bluuebunny/crossref_metadata_2025_split huggingface dataset:
Organoapatites: Materials for artificial bone. I. Synthesis and microstructure
Authors: Stupp Samuel I. , Ciegler Glenn W. 
Published: December 2004

AbstractWe have synthesized a new family of materials we termed organoapatites which may be useful in the formulation of artificial bone. These materials are synthesized by nucleation and growth of apatite crystals in media containing poly(amino acids) or synthetic organic polyelectrolytes using strict atmospheric, temperature, and pH control. The macromolecules used to synthesize the organoapatites include poly(L‐lysine), poly(L‐glutamic acid), and poly(sodium acrylate). The products were characterized by x‐ray diffraction, scanning electron microscopy, surface area measurements, elemental analysis, and spectroscopic techniques. Organoapatites were found to contain large surface area morphologies with small crystallites which mature slowly based on analysis of Ca/P ratios. The organic macromolecules are thought to induce nucleation of crystals but also to quench their growth, thus becoming intimately dispersed in a mineral network. The organomineral particles harvested from the reaction medium contain polymernetted microcrystals, and for this reason the synthetic approach can be used to modulate crystal maturation and biological response. It is likely that the preparative approach mimics some aspects of natural bone matrix synthesis and could be specially useful in the preparation of mineral implants containing intimate dispersions of small amounts of biomolecules such as growth factors, special drugs, or bioadhesives.
Microbial adherence on poly(methyl methacrylate) (PMMA) surfaces
Authors: Chang Chung‐Che , Merritt Katharine 
Published: December 2004

AbstractInfection remains a major complication following the use of implanted biomaterials. Often these infections are caused by low‐virulence organisms or by a mixture of organisms (polymicrobial). In this study two methods were used to quantitate the bacteria which had adhered to poly(methyl methacrylate) (PMMA) samples. The bacteria were eluted from the sample using an ultrasonic cleaner. The number eluted was then counted by colony counts, which determines viable organisms and by particle counting which counts both viable and nonviable organisms. A known adherent strain of Staphylococcus epidermidis and a strain of Proteus mirabilis were used. In general the adherence of S. epidermidis was greater than that of Proteus. When the two organisms were used together, there was an alteration in the adherence pattern which generally increased the adherence of Proteus and had no effect or decreased the adherence of S. epidermidis. The use of both quantitation techniques provided important information on the adherence of organisms to PMMA to which gentamicin had been added. It was evident that organisms did adhere to the PMMA plus gentamicin samples but were not viable when eluted. The amount of adherence to PMMA plus gentamicin was similar to that of PMMA alone at 30 min but was markedly decreased at 24 h. There was a considerable dead biofilm mass on the PMMA plus gentamicin samples which might be a significant promoter of late infections by providing a surface attractive to other strains of bacteria.
Leucite content of selected dental porcelains
Authors: Piché Peter W. , O'Brien William J. , Groh Carole L. , Boenke Kenneth M. 
Published: December 2004

AbstractLeucite is a major crystalline component of dental porcelains. The presence of tetragonal leucite in dental porcelains increases their coefficients of thermal expansion due to its high coefficient of thermal expansion (20–25 × 10−6/°C). This is particularly useful for those porcelains designed for bonding to precious metals and nickel alloys. The purpose of this study was to determine the leucite content of selected commercial dental porcelains in relation to their coefficient of thermal expansion values. The weight fraction of leucite was determined with quantitative x‐ray diffraction using copper as an internal standard. Coefficient of thermal expansion values were determined using a thermal dilatometric analyzer. Five commercial body porcelains were studied. Leucite was not detected in samples of Vitadur N and Duceram LFC. An ANOVA showed that there was a significant difference in the weight fraction of leucite for Silhouette, Ceramco II, and Optec HSP porcelains. Linear regression revealed a correlation (R = 0.91) between weight fraction of leucite and the coefficient of thermal expansion for those samples containing leucite. Ducram LFC, which is recommended by the manufacturer for use with metals and leucite‐containing porcelains, had no detectable leucite although the coefficient of thermal expansion was found to be 13.2 ± 0.4 × 10−6/°C at 25–472°C. A low glass transition temperature contributed to the high average coefficient of thermal expansion value. © 1994 John Wiley & Sons, Inc.
In vivo induction of macrophage Ia antigen (MHC class II) expression by biomedical polymers in the cage implant system
Authors: Petillo O. , Peluso G. , Ambrosio L. , Nicolais L. , Kao W. J. , Anderson J. M. 
Published: December 2004

AbstractExamination of the cellular components in the inflammatory exudate, which infiltrates subcutaneous cages, can be used to monitor the progress of an inflammatory response to an implanted material. Of particular interest is the study of monocyte/ macrophage infiltration into the implanted cages containing biomaterials, as macrophages may initiate a wide spectrum of responses upon interaction with a foreign material. In this study, the authors propose a technique using subcutaneous tissue cages in conjunction with cytofluorimetric analysis of exudate leukocytes to evaluate the monocyte/macrophage cell activation in response to different materials. The studies reported here used several materials. The studies reported here used several materials (thermoplastic and elastomeric polymers) as the challenging agent, to demonstrate whether polymers, chemically different from each other, could differentially activate macrophages to carry out their proinflammatory role more effectively. The materials tested included: poly(etherurethane ureas) (PEUU A'), poly(etherurethane ureas) with a surface active additive, Methacrol®, (PEUUC'), polymethylsiloxane (PDMS), polyetherimide, (PEI), and polyetheretherketone, (PEEK). For all the tested materials, the maximum numbers of exudate cells and of Ia‐positive macrophages were found on day 7, although the entity of the cell increase was associated with the material used for the implant. Similarly, the percentage of Ia‐positive macrophages varied according to the specific polymer present in the cages after 7 days. By day 14, the percentage of Ia‐positive macrophages decreased with individual exudates showing 19–32% Ia‐positive cells depending on the different type of material. Only in the case of PDMS did the percentage of Ia‐positive macrophages remain the same as compared with control empty cage macrophages. © 1994 John Wiley & Sons, Inc.
A systematic preparation of new contracted Gaussian‐type orbital sets. IV. The effect of additional 3s functions introduced by the use of the six‐membered 3dGTOs
Authors: Tatewaki Hiroshi , Sakai Yoshiko , Huzinaga Sigeru 
Published: December 2004

AbstractThe Gaussian‐type basis sets for molecular calculations are usually prepared by an atomic SCF program in which spherical coordinates are used. On the other hand, many molecular SCF and CI programs are written by using the Cartesian coordinates and as a result six‐membered d‐type functions (x2, y2, z2, xy, yz, zx)e−γr2 are often used. They contain one additional 3s function which does not exist in the atomic calculation. Therefore, we shall have an incorrect, deeper molecular binding energy, unless we readjust the atomic total energy by adding the 3s orbital to the original basis set. Some examples are shown in the case of Cu2 molecule, where we have found that the correction is quite appreciable, which was overlooked in previous work.
Isomer discrimination by topological information approach
Authors: Bonchev D. , Mekenyan O. V. , Trinajstić N. 
Published: December 2004

AbstractA comparative analysis of ten topological indices is made. No index is found to discriminate isomers uniquely. A combined topological index, named the superindex, consisting of a number of topological indices is proposed. Information theory is applied to express all components of the superindex on a common quantitative scale. The superindex is tested on the sample of 427 graphs consisting of all acyclic, monocyclic, and bicyclic graphs with 4–8 vertices.
The conformation of alkyl sulfoxides
Authors: Tai Julia C. 
Published: December 2004

AbstractParameters for sulfoxides used in force field MM1 were modified to be incorporated into force field MM2. The conformations of ten alkyl sulfoxides were then calculated using MM2 with these new parameters. The alkyl groups used were methyl, ethyl, isopropyl, and t‐butyl. It was found that of the many possible conformations for these compounds, only one or two stable conformers exist, and that the number of these conformers agrees with the number of reported SO stretching frequencies in almost every case. No apparent correlation between the vibration frequency and the molecular structure was found.
Molecular orbital theory of the hydrogen bond. 24. Ground‐state water–uracil complexes
Authors: Del Bene Janet E. 
Published: December 2004

AbstractAb initio SCF calculations with the STO‐3G basis set have been performed to investigate the structural, energetic, and electronic properties of mixed water–uracil dimers formed at the six hydrogen‐bonding sites in the uracil molecular plane. Hydrogen‐bond formation at three of the carbonyl oxygen sites leads to cyclic structures in which a water molecule bridges N1H and O2, N3H and O2, and N3H and O4. Open structures form at O4, N1H, and N3H. The two most stable structures, with energies of 9.9 and 9.7 kcal/mole, respectively, are the open structure at N1H and the cyclic one at N1H and O2. These two are easily interconverted, and may be regarded as corresponding to just one “wobble” dimer. At 1 kcal/mole higher in energy is another “wobble” dimer consisting of an open structure at N3H and a cyclic structure at N3H and O4. The third cyclic structure at N3H and O2 collapses to the “wobble” dimer at N3H and O4. The two “wobble” dimers are significantly more stable than the open dimer formed at O4, which has a stabilization energy of 5.4 kcal/mole. Uracil is a stronger proton donor to water through N1H than N3H, owing to a more favorable molecular dipole moment alignment when association occurs through H1. Hydration of uracil by additional water molecules has also been investigated. Dimer stabilization energies and hydrogen‐bond energies are nearly additive in most 2:1 water:uracil structures. There are three stable “wobble” trimers, which have stabilization energies that vary from 7 to 9 kcal/mole per water molecule. Hydrogen‐bond strengths are slightly enhanced in 3:1 water:uracil structures, but the cooperative effect in hydrogen bonding is still relatively small. The single stable water–uracil tetramer is a “wobble” tetramer, with two water molecules which are relatively free to move between adjacent hydrogen‐bonding sites, and a stabilization energy of approximately 8 kcal/mole per water molecule. Within the rigid dimer approximation, successive hydration of uracil is limited to the addition of one, two, or three water molecules.
Molecular orbital theory of the hydrogen bond. 25. Water–uracil complexes in excited n → π states
Authors: Del Bene Janet E. 
Published: December 2004

AbstractHydrogen bonding of uracil with water in excited n → π* states has been investigated by means of ab initio SCF‐CI calculations on uracil and water–uracil complexes. Two low‐energy excited states arise from n → π* transitions in uracil. The first is due to excitation of the C4O group, while the second is associated with excitation of the C2O group. In the first n → π* state, hydrogen bonds at O4 are broken, so that the open water–uracil dimer at O4 dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N3H and a cyclic structure at N3H and O4 in the ground state, collapses to a different “wobble” dimer at N3H and O2 in the excited state. The third dimer, a “wobble” dimer at N1H and O2, remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O2 are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N1H and O2 changes to an excited open dimer at N1H. The “wobble” dimer at N3H and O4 remains intact, and the open dimer at O4 is further stabilized upon excitation. Dimer blue shifts of n → π* bands are nearly additive in 2:1 and 3:1 water:uracil structures. The fates of the three 2:1 water:uracil trimers and the 3:1 water:uracil tetramer in the first and second n → π* states are determined by the fates of the corresponding excited dimers in these states.
Model for diffusion‐limited electrochemiluminescence
Authors: Akins Daniel L. , Snider Arthur David 
Published: December 2004

AbstractAn approximate analytic expression is derived, based on a novel model for the reactive–diffusive processes in a electrochemiluminescence (ecl) experiment. The physical system treated is one in which ecl is produced by stepping the voltage of a planar electrode to the diffusion‐limited reduction value for an emitter species in the presence of a bulk oxidant precursor (9,10‐DPA with benzoyl peroxide). The unknown rate constants are determined by a least‐squares curve‐fitting computer code. The model, though crude, provides an excellent fit to the ecl intensity versus time data, including the rise portion of the curve.
The electronic structure, spectra, and reactivity of nitrophenols
Authors: Namboodiri K. P. Krishnan , Viswanathan S. , Ganesan R. , Bhasu V. C. Jyothi 
Published: December 2004

AbstractThe all‐valence‐electron CNDO/2 calculations were performed for the three isomeric nitrophenols. Using the newly derived σ‐core charges and subsequently revising the valence‐state ionization potentials and one‐center two‐electron repulsion integrals, Pariser–Parr–Pople (PPP) CI calculations were performed on the title compounds following the Nishimoto–Forster scheme. A better agreement between theory and experiment has been observed in spectral assignments compared to the conventional PPP approach. Information from the CNDO/2 calculations was used to obtain useful electronic structural parameters and to get a quantitative insight into the chemical reactivity of these molecules. All the results were compared with the basic compounds, phenol and nitrobenzene. The electronic spectra of these isomers were recorded in both polar and nonpolar solvents.
Three‐Electron bonds. II. SS and SCL three‐electron bonds
Authors: Clark Timothy 
Published: December 2004

AbstractAb initio molecular orbital calculations are reported for the H2SSH· and H2SCl· radicals and for the H2SClH+· radical cation. The two neutral species are found to be very weakly bound van der Waals' complexes, whereas the H2SClH+· radical cation is bound by 11.9 kcal mol−1 at MP2/4‐31G. The importance of charge in σ* radicals is discussed.
Geometry and energy of overcrowded ethylenes. II. Bornanylidene, fenchylidene, and bifluorenylidene derivatives
Authors: Favini G. , Simonetta M. , Sottocornola M. , Todeschini R. 
Published: December 2004

AbstractThe geometry and energy of some bornanylidene, fenchylidene, and bifluorenylidene derivatives have been determined by the molecular mechanics method. Low twisting of the double bond was found for the compounds of the first two series. The difficulties found in the synthesis of 2‐di‐tert‐butylmethylenefenchane can be related to a remarkable strain energy which is only slightly lower than that of tetra‐tert‐butylethylene. A twisted conformation is the preferred one in bifluorenylidene and its 1,1′‐dimethyl derivative (E isomer) in agreement with experimental data. Conformational inversion and E–Z interconversion paths are discussed for the latter compound and theoretical results are compared with dynamic NMR data.
Theoretical studies on “acetylenic zipper” reaction intermediates
Authors: Hass Ernst C. , Mezey Paul G. , Abrams Suzanne R. 
Published: December 2004

AbstractNonempirical molecular orbital studies on a series of isomeric octyn‐1‐ols indicated that the anomalous stability of the alkoxide of 3‐octyn‐1‐ol isomer is probably related to stereochemical and polarization properties of the molecule. The calculations correlated with the experimental observation that the least stable isomer should be the terminal alkyne, 7‐octyn‐1‐ol.
Evaluation of MINDO/3 calculated structures. III. Saturated acyclic compounds with chlorine, nitrogen, oxygen, or sulfur
Authors: McManus Samuel P. , Smith Maurice R. , Shafer Steven G. 
Published: December 2004

AbstractFifty‐one structures have been calculated by the MINDO/3 method to evaluate the errors introduced by branching and by the presence of the heteroatom. The structures are evaluated by comparisons which reveal that the calculated ΔHf values reflect a bias because of the presence of the heteroatom. With two carbons or more in a chain attached to a heteroatom group, a linear relationship exists which makes possible the calculation of reasonably accurate ΔHf values for unbranched alcohols, primary or secondary amines, ethers, thioethers, thiols, and alkyl chlorides. Branching errors do not seem to be linearly related among the systems. Some errors in calculations of geometries are also discussed.
Computer enumeration and generation of benzenoid hydrocarbons and identification of bay regions
Authors: Knop J. V. , Szymanski K. , Jeric̃ević Z̃. , Trinajstić N. 
Published: December 2004

AbstractA computer‐oriented algorithm is developed for enumerating and generating all rigorously planar benzenoid hydrocarbons. An additional algorithm is also developed for identification and enumeration of carcinogenic bay regions. The results are reported for undisrupted benzenoid structures up to ten fused rings. The reported numbers of isomeric benzenoids do not agree with the results in the literature, because other authors included in their considerations some polyhexes which are not strictly planar structures (e.g., helicenes) or which represent rings of hexagons (e.g., circulenes). The reported results for enumeration of bay regions fully agree with those of Balasubramanian et al.
Restricted Hartree–Fock approximation. II. Computational aspects of the direct minimization procedure
Authors: Fern Rico J. ÁNdez , Paniagua M. , Fern Alonso J. I. ÁNdez , Fantucci P. 
Published: December 2004

AbstractA RHF energy minimization procedure based on the treatment outlined in Part I of this series of articles is presented. Test calculations performed on several closed‐ and open‐shell systems show that the present procedure is definitely superior to the conventional SCF methods. In particular, the convergence of this procedure is ensured, the rate of convergency is high, and the computational cost of each cycle is low.
Symmetry properties of chemical graphs. V. Internal rotation in XY⋅3XY⋅2XY3
Authors: Randić Milan 
Published: December 2004

AbstractStructures XY⋅3XY⋅2XY3 of symmetry C2v (of which propane is an example) are examined and the rearrangement due to the internal rotation of the end groups XY3 studied. The isomerization graph is constructed, various forms of which are displayed and the symmetry of which has been determined. The order of the group is 72. There are nine prime (irreducible) representations (4A + E + 4G) with the following partitioning of the elements into classes: 1, 42, 62, 9, 122, 18. When the mechanism for rearrangement is generalized to include enantiomers, a duplex graph is produced with the order of the group 144 which is isomorphic to the group S2(S3,S2) (generalized wreath product of the symmetric group S2 and S3). The corresponding graph has been constructed and displayed in one of more symmetrical forms. Isomorphism of groups of order 144 is discussed and a procedure is outlined in which correspondence between distinctive combinatorial objects is established by inducing permutations of m elements from available permutations of n elements. The scheme is based on selection of suitable graph invariants in one system and their labeling as m objects which form the basis for representation of the symmetry for the other system.
CHARMM: A program for macromolecular energy, minimization, and dynamics calculations
Authors: Brooks Bernard R. , Bruccoleri Robert E. , Olafson Barry D. , States David J. , Swaminathan S. , Karplus Martin 
Published: December 2004

AbstractCHARMM (Chemistry at HARvard Macromolecular Mechanics) is a highly flexible computer program which uses empirical energy functions to model macromolecular systems. The program can read or model build structures, energy minimize them by first‐ or second‐derivative techniques, perform a normal mode or molecular dynamics simulation, and analyze the structural, equilibrium, and dynamic properties determined in these calculations. The operations that CHARMM can perform are described, and some implementation details are given. A set of parameters for the empirical energy function and a sample run are included.
Efficient diffuse function‐augmented basis sets for anion calculations. III. The 3‐21+G basis set for first‐row elements, Li–F
Authors: Clark Timothy , Chandrasekhar Jayaraman , Spitznagel Günther W. , Schleyer Paul Von Ragué 
Published: December 2004

AbstractThe relatively small diffuse function‐augmented basis set, 3‐21+G, is shown to describe anion geometries and proton affinities adequately. The diffuse sp orbital exponents are recommended for general use to augment larger basis sets.
Uracil–4‐Hydroxyuracil tautomerism revisited
Authors: Zielinski Theresa Julia 
Published: December 2004

AbstractThe tautomerism of the uracil‐4‐hydroxyuracil system is examined with the 3–21G basis set and full geometry optimization using gradient techniques. Single‐point calculations at the 6‐31G level were also done and the results compared with the 2‐pyridone–2‐hydroxypyridine system. The best ΔE(taut) obtained in this work is 74.1 kJ/mol.
Ab initio study of catalyzed and uncatalyzed amide bond formation as a model for peptide bond formation: Ammonia‐Glycine reactions
Authors: Oie Tetsuro , Loew Gilda H. , Burt Stanley K. , Macelroy Robert D. 
Published: December 2004

AbstractAs a model reaction for peptide and bond formation, the SN2 reactions between glycine and ammonia have been studied with and without amine catalysis:

using ab initio molecular‐orbital methods. For each of the catalyzed and uncatalyzed reactions, two reaction mechanisms have been examined: a two‐step and a concerted mechanism. The stationary points of each reaction, including intermediate and transition states, have been identified and free energies calculated for all geometry‐optimized reaction species to determine the thermodynamics and kinetics of the reaction. The calculations demonstrate that a second ammonia molecule catalyzes amide bond formation, and that the two‐step mechanism is more favorable than the concerted one for the catalyzed reaction, while for the uncatalyzed reaction both mechanisms are competitive.
Solvent dielectric attenuation of substituent effects. Dependence on boundary representation in prolate spheroidal cavity models
Authors: Ehrenson S. 
Published: December 2004

AbstractExtension of the inhomogeneous continuum solvent model to prolate spheroidal cavity systems in the context of Kirkwood–Westheimer substituent‐reactivity theory is described. Reasonable effects attributable to electrical saturation and electrostriction, which are modeled by relatively simple spatial dielectric functions outside the solute–solvent boundary, may be demonstrated. It is also shown that choices of proper (i.e., nonaveraged) location of the interacting sites and magnitude of substituent dipole moments are comparably important to the quality of theoretical prediction.