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PubMed2005

Water sorption, solubility, and tensile bond strength of resilient denture lining materials polymerized by different methods after thermal cycling.

STATEMENT OF PROBLEM: The clinical properties of resilient denture lining materials may be influenced by the methods used to polymerize them. PURPOSE: This study evaluated and compared water sorption, solubility, and tensile bond strength of 2 resilient liner materials polymerized by different methods after being thermal cycled. MATERIAL AND METHODS: Two acrylic resin-based resilient liner materials were evaluated: 1 (Light Liner) polymerized by visible light, and 1 (Ever-Soft) processed by 2 different methods: hot water bath and microwave energy. To evaluate water sorption and solubility, 10 disc-shaped specimens (50 x 0.5 mm) were fabricated for each polymerization method, using acetate matrices. The specimens were dried and thermal cycled (2000 cycles) between baths of 5 degrees C and 55 degrees C. After thermal cycling, the specimens were weighed, dried, and weighed again to calculate the water sorption values and solubility. For tensile bond strength assessment, 12 rectangular specimens measuring 83 mm in total length and with a cross-sectional area of 10 x 10 mm were fabricated using each polymerization method. The specimens were also thermal cycled (2000 cycles) and, afterwards, tested in a universal testing machine at a crosshead speed of 6 mm/min with 100 Kg load cell. The type of failure was determined by use of stereoscopic microscopy. The data (for water sorption and solubility, n = 30, and for tensile bond strength, n = 36) were submitted to 1-way analysis of variance. The means were compared using the Bonferroni test (alpha=.05). RESULTS: For water sorption the results showed no significant differences among the polymerization methods. Light Liner showed the lowest solubility values (5.3% +/- 0.3%) when compared to Ever-Soft (hot water bath: 7.3% +/- 1.1% and microwave energy: 7.8 +/- 0.9%). For tensile bond strength, Ever-Soft polymerized by microwave energy (0.7 +/- 0.0 MPa) showed the greatest and most significant ( P <.05) value when compared to Ever-Soft polymerized by hot water bath (0.5 +/- 0.1 MPa) and Light Liner polymerized by visible light (0.5 +/- 0.0 MPa). Most Ever-Soft specimens polymerized by microwave energy and Light Liner specimens showed adhesive/cohesive failures. However, Ever-Soft polymerized by hot water bath showed 50% adhesive/cohesive and 50% adhesive failures. The results at the reline material-resin bond interface showed the absence of cohesive failures. CONCLUSION: Within the limitations of this study, Light Liner showed the lowest solubility values. Ever-Soft should be polymerized by microwave energy to obtain the greatest tensile bond strength values. Materials polymerized by microwave energy and visible light showed predominantly adhesive/cohesive failures.

Absorption↗
PubMed1988

Characterization of the actin polymerization-inhibiting protein from chicken gizzard smooth muscle.

An actin polymerization-inhibiting protein, occurring in crude preparations of vinculin from chicken gizzard, has been found to be heterogeneous. The molecular masses of the polymerization-inhibiting peptides have been reported to range from 20 kDa to 80 kDa [Schröer, E. & Wegner, A (1985) Eur. J. Biochem. 153, 515-520]. In this paper, a 21-kDa peptide was isolated from the bulk of the other peptides by gel chromatography. The 21-kDa peptide was identified as a polymerization-inhibiting peptide by its ability to retard nucleated actin polymerization and to bind polymeric actin when it was blotted onto nitrocellulose. Antiserum raised to the 21-kDa peptide was found to react with almost all peptides of the blotted heterogeneous polymerization-inhibiting protein. The same peptides which reacted with antiserum cosedimented with polymeric actin. The major peptides of the blotted polymerization-inhibiting protein bound polymeric actin. The largest peptide which reacted with antiserum and cosedimented with polymeric actin had a molecular mass of 85 kDa. The results suggest that the preparation of polymerization-inhibiting protein contains mainly polymerization-inhibiting peptides and only some contaminants, and that all the polymerization-inhibiting peptides are proteolytic fragments stemming from a common precursor.

Actin Depolymerizing Factors↗
PubMedDate not supplied

Polymerization shrinkage of flowable resin-based restorative materials.

This study measured the linear polymerization displacement and polymerization forces induced by polymerization shrinkage of a series of flowable resin-based restorative materials. The materials tested were 22 flowable resin-based restorative materials (Admira Flow, Aelite Flow, Aeliteflow LV, Aria, Crystal Essence, Definite Flow, Dyract Flow, Filtek Flow, FloRestore, Flow-it, Flow-Line, Freedom, Glacier, OmegaFlo, PermaFlo, Photo SC, Revolution 2, Star Flow, Synergy Flow, Tetric Flow, Ultraseal XT and Wave). Measurements for linear polymerization displacement and polymerization forces were performed using custom made measuring devices. Polymerization of the test materials was carried out for 60 seconds by means of a light curing unit, and each property was measured for 180 seconds from the start of curing in eight specimens for each material. Statistical evaluation of the data was performed with one-way analysis of variance (ANOVA), Tukey's Studentized Range (HSD) test (p=0.05) and simple linear regression. A wide range of values was recorded for linear polymerization displacement (26.61 to 80.74 microns) and polymerization forces (3.23 to 7.48 kilograms). Statistically significant differences among materials were found for both properties studied. Very few materials (Freedom, Glacier, and Photo SC) presented low values of linear polymerization displacement and polymerization forces (similar to hybrid resin composites), while the majority of materials presented very high values in both properties studied. Study of the shrinkage kinetics revealed the exponential growth process of both properties. The polymerization forces development exhibited a few seconds delay over linear polymerization displacement. Simple linear regression showed that the two polymerization shrinkage properties that were studied were not highly correlated (r2=0.59).

Analysis of Variance↗
PubMed1988

Quantitation of human serum polymeric IgA, IgA1 and IgA2 immunoglobulin by enzyme immunoassay.

This report concerns the relative quantitation of serum polymeric IgA and polymeric IgA subclass concentrations by enzyme immunoassay (EIA). The assay relies on the specific binding of polymeric IgA to secretory component. Competition between pentameric IgM and polymeric IgA for binding to secretory component was observed. Thus, samples were adsorbed for IgM by affinity chromatography before the EIA was performed. The assay was used to determine an age-related range of serum polymeric IgA concentrations and to compare the polymeric IgA concentrations in patients with IgA nephropathy (n = 50) to those of controls (n = 50). The serum concentrations of both polymeric IgA and polymeric IgA1 increased with age reaching adult values of around 12 years of age. Polymeric IgA2 concentrations did not reach adult levels until 18 years of age. The ratio of the polymeric IgA concentration to the total serum IgA concentration was found to be significantly increased in children under 2 years of age compared with those over 4 years of age (Mann-Whitney U-test, P less than 0.01). Patients with IgA nephropathy had significantly increased concentrations of polymeric IgA (P = 0.001) and polymeric IgA1 (P = 0.001) but similar polymeric IgA2 concentrations to controls.

Adolescent↗
PubMed1991

Actin polymerization in murine B lymphocytes is stimulated by cytochalasin D but not by anti-immunoglobulin.

One might predict that cytochalasin D, which slows polymerization of actin in solution and which inhibits actin-containing microfilament function in live B lymphocytes, would also prevent actin polymerization in these cells. However, we have used the NBD-Phallacidin flow cytometric assay for F-actin and the DNase I inhibition assay for G-actin to demonstrate that cytochalasin D (at 20 micrograms/ml and higher) stimulates actin polymerization in murine B lymphocytes within the first 30 sec of exposure. A similar response was seen in human neutrophils. Actin polymerization induced in neutrophils by chemotactic peptides has been linked to activation of the polyphosphoinositide-calcium increase-protein kinase C signal transduction pathway. As B lymphocytes also transduce signals using this pathway, we investigated whether cytochalasin D induced actin polymerization by activating this pathway. Cytochalasin D and ionomycin both stimulated a rapid increase in internal calcium (by 1 min) in the B cell which was inhibitable by EGTA, implicating calcium influx. Ionomycin also induced actin polymerization, detectable later, by 10 min. EGTA blocked the ionomycin-induced actin polymerization, but not that induced by cytochalasin D. Cytochalasin D-induced actin polymerization was not associated with detectable hydrolysis of polyphosphoinositides, nor was it inhibited by H7 (a protein kinase C inhibitor) or by HA1004 (an inhibitor of cyclic nucleotide-dependent kinases). Furthermore, anti-immunoglobulin antibodies, which stimulate B lymphocytes through the polyphosphoinositide hydrolysis-calcium increase-protein kinase C pathway, failed to induce actin polymerization in these cells. These antibodies did, however, stimulate the cells to perform activities that involve actin-containing microfilaments. Other primary activators of B lymphocytes (dextran sulfate, PMA, and LPS) and a panel of lymphokines previously shown to enhance B lymphocyte activation (IL-1, IL-2, IL-4, IL-5) were also screened in the F-actin assay and no evidence for actin polymerization was found. We conclude that the actin polymerization response to cytochalasin D in the B cell does not involve the polyphosphoinositide hydrolysis-calcium increase-protein kinase C pathway, nor does it depend on cyclic nucleotide-dependent kinases. Furthermore, our studies failed to provide any evidence that early actin polymerization occurs in murine B lymphocyte activation.

Actins↗
PubMed1978

Quantification and properties of tubulin polymerization in crude brain extracts and preparations of microtubular and purified tubulin.

Removal of assembled tubulin by centrifugation, followed by measurement in the supernatant of the residual colchicine binding capacity of the non-polymerized, non-precipitable tubulin, is a sensitive and reliable method of measuring tubulin polymerization. This method can be used in both crude and purified preparations of brain tubulin and allows the molar quantification of the total, polymerized and non-polymerized tubulin species in each sample. Only 40--50% of the total tubulin present in crude adult brain extracts is capable of polymerizing when incubated with GTP. The percentage of tubulin polymerizing with GTP is slightly higher in crude foetal brain extracts than in the adult. Incubation of first polymerization supernatants, containing exclusively the GTP-insensitive tubulin, with guanosine 5'-[alpha, beta-methylene]triphosphate (GuoP[CH2]PP) but not with 2.4 M glycerol results in tubulin polymerization. High concentrations of glycerol (2.4 M) promote the polymerization of tubulin in adult but not in crude foetal brain extracts. Both ATP and adenosine 5'-[alpha, beta-methylene]triphosphate (AdoP[CH2]PP) are effective in promoting the polymerization of GDP-free (90%) microtubular protein. The microtubular protein assembled with GuoP[CH2]PP or AdoP[CH2]PP has the unique character of being resistant to calcium concentrations (2mM), which cause complete depolymerization of the tubulin assembled with GTP or ATP. Phosphocellulose-purified tubulin significantly assembles when incubated with GuoP[CH2]PP or AdoP[CH2]PP at protein concentrations at which GTP or ATP do not promote polymerization. 2 micrometer tubulin is the minimal concentration required for polymerization in both crude and purified preparations of adult rat brain tubulin.

Animals↗
PubMed1998

Effect of polymerization mode on bond strengths of resin adhesive/cement systems.

PURPOSE: The purpose of this study was to examine the effect of resin adhesive/cement system primer and cement polymerization mode (auto- vs dual-polymerized) on the shear bond strength to dentin. MATERIALS AND METHODS: Dentin surfaces of 90 bovine teeth were polished to 600 grit. The dentin was etched for 15 seconds with phosphoric acid, rinsed, and blotted. Three adhesive systems, All-Bond 2, Prime & Bond 2.1 Dual-Cure, or Scotchbond Multi-Purpose Plus, were applied. Three primer polymerization methods were used with each system: dual-polymerized, autopolymerized, or strictly following manufacturers' recommendations. Resin cements, which were also either auto- or dual-polymerized, were applied to dentin using gelatin capsules. Shear bond strengths were determined using an Instron universal testing machine. RESULTS: The greatest bond strength, 15.4 MPa, was found with the Scotchbond system when the primer was dual-polymerized and the cement was autopolymerized. The lowest bond strength, 7.5 MPa, was found with All-Bond 2/Duo-Link, when the primer and cement were dual-polymerized. The autopolymerization mode produced the highest mean bond strength for All-Bond 2, while dual-polymerization of primer and cement resulted in the highest mean bond strength for Prime & Bond 2.1. Scotchbond Multi-Purpose Plus was least affected by the polymerization method. Two-way ANOVA showed that the interaction between the type of adhesive system and the method of polymerization significantly influenced bond strength (p = .0001). CONCLUSIONS: The effect of the primer polymerization method on ultimate bond strength was different for each adhesive system evaluated. More research is needed to elucidate the interaction between adhesive system and method of polymerization on bonding of resin cements to dentin.

Animals↗
PubMed1985

Kinetics of citrate-induced activation and polymerization of chick liver acetyl-CoA carboxylase.

The kinetic time course of citrate-induced activation and polymerization (into filaments) of the protomeric form of acetyl-CoA carboxylase were compared to assess the concertedness of the two processes. Rapid-quench techniques were employed to measure the kinetics of activation of the carboxylase-catalyzed reaction by citrate. When enzyme was preincubated with citrate prior to initiating the steady state turnover reaction with acetyl-CoA in the rapid-quench device, the observed rate of carboxylation of acetyl-CoA was apparently linear from the moment of mixing. However, when enzyme was mixed with citrate to initiate the reaction, a lag (t1/2 = 0.7 s) occurred in the approach to steady state carboxylation rate. This lag was independent of enzyme concentration over a 230-fold range and was marginally dependent upon citrate concentration. Over the same range of enzyme concentration, polymerization of carboxylase protomers, as determined by right angle light scattering, was enzyme concentration-dependent in a manner predicted by a single protomer activation step, followed by a rate-limiting dimerization of active protomer and subsequent polymerization. Polymerization is a second order process, with a second order rate constant of 597,000 M-1 s-1. There appear to be two steps that limit polymerization of the inactive carboxylase protomer: a rapid citrate-induced conformational change, which is independent of enzyme concentration and leads to an active protomeric form of the enzyme and the dimerization of the active protomer, which constitutes the first step of polymerization and is enzyme concentration-dependent. Dimerization is the rate-limiting step of acetyl-CoA carboxylase polymerization. On the basis these results, it is concluded that activation of catalysis and the polymerization of carboxylase protomers are not concerted. Furthermore, activation of carboxylation leading to the formation of an active protomer was faster than polymerization under all conditions, and therefore precedes polymerization. It was also shown that the activation constant (Kact) for citrate is altered in a predictable manner by the accumulation of the reaction product, malonyl-CoA, the Kact increasing with malonyl-CoA concentration. Depolymerization of fully polymerized acetyl-CoA carboxylase is caused by malonyl-CoA or ATP.Mg (and HCO3-). Both malonyl-CoA and ATP.Mg (and HCO3-) compete with citrate in the maintenance of a given state of the protomer-polymer equilibrium apparently by carboxylating the enzyme to form enzyme-biotin CO2- which destabilizes the polymeric form.

Acetyl-CoA Carboxylase↗
PubMed2002

Quantitative assessment of polymerization-binding mechanics of cyanoacrylates: model development and validation.

BACKGROUND AND PURPOSE: Although commonly acknowledged as paramount in significance, the mechanics of cyanoacrylate polymerization remain poorly characterized and quantified for clinical applications. This prompted development of a simplistic model for the systematic study of polymerization and binding behaviors of cyanoacrylates. METHODS: A sliding bed apparatus was constructed that linked a strain gage with a vessel that could be filled with liquid medium, cyanoacrylate, and a microcatheter. As the cyanoacrylate polymerized, the microcatheter was mechanically drawn away very slowly from the fixed vessel, resulting in the development of forces that were recorded to characterize the dynamics of polymerization and binding. Optimization of the model required manipulation of several variables that could influence polymerization. Three different formulations of cyanoacrylate were also tested to determine whether there are significant differences in polymerization dynamics. RESULTS: After experimenting with a few basic physical parameters of the test apparatus, consistent measurements of binding forces during cyanoacrylate polymerization could be recorded and measured. Polymerization produced a multiphasic pattern of binding forces, in which three distinct phases were observed. Furthermore, the rates of polymerization were significantly influenced by a variety of parameters, including the type of fluid within the model vessel, geometry of the model vessel, and rate of injection of acrylic into the well. Furthermore, there were significant differences in the pattern of dynamic binding forces among the various formulations of cyanoacrylate tested. CONCLUSIONS: A standardized bench top testing apparatus has been developed, which can consistently show dynamic binding related to polymerization of cyanoacrylates. This preliminary study shows a clear multiphasic pattern of polymerization binding, which may have important clinical implications. The apparatus may be useful for gaining better insight into a variety of clinically important phenomena related to cyanoacrylate polymerization.

Cyanoacrylates↗
PubMed1998

Polymerization of human prion peptide HuPrP 106-126 to amyloid in nucleic acid solution.

The human prion peptide PrP106-126 polymerizes in the presence of DNA both in its circular and linearized forms under solution conditions where the peptide alone does not polymerize. The polymerization process has been monitored by the increase in the fluorescence of anilino naphthalene sulfonic dye which detects the availability of the hydrophobic surface(s) in the aggregate as a consequence of polymerization. The polymerization is a nucleation dependent phenomenon as is evidenced from an existence of a lag period before the onset of the polymerization and a strong dependence of the polymerization on the prion peptide concentrations. The reaction is dependent on the pH as seen from rapid polymerization at pH 5 compared to the reaction at neutral pH where no polymerization is observed after a relatively long period of incubation. The polymer has been characterized as amyloid by using new absorbing and emitting species resulting from the interaction of the polymer with the amyloid specific fluorescent dye, Thioflavine S. This is probably the first demonstration that an endogenous macromolecule can influence the polymerization of a prion peptide. We have previously shown that there is a conformational change in the nucleic acid as a consequence of this interaction. This prion peptide is considered as a model to understand prion diseases as is evidenced from its toxicity towards primary brain cells in culture. The peptide encompasses one of the important amyloidogenic regions of the normal cellular prion protein. Demonstration of nucleic acid induced polymerization of the normal and scrapie prion isoforms accompanying a change in the nucleic acid conformation can establish a possible role of nucleic acid in prion disease.

Amino Acid Sequence↗
PubMed2004

The effect of polymerization cycles on porosity of microwave-processed denture base resin.

STATEMENT OF PROBLEM: Although most of the physical properties of denture base resin polymerized by microwave energy have been shown to be similar to resins polymerized by the conventional heat polymerization method, the presence of porosity is a problem. PURPOSE: This study evaluated the effect of different microwave polymerization cycles on the porosity of a denture base resin designed for microwave polymerization. MATERIAL AND METHODS: Thirty-two rectangular resin specimens (65 x 40 x 5 mm) were divided into 3 experimental groups (A, B, and C; Onda-Cryl, microwave-polymerized resin) and 1 control group (T; Clássico, heat-polymerized resin), according to the following polymerization cycles: (A) 500 W for 3 minutes, (B) 90 W for 13 minutes+500 W for 90 seconds, (C) 320 W for 3 minutes+0 W for 4 minutes+720 W for 3 minutes, and (T) 74 degrees C for 9 hours. Porosity was calculated by measurement of the specimen volume before and after its immersion in water. Data were analyzed using 1-way analysis of variance (alpha=.05). RESULTS: The mean values and SDs of the percent mean porosity were: A=1.05%+/-0.28%, B=0.91%+/-0.15%, C=0.88%+/-0.23%, T=0.93%+/-0.23%. No significant differences were found in mean porosity among the groups evaluated. CONCLUSION: Within the limitations of this study, a denture base resin specifically designed for microwave polymerization tested was not affected by different polymerization cycles. Porosity was similar to the conventional heat-polymerized denture base resin tested.

Acrylic Resins↗
PubMed2005

Influence of ceramic thickness and polymerization mode of a resin luting agent on early bond strength and durability with a lithium disilicate-based ceramic system.

STATEMENT OF PROBLEM: Attenuation of polymerization light energy by translucent all-ceramic materials may result in insufficient polymerization of underlying resin luting agents and inadequate early bond strength and durability. There is little information regarding the selection of an appropriate polymerization mode for cementing translucent all-ceramic restorations. PURPOSE: The purpose of this study was to evaluate the influence of ceramic thickness and polymerization mode on the early bond strength and bond durability of a lithium disilicate-based ceramic system. MATERIAL AND METHODS: The occlusal surfaces of 120 extracted, intact, human third molars were sectioned to expose a flattened area of dentin. The surface was etched with 32% phosphoric acid, and a single-step adhesive (One-Step) was applied to the etched dentin surfaces. Ceramic specimens (Empress 2), 6 mm in diameter and 1 mm, 1.5 mm, or 2 mm thick (n=40 per group), were fabricated using fluoropolymer resin matrixes. Each specimen was ground flat. Following hydrofluoric acid etching and silane treatment, ceramic discs of each thickness were further divided into 2 groups (n=20 per group) and bonded to the dentin surfaces with a dual-polymerized resin luting agent (Illusion), either with a catalyst (dual polymerization) or without a catalyst (light polymerization). A shear bond test was performed after 10 minutes (n=10) or after 24 hours following 1000 thermal cycles between 5 degrees C and 55 degrees C and a dwell time of 30 seconds (n=10). Debonded dentin surfaces were examined with SEM. The data were analyzed with 3-way analysis of variance (ANOVA) (alpha=.05). RESULTS: The shear bond strengths ranged between 13.2 +/- 4.1 MPa and 15.9 +/- 2.0 MPa. Three-way ANOVA revealed that ceramic thickness, polymerization mode, storage time, or combinations of these parameters did not influence shear bond strength. The location of failure for all specimens was adhesive, between the dentin surface and bonding agent. CONCLUSION: Both light polymerization and dual polymerization provided similar early shear bond strengths for the lithium disilicate-based ceramic system (Empress 2). The bond strength was not dependent on the thickness of the ceramic material tested. Durability of the bond was similar for both of the polymerization modes.

Analysis of Variance↗
PubMed2002

Sterically and electrosterically stabilized emulsion polymerization. Kinetics and preparation.

The principal subject discussed in the current paper is the radical polymerization in the aqueous emulsions of unsaturated monomers (styrene, alkyl (meth)acrylates, etc.) stabilized by non-ionic and ionic/non-ionic emulsifiers. The sterically and electrosterically stabilized emulsion polymerization is a classical method which allows to prepare polymer lattices with large particles and a narrow particle size distribution. In spite of the similarities between electrostatically and sterically stabilized emulsion polymerizations, there are large differences in the polymerization rate, particle size and nucleation mode due to varying solubility of emulsifiers in oil and water phases, micelle sizes and thickness of the interfacial layer at the particle surface. The well-known Smith-Ewart theory mostly applicable for ionic emulsifier, predicts that the number of particles nucleated is proportional to the concentration of emulsifier up to 0.6. The thin interfacial layer at the particle surface, the large surface area of relatively small polymer particles and high stability of small particles lead to rapid polymerization. In the sterically stabilized emulsion polymerization the reaction order is significantly above 0.6. This was ascribed to limited flocculation of polymer particles at low concentration of emulsifier, due to preferential location of emulsifier in the monomer phase. Polymerization in the large particles deviates from the zero-one approach but the pseudo-bulk kinetics can be operative. The thick interfacial layer can act as a barrier for entering radicals due to which the radical entry efficiency and also the rate of polymerization are depressed. The high oil-solubility of non-ionic emulsifier decreases the initial micellar amount of emulsifier available for particle nucleation, which induces non-stationary state polymerization. The continuous release of emulsifier from the monomer phase and dismantling of the non-micellar aggregates maintained a high level of free emulsifier for additional nucleation. In the mixed ionic/non-ionic emulsifiers, the released non-ionic emulsifier can displace the ionic emulsifier at the particle surface, which then takes part in additional nucleation. The non-stationary state polymerization can be induced by the addition of a small amount of ionic emulsifier or the incorporation of ionic groups onto the particle surface. Considering the ionic sites as no-adsorption sites, the equilibrium adsorption layer can be thought of as consisting of a uniform coverage with holes. The de-organization of the interfacial layer can be increased by interparticle interaction via extended PEO chains--a bridging flocculation mechanism. The low overall activation energy for the sterically stabilized emulsion polymerization resulted from a decreased barrier for entering radicals at high temperature and increased particle flocculation.

Emulsions↗
PubMed1975

The role of the bound nucleotide in the polymerization of actin.

Three mucleotides, ATP, ADP, and an unsplit-table analog of ATP (adenylyl imidodiphosphate (AMPPNP)), were bound to monomeric actin, and their effects on the rate and extent of the actin polymerization were studied. The kinetics of polymerization, assayed by the change in OD232, followed a simple exponential curve. The rates of polymerization were equal for bound ATP and AMPPNP; both of which were three to five times faster than the rate for ADP. The concentration of actin monomers in apparent equilibrium with the polymer, G(180 degrees longitude), was determined. Values of G(180 degrees longitude) in 100 mM KCl were found for different nucleotides to be: G-ATP(180 degrees longitude) = 0.7 mu-M, G-AMPPNP(180 degrees longitude) = 0.8 MU-M, and G-ADP(180 degrees longitude) = 3.4 mu-M. The equilibrium constant of the polymerization is given by K = [G(180 degrees longitude)]-minus 1 when no nucleotide is split. The polymerization of actin-ATP is more complex due to the splitting of the nucleotide and our data require that this polymerization involves more than one step. The kinetic parameters for the polymerization of actin-ATP can be explained by a simple scheme in which the nucleotide dephosphorylation occurs in a step following the polymerization step. The conclusions are: (1) the binding of ATP to actin monomer promotes polymerization slightly more than the binding of ADP, (2) actin bound ATP provides less than 4 kJ/mol of free energy to promote polymerization, and (3) the dephosphorylation of the nucleotide is not coupled to polymerization.

Actins↗
PubMed2001

Radical polymerization of styrene controlled by half-sandwich Mo(III)/Mo(IV) couples: all basic mechanisms are possible.

Density functional calculations of bond dissociation energies (BDEs) have been used as a guide to the choice of metal system suitable for controlling styrene polymerization by either the stable free radical polymerization (SFRP) or the atom transfer radical polymerization (ATRP) mechanism. In accord with the theoretical prediction, CpMo(eta(4)-C(4)H(6))(CH(2)SiMe(3))(2), 2, is not capable of yielding SFRP of styrene. Still in accord with theoretical prediction, CpMo(eta(4)-C(4)H(6))Cl(2), 1, CpMo(PMe(3))(2)Cl(2), 3, and CpMo(dppe)Cl(2) (dppe = 1,2-bis(diphenylphosphino)ethane), 4, yield controlled styrene polymerization by the SFRP mechanism in the presence of 2,2'-azobisisobutyronitrile (AIBN). This arises from the generation of a putative Mo(IV) alkyl species from the AIBN-generated radical addition to the Mo(III) compound. The controlled nature of the polymerizations is indicated by linear M(n) progression with the conversion in all cases and moderate polydispersity indices (PDIs). Controlled polymerization of styrene is also given by compounds 3 and 4 in combination with alkyl bromides. These complexes then operate by the ATRP mechanism, again in accord with the theoretical predictions. Controlled character is revealed by linear increase of M(n) versus conversion, low PDIs, a stop-and-go experiment, and (1)H NMR and MALDI-TOF analyses of the polymer end groups. The same controlled polymerization is given by a "reverse" ATRP experiment, starting from AIBN and CpMo(PMe(3))(2)Cl(2)Br, 5. On the other hand, when compound 1 or 2 is used in combination with an alkyl bromide (as for an ATRP experiment), the isolated polystyrene shows by M(n), (1)H NMR, and MALDI-TOF analyses that catalytic chain transfer (CCT) radical polymerization takes place in this case. Kinetics simulations underscore the conditions regulating the radical polymerization mechanism and the living character of the polymerization. The complexes herein described are ineffective at controlling the polymerization of methyl methacrylate.

Journal Article↗
PubMed2002

Comparison of post-gel shrinkage strains in light-polymerized composite resins.

STATEMENT OF PROBLEM: Polymerization shrinkage is associated with light-polymerized and polyacid-modified composites. PURPOSE: This in vitro study compared the polymerization shrinkage of a light-polymerized hybrid composite, a posterior composite, and a polyacid-modified composite. MATERIAL AND METHODS: Five disk-shaped specimens (1 cm in diameter and 2 mm in thickness) were prepared in a mold from each of the following test materials: a light-polymerized hybrid composite (3M Valux Plus), a posterior composite (Filtek P60), and a polyacid-modified composite (Dyract AP). The hybrid composite served as the control material. A linear strain gauge was placed at the center of each specimen. After the specimens were light-polymerized for 60 seconds under 400-mW/cm(2) light intensity, microstrains were recorded with the strain indicator continuously every 4 seconds for 120 seconds. The data were evaluated with 1-way analysis of variance (P<.05) followed by a post hoc least significant difference test. RESULTS: During the first 20 seconds of light polymerization, all materials exhibited tensile strains between 6 and 55 mu epsilon that quickly converted into compressive strains and continued rising after the light source was removed. The strains for all materials increased at a declining rate until they reached a constant level between 148 and 180 seconds. The polymerization shrinkage recorded for Filtek P60 was the lowest (-190.56 mu epsilon), followed by 3M Valux Plus (-303.94 mu epsilon) and Dyract AP (-345.64 mu epsilon) (P<.05). The dimensional change between Filtek P60 and Dyract AP and between Filtek P60 and 3M Valux Plus was significant (P=.001). However, the difference between Dyract AP and 3M Valux Plus specimens was not significant (P=.208). CONCLUSION: Within the limitations of this study, the light-polymerized posterior composite tested demonstrated less polymerization shrinkage than the polyacid-modified and hybrid composites tested. Polymerization shrinkage velocity significantly affected the magnitude of strains.

Analysis of Variance↗
PubMed1989

An in vitro investigation of the effects of glass inserts on the effective composite resin polymerization shrinkage.

We placed an MOD preparation in each of 12 permanent molars, then restored each tooth with a posterior composite resin by means of six different application techniques (I-polymerization as one complete unit; II--polymerization as one complete unit with glass inserts; III--polymerization in gingivo-occlusal increments; IV-polymerization in gingivo-occlusal increments with glass inserts; V--polymerization in bucco-lingual increments; and VI--polymerization in a gingival increment with glass inserts, then bucco-lingual increments). A precision strain gauge was attached to the buccal surface of each tooth and balanced at zero. After each increment was polymerized, the strain appearing on the strain gauge indicator was recorded. Each tooth was restored by use of all techniques; two teeth started with each technique. Results demonstrated the average microstrain units to be 127-I, 102-II, 105-III, 86-IV, 72-V, and 66-VI. A randomized block design was the format used for data evaluation. Scheffé's Test indicated that composite resin placement and polymerization in bucco-lingual increments (V) created significantly less cuspal deflection than polymerization as one complete unit, with or without glass inserts (I and II), p less than 0.001, and gingivo-occlusal increments (III), p less than 0.05. Placement and polymerization in a gingival increment with glass inserts, then bucco-lingual increments (VI), also created significantly less internal deflection than polymerization as one complete unit, with or without glass inserts (I and II), p less than 0.001, and gingivo-occlusal increments (III), p less than 0.005.

Analysis of Variance↗
PubMed2002

[Effect of techniques of composite resin insertion and polymerization on microleakage and microhardness].

The aim of this study was to evaluate the influence of techniques of composite resin polymerization and insertion on microleakage and microhardness. One hundred and eighty class II cavities were prepared in bovine teeth and assigned to six groups: G1 - bulk filling + conventional polymerization; G2 - bucco-lingual increments + conventional polymerization; G3 - bulk filling + soft-start polymerization; G4 - bucco-lingual increments + soft-start polymerization; G5 - bulk filling + progressive polymerization; G6 - bucco-lingual increments + progressive polymerization. All cavities were restored with the Z100/Single Bond system (3M). After thermocycling, the samples were immersed in 2% methylene blue dye solution for 4 hours. Half of the samples were embedded in polystyrene resin, and Knoop microhardness was measured. The Kruskal-Wallis test did not reveal statistical differences (p > 0.05) between the polymerization and insertion techniques as to microleakage. Regarding microhardness, the two-way ANOVA and the Tukey test did not reveal statistical differences between the restorative techniques (p > 0.05), but progressive polymerization (G5 and G6) was associated with smaller Knoop microhardness values (p < 0.05): G = 144.11; G2 = 143.89; G3 = 141.14; G4 = 142.79; G5 = 132.15; G6 = 131.67. It was concluded that the evaluated polymerization and insertion techniques did not affect marginal microleakage, but a decrease in microhardness occurred when progressive polymerization was carried out.

Acrylic Resins↗