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PubMed2000

Contemporary resin-based composite materials for direct placement restorations: packables, flowables and others.

Resin-based composite (RBC) materials are composed of principal and diluent monomers, fillers, coupling agents and other components. These materials have been developed in recent years to produce packable and flowable materials, and materials which may release ions under specific conditions and which may contain fibres. Materials which are organic-inorganic copolymers have also become available. Long term clinical trial data are not available for the most recently introduced materials.

Aluminum Silicates↗
PubMed1991

Phenomenological theory of GC/AT pressure on DNA base composition.

We present a phenomenological theory expressing the constraints operating on the (G + C) contents of the three codon positions, i.e., first, second, and third bases of codons, by using the smallest number of constraint parameters having clear physical and genetic meaning. Theoretical curves displaying base composition at each of the three codon sites are given. The agreement between the theoretical curves and the data points of 1277 genes is quite good irrespective of the species from which the DNAs originated; the curves might be universal ones and the constraint parameters might have general biological meanings in relation to the DNA/RNA and protein functions.

Base Composition↗
PubMed2004

Water sorption characteristics of light-cured dental resins and composites based on Bis-EMA/PCDMA.

The water uptake characteristics of resins and composites based on an ethoxylated bisphenol A glycol dimethacrylate (Bis-EMA) and a polycarbonate dimethacrylate (PCDMA) were studied in detail. Polydimethacrylate resins were prepared by photopolymerization of the neat monomers and mixtures of them with various weight ratios, using the camphoroquinone/N,N-dimethylaminoethyl methacrylate system as initiator, while the composites were prepared from the light-curing of commercial samples (Sculpt-It and Alert). Water sorption/desorption was examined both in equilibrium and dynamic conditions in two adjacent sorption-desorption cycles. The equilibrium water uptake from all resins was very small with a trend to increase as the amount of PCDMA was increased. The inverse effect was observed in the solubility values. The composites studied exhibited also very low water uptake values in comparison to other composite materials reported in the literature. It was also observed that the equilibrium uptake decreased with increasing filler loading. Slightly larger equilibrium water uptake and much smaller solubility values were obtained during the second sorption-desorption cycle in comparison to the first one. Concerning the sorption rate data, it was observed that the resin materials followed Fickian diffusion during almost the whole sorption or desorption curve, while the composites showed this behavior until only M(t)/M( infinity ) congruent with 0.5. The diffusion coefficients calculated for the resins were larger than those of the composites and always higher during desorption compared to sorption. The values of the diffusion coefficients for both resins and composites were in the same order of magnitude with the values of the corresponding materials reported in the literature.

Composite Resins↗
PubMed1984

High performance liquid chromatography preparation of the molecular species of GM1 and GD1a gangliosides with homogeneous long chain base composition.

A semi-preparative, analytical high performance liquid chromatographic (HPLC) procedure is described for the isolation of molecular species of GM1 and GD1a gangliosides containing a single long chain base, C18 or C20 sphingosine, C18 or C20 sphinganine, each in its natural erythro or unnatural threo form. The threo forms were obtained from 2,3-dichloro-5,6-dicyanobenzoquinone/NaBH4 -treated gangliosides. The ganglioside molecular species separated by HPLC were analyzed for carbohydrate, fatty acid, and long chain base composition. In particular, long chain bases were submitted to gas-liquid chromatographic-mass spectrometric analyses as their trimethylsilyl (TMS) or N-acetyl-TMS derivatives, and chain length, presence or absence of C4-C5 double bond, and C-3 steric configuration were ascertained. The final preparations of individual molecular species of GM1 and GD1a gangliosides were more than 99% homogeneous in their saccharide moiety, contained a single long chain base (homogeneity higher than 99%), and had a fatty acid composition primarily of stearic acid (92 to 97%). All the individual molecular species of GM1 and GD1a gangliosides were also prepared in radioactive form by selective tritiation at C-3 of the long chain base. Their specific radioactivity ranged from 1.3 to 1.45 Ci/mmol. The availability of these molecular species of gangliosides is expected to facilitate studies aimed at ascertaining the role played by the hydrophobic portion in the functional behavior of gangliosides.

Chromatography, High Pressure Liquid↗
PubMed2004

Raman spectroscopy of carbon-nanotube-based composites.

Recent developments in the application of Raman spectroscopy to carbon-nanotube-based composite materials are reviewed. This technique may be used to identify carbon nanotubes, access their dispersion in polymers, evaluate nanotube/matrix interactions and detect polymer phase transitions. The Raman spectra of nanotubes can also be used to quantify the strain or stress transferred to nanotubes from the surrounding environment and to investigate local stresses and strains in polymers and composites. A polarized Raman technique was developed to detect the stress or strain in a matrix using randomly dispersed single-walled nanotubes. This technique has been used to detect and map stress fields in model fibre-polymer composites. The stress distributions around fibre breaks were mapped and compared with classical load transfer models.

Carbon↗
PubMed2006

Neutral evolution of synonymous base composition in the Brassicaceae.

The GC content of synonymous sites is elevated in genes from both Brassica oleraceae and Arabidopsis lyrata compared with Arabidopsis thaliana. However, this shift in base composition is independent of gene expression level, and there is no evidence for a similar difference in the frequency of codons preferred by translational selection. The results suggest that composition evolution is caused by a change in mutation bias or biased gene conversion, rather than by a reduction in the efficacy of natural selection in selfing Arabidopsis.

Arabidopsis↗
PubMed1963

BASE COMPOSITION OF THE DEOXYRIBONUCLEIC ACID OF SULFATE-REDUCING BACTERIA.

Sigal, Nicole (Laboratoire de Chimie Bactérienne du CNRS, Marseille, France), Jacques C. Senez, Jean Le Gall, and Madeleine Sebald. Base composition of the deoxyribonucleic acid of sulfate-reducing bacteria. J. Bacteriol. 85:1315-1318. 1963-The deoxyribonucleic acid constitution of several strains of sulfate-reducing bacteria has been analytically determined. The results of these studies show that this group of microorganisms includes at least four subgroups characterized by significantly different values of the adenine plus thymine to guanine plus cytosine ratio. The nonsporulated forms with polar flagellation, containing both cytochrome c(3) and desulfoviridin, are divided into two subgroups. One includes the fresh-water, nonhalophilic strains with base ratio from 0.54 to 0.59, and the other includes the halophilic or halotolerant strains with base ratio from 0.74 to 0.77. The sporulated, peritrichous strains without cytochrome and desulfoviridin ("nigrificans" and "orientis") are distinct from the above two types and differ from each other, having base ratios of 1.20 and 1.43, respectively.

Adenine↗
PubMed1998

Base-compositional biases and the bat problem. II. DNA-hybridization trees based on AT- and GC-enriched tracers.

We conducted a series of parallel DNA-hybridization experiments on a small group of bats (species of Pteropus, Rhinolophus, Noctilio and Pteronotus) and outgroups (Lemur, Cynocephalus, Didelphis), using whole-genome labels and tracers made from extracts enriched with AT and two levels of GC content. FITCH (additive phylogenetic trees) topologies were constructed from the four sets of comparisons, indexed as both delta Tmode and delta NPHs (normalized percentage of hybridization). Based on our previous work showing that the shared AT bias of pteropodids and some microchiropterans may affect the rank-ordering of taxa based on either AT- or GC-rich labels, our expectation was that the resulting trees would show differing topologies when generated from tracers made with the variously enriched DNA extracts. Whereas there was some variation among the trees, most of them grouped the bats together, and almost all paired the representative megachiropteran and rhinolophoid microchiropteran as sister-taxa in contrast to the other microchiropterans. As the pteropodid-rhinolophoid relationship is an unexpected and unlikely one, we attribute this association to an AT bias that was not obviated even by our most GC-rich labels, and suggest that such a bias may compromise the truth of some molecular trees. Accordingly, we believe the broader issue of bat monophyly remains unresolved by DNA-hybridization and probably also by gene-sequencing studies.

Animals↗
PubMed1970

Reexamination of the association between melting point, buoyant density, and chemical base composition of deoxyribonucleic acid.

The equations currently used for the calculation of the chemical base composition of deoxyribonucleic acid (DNA), expressed as moles per cent guanine plus cytosine (% GC), from either buoyant density (rho) or midpoint of thermal denaturation (T(m)) were recalculated by using only sets of data on DNA determined with the same strains. All available information from the literature was screened and supplemented by unpublished data. The results were calculated by regression and correlation analysis and treated statistically. From the data on 96 strains of bacteria, it was calculated that% GC = 2.44 (T(m) - 69.4). T(m) appears to be unaffected by the substitution of cytosine by hydroxymethylcytosine. This equation is also valid for nonbacterial DNA. From the data on 84 strains of bacteria, the relation% GC = 1038.47 (-1.6616) was calculated. The constants in this equation are slightly modified when data on nonbacterial DNA are included. Both correlations differ only slightly from those currently used, but now they lean on a statistically sound basis. As a control, the relation between rho and T(m) was calculated from data of 197 strains; it agrees excellently with the above two equations.

Bacteria↗
PubMed1972

Deoxyribonucleic acid base composition of the yeastlike and mycelial phases of Histoplasma capsulatum and Blastomyces dermatitidis.

The base composition in moles percent guanine plus cytosine (%GC) of both nuclear and mitochondrial deoxyribonucleic acid (DNA) isolated from the yeastlike and mycelial phases of the dimorphic fungal pathogens Histoplasma capsulatum and Blastomyces dermatitidis was determined by techniques of thermal denaturation and CsCl buoyant density gradient equilibrium centrifugation. The mean observed values for GC content of nuclear DNA from H. capsulatum and B. dermatitidis were 47.3 and 48.2%, respectively. What is speculated to be mitochondrial DNA was found to be 34.0% for H. capsulatum and 34.3% for B. dermatitidis. Thermal denaturation curves for Blastomyces DNA indicated a bimodality in thermal denaturation profiles, thereby suggesting a significant mitochondrial DNA contamination. Mitochondrial DNA appeared to represent a smaller percentage of the total DNA prepared from Histoplasma, and was not observed consistently to affect%GC values as determined by thermal denaturation profiles. On the basis of the now known perfect stage of B. dermatitidis (Ajellomyces dermatitidis) as a member of the family Gymnoascaceae, the close approximation of%GC content of nuclear DNA of this fungal organism with that of H. capsulatum suggests possible phylogenetic relationship. It is suggested that the just reported, but as yet unclassified, perfect stage of H. capsulatum may be found to be phylogenetically a primitive form of the Gymnoascaceae.

Biological Evolution↗
PubMed1969

Characterization and relatedness of marine vibrios pathogenic to fish: deoxyribonucleic acid homology and base composition.

Deoxyribonucleic acid (DNA) isolated from several pathogenic marine vibrios was utilized in studies of base composition and nucleotide sequence relationships. Patterns of relatedness inferred from DNA criteria were found to correlate closely with those inferred from morphological, physiological, and serological analysis of the same organisms. The utility of the phenotypic and genotypic approaches to the determination of relatedness is discussed.

Adenine↗
PubMed2000

Microleakage of Class II resin-based composite restorations with flowable composite in the proximal box.

PURPOSE: To compare microleakage of Class II hybrid resin-based composite (RBC) restorations with and without low viscosity (flowable) RBC in the proximal box. MATERIALS AND METHODS: Conservative Class II cavity preparations were made in 50 extracted premolars randomly assigned to five groups (n = 10). OptiBond FL dentin bonding agent, and Prodigy hybrid RBC were used to restore all cavity preparations. Preparations in four of the five groups were restored with flowable RBC as the initial increment in the proximal box. The four flowable RBCs tested were: AEliteFlo, Revolution, UltraSeal XT and Flow-It. In the control group, the entire preparation was restored only with hybrid RBC. Specimens were stored in water at 37 degrees C for 24 hrs after restoration, thermocycled 1,000x, immersed in 2% basic fuchsin, sectioned and evaluated for microleakage. Microleakage was scored on a 0-4 scale. RESULTS: Non-parametric Kruskal-Wallis one-way ANOVA followed by Wilcoxon signed ranks test was used for statistical analysis. The use of any of the four flowable RBCs tested did not influence microleakage significantly (P > 0.05) in Class II RBC restorations.

Analysis of Variance↗
PubMedDate not supplied

Polymerization efficiency of curing lamps: a universal energy conversion relationship predictive of conversion of resin-based composite.

A universal energy-conversion relationship (ECRu) predictive of conversion of a resin-based composite (RBC) polymerized with any light source has been described. This relationship was derived from an energy conversion relationship for RBC polymerized with a tungsten-halogen lamp and the lamp's efficiency relative to a hypothetical standard lamp. The ECRu was then used to predict conversion throughout RBC polymerized with an LED lamp using the lamp's relative efficiency compared to the standard lamp. The universal energy scale has also been described as predictive of scrape-back lengths for this RBC family when polymerized with any light source. Despite a 31% greater relative efficiency, scrape-back lengths from RBC polymerized using the LED lamp were predicted to be only 6% greater than those polymerized with the tungsten-halogen lamp when RBC is polymerized on an equal energy basis. This result was experimentally verified.

Algorithms↗
PubMed1986

[Relationship between base composition in non-coding DNA of genes and codon composition].

The C + G percentage in third position of codons is linearly dependent on the C + G composition of flanking regions and introns. A similar relationship is shown for the first and second position which significantly influence the nature of amino acid sequence. If mutations would be oriented according to the local base composition, this will imply that genes of the same multigenic family would evolve at different rate.

Base Composition↗
PubMed1983

Interaction of purified rat liver glucocorticoid-receptor complexes with polynucleotides: strong base composition dependence.

The binding of 10(3)-fold purified rat liver glucocorticoid receptors to natural and synthetic polynucleotides has been studied. The receptors may bind to RNA as well as to DNA. The binding effectiveness of nucleic acids is strongly base composition dependent, with natural DNAs (double-stranded or denatured) greater than E. coli rRNA greater than poly(dA).poly(dT) greater than or equal to poly d(A,T,G)7 greater than or equal to poly(U) greater than poly(A).poly(U) greater than poly(A) approximately poly(C). The equilibrium (apparent) constants of receptor binding to a number of polynucleotides were calculated by a method proposed.

Animals↗
PubMed2003

Effect of bifunctional comonomers on mechanical strength and water sorption of amorphous calcium phosphate- and silanized glass-filled Bis-GMA-based composites.

This study seeks to elucidate structure-property relationships in a series of unfilled dental copolymers and their composites. The copolymers/composites were derived from photo-activated binary monomer systems based on 2,2-bis[p-2'-hydroxy-3'-methacryloxypropoxy)phenyl] propane (Bis-GMA) and equimolar amounts of a bifunctional, surface-active comonomer, i.e., 2-hydroxyethyl methacrylate (HEMA), glycerol dimethacrylate (GDMA) or ethylene glycol methacrylate phosphate (PHEMA). Triethyleneglycol dimethacrylate, a widely used comonomer for Bis-GMA, was used as a control. Two types of fillers were investigated: (1) a hydrophilic, silica-modified amorphous calcium phosphate (Si-ACP) and (2) a more hydrophobic, silanized nanosized silica (n-SiO(2)). Both the unfilled copolymers and their composites were evaluated for biaxial flexure strength (BFS), both dry and wet after 30 days immersion in buffered saline, and for water sorption (WS) and their WS kinetic profiles. The Bis-GMA copolymers and composites derived from HEMA and GDMA had BFS and WS values, as well as WS kinetic profiles, similar to the controls. Copolymers and composites based on Bis-GMA/PHEMA had lower BFS and higher WS values. Si-ACP composites had significantly lower BFS values (that were further diminished on soaking) than their copolymers. WS increased as the level of this filler was increased except for Bis-GMA/PHEMA composites. With n-SiO(2) as the filler, a more moderate reduction in BFS occurred compared to the unfilled copolymers. By contrast to Si-ACP composites, the WS of all the n-SiO(2) composites decreased with increasing filler level. From this study it is evident that both the chemical structure of the polymer matrix and the type of filler system can have significant effects on the strength and water-related properties of dental composites.

Absorption↗