparison

A quantitative electrochemical comparative study is reported which demonstrates that Costa's B₁₂ model is a closer electrochemical mimic of B₁₂ than is the widely used cobaloxime model. Also reported are conditions allowing chemically reversible cyclic voltammetry of cobalt-alkyls of Costa’s B₁₂ model and the surprising effect of axial base concentration upon the apparent cobaltcarbon bond stability subsequent to electrochemical reduction.     

The catalytic power of pyruvate decarboxylase. A stochastic model for the molecular evolution of enzymes.

Pyruvate decarboxylase (PDC) catalyzes the decarboxylation of pyruvate anion by a factor of around 10(12), compared with the non-enzymic decarboxylation by thiamine, under standard state conditions of 1 mM pyruvate and thiamine diphosphate (TDP), pH 6.2. Free-energy diagrams constructed on the basis of earlier measurements for the enzymic and non-enzymic reactions give some information on catalysis by PDC. PDC stabilizes the reactant state preceding TDP addition to pyruvate by 76 kJ mol-1 and the transition state for the addition by 83 kJ mol-1. PDC stabilizes the reactant state preceding decarboxylation (presumably alpha-lactyl-TDP) by 27 kJ mol-1 and the transition state for decarboxylation by 68 kJ mol-1. In addition, the free-energy diagrams reveal a leveling of reactant-state free energies in the enzymic reaction compared with the non-enzymic reaction, in that the former are nearly equal to each other. The enzyme-bound transition-state energies are similarly leveled. The energetic leveling of reactant states has been noted by Albery, Knowles and their coworkers in many enzymic reactions and termed 'matched internal thermodynamics.' They showed that the result would arise naturally (and inevitably) in the 'evolution to perfection' of enzymes, when the evolutionary process was treated by a deterministic model. The critical assumption of this model was the validity of a Marcus-type or Br?nsted-type linear free-energy relationship between rate and equilibrium constants for reactions occurring wholly within enzyme complexes. Here a completely stochastic simulation of molecular evolution, with no deterministic assumptions, is shown to reproduce both 'matched internal thermodynamics' and the 'matched internal kinetics' or leveling of transition-state energies noted here. The Albery-Knowles result is thus more general than might have been supposed.     

Monte Carlo study of the coil-to-globule transition of a model polymeric system

Monte Carlo computer simulations of single, flexible, self-avoiding chains on a cubic lattice have been performed upon conditions of increasing segment–segment cohesive energy (deteriorating solvent quality). The simulations spanned a wide range of chain lengths (20–10,000, i.e., up to molecular weights of a few millions) and cohesive energies (0.0–0.45k_BT, i.e., from athermal to very poor solvents). The chain length dependence of the chain size in poor solvents was characterized by a wide plateau of almost null growth for intermediate chain lengths. This plateau was linked to the development of the incipient constant density core, while genuine power law dependence (1/3) was not reached even for the longest chains and poorest solvents simulated here. The mere appearance of a core required substantial chain lengths (higher than 1000; molecular weights of a few hundred thousands), while short chains underwent a gradual densification devoid of any qualitative changes in the density distribution. Sufficiently long chains became more but not quite spherical and underwent a reasonably sharp second order phase transition. The findings were generally in agreement with predictions of mean-field theory and with the use of the standard scaling variables, despite slight inconsistencies. Nevertheless, the results stress the fact that short chains never form a constant density core and that core-dominance on the globule's properties (“volume approximation”) is only valid for extraordinarily long chains [molecular weight of O(10⁹)], an effect linked to the relatively diffuse nature of the surface layer and originating from chain connectivity in conjunction with spherical geometry. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3651–3666, 2006     

A novel model for quantification of the moisture absorption of polymeric laminated composites

In the present study, a new model was developed that considers the amount of the environmental fluid absorption by different constituents of polymeric laminated composites including fibers, resin, fiber-matrix interphase region, ply interface region, and voids. By knowing the fluid absorption behavior of the composite constituents, the present model can predict the amount of fluid absorption of different constituents of polymeric laminated composites with an arbitrary resin volume fraction and stacking sequence. Test specimens were fabricated by glass fibers and vinyl ester resin. The environmental fluids, examined in this study, were distilled and saline water under different temperatures and salt concentrations. To investigate the absorption behavior of different constituents of polymeric composite, various tests were conducted on fibers, pure cured resin, unidirectional composite specimens, and laminated composites. Based on the results of the tests, a new theoretical model was developed to quantify and predict the amount of fluid absorption of different constituents of laminated polymeric composites. The thickness of the interphase region between the fiber and matrix was also measured using the scanning electron microscope (SEM) images and nano-indentation tests. The consistency of experimental results with the outcomes of the theoretical model indicates the accuracy of the model.     

A five-parameter fractional derivative temperature spectrum model for polymeric damping materials

To capture viscoelastic behavior of polymeric damping materials based on limited dynamic mechanical analysis tests, a simple fractional temperature spectrum model representing the viscoelastic materials is proposed in this paper and experimental tests aims at stressing the validity of the model. The storage modulus, the loss modulus, and the loss factor, are established based on the five-parameter fractional derivative model and the time–temperature superposition principle. The dynamic mechanical tests of two polymeric materials are carried out to verify this temperature spectrum model. Results indicate a good agreement between the temperature spectrum model and experimental tests at various temperature conditions. Furthermore, thermodynamic coupling of the viscoelastic material is investigated by temperature rise calculation and vibration experiment test. Comparison analysis shows that the temperature rise model can simulate the temperature rise process for the shear vibration of the constrained damping, which provide references for the damping capability, thermal damage and failure of viscoelastic material.     

A genetic selection system for evolving enantioselectivity of enzymes

As an alternative to screening in the directed evolution of enantioselective enzymes, a selection system has been implemented for a lipase-catalyzed hydrolytic kinetic resolution of a chiral ester.     

Paramagnetic active site models for the molybdenum-copper carbon monoxide dehydrogenase

New paramagnetic, heterobimetallic Mo/Cu complexes featuring the Mo(=O)(mu-S)Cu core of O. carboxidovorans carbon monoxide dehydrogenase have been synthesized and structurally and spectroscopically characterized. The complexes exhibit EPR spectra (left graphic) indicative of extensive electron delocalization across the Mo-S-Cu core, in agreement with computational studies identifying the singly-occupied molecular orbital (right graphic).     

A general model for the release of active agents incorporated in swellable polymeric matrices

A theoretical model, which takes proper account of the simultaneous uptake of a liquid leachant by a polymer matrix and the consequent release of a bioactive or other solute incorporated therein, is presented and the kinetics of release predicted under various conditions is investigated. This model is shown to be both more rigorous and much more widely applicable than previous attempts to approach the above problem. Furthermore, it can be easily modified or extended, in accordance with the information available about any particular system. It is, therefore, expected that the model introduced here will prove useful as a basis for the design of monolithic controlled-release devices of this type or for the evaluation of the leachability of low-level and medium-level radioactive wastes “immobilized” in polymeric matrices.     

One-dimensional models of polymer dynamics. II. A dashpot-spring model

The assumption of Clark and Zimm that coupled dashpots and springs can be used to model the dynamics of polymer molecules is here applied to a model different from that of Clark and Zimm. The precise differences are given in the preceding paper. The dielectric relaxation spectrum of the model is computed in time and frequency domains. The relaxation spectrum can be fitted reasonably well by the empirical Williams–Watts and Havriliak–Negami functions. The best-fit Williams–Watts and Havriliak–Negami parameters are given as functions of the parameters of the model. The model is compared with several related models found in the literature and possible interpretations are given.     

A harmonic oscillator model of the conformationally flexible helical polymeric molecules

The approach of the persistence vector to its asymptotic limit can be characterized by the average projection of a unit vector along the last bond onto a unit vector along the first bond. The behavior of this projection has been examined in simple, predominantly helical chains in which limited flexibility is introduced by either of two devices: occurrence in an average way of a configuration much different from that required for helix propagation, and use of a square-well potential centered at the dihedral angle utilized in a rigid helix. The manner in which flexibility is introduced determines the behavior of the angular frequency as well as the relative decay rates for oscillations and the midpoint about which oscillations occur.     

Thermodynamic model of the effect of nanoadhesion for polymeric nanocomposites

A thermodynamic model of the effect of nanoadhesion in filled polymeric nanocomposites is discussed. The model demonstrates good agreement with experimental data. It is shown that the effect of nanoadhesion has a dimensional origin, i.e., it is a true nanoeffect, which is greatly influenced by the aggregation of filler nanoparticles.     

A self-contained polymeric 2-DE chip system for rapid and easy analysis

We developed a polymeric 2-DE chip system. The chip consisted of an IEF region, an SDS-PAGE region, a valveless connection port, and a sample introduction port. A "junction structure" as a valveless connection port, which allowed separating and connecting the first- and second-dimensional gels, was fabricated between their regions. A "solution inlet" as a sample introduction port was fabricated to perform the liquid and sample introductions without solution leakage. Simultaneous sample monitoring was performed using the on-chip detection system. The performances of the system were demonstrated using commercially available proteins as a standard specimen and tissue-extracted proteins as the real samples. All procedures were employed without any movement of relocation part. This new 2-D separation system realized improved labor-intensive operations and a reduced experimental time.     

Tube relaxation: A quantitative molecular model for the viscoelastic plateau of entangled polymeric media

The relaxation of an entangled polymeric medium in the viscoelastic plateau is investigated theoretically by using the slip-link representation of topological constraints. In addition to the chain retraction process introduced by Daoudi and investigated theoretically by Doi, we show that two processes contribute significantly to the relaxation: The first, “equilibration across slip-links,” is a longitudinal reequilibration between parts of the chain which have been differently extended or compressed, depending on their initial orientation relatively to the strain tensor. The second, “tube relaxation,” is a mean-field representation of the loss of topological constraints on one chain due to the retraction of the others. Closed analytical expressions for the stress accounting for these three processes are derived and compared with previous theories: the relaxation should be much more progressive than previously predicted, and the terminal time for retraction is reduced significantly by tube relaxation.