The reactions of O(3P) with ozone and carbonyl sulfide

The competitive reactions between 2-trifluoromethylpropene (TMP) and OCS for O(³P) atoms were studied between 300° and 523°K, using the mercury-senstitized photolysis of N₂O as a source of O(³P). From the known value for the rate constant of the O(³P) + TMP reaction, k₃ was found to be 1.6 × 10^?11 exp (?4500/RT) cm³/particle-sec, where reaction (3) is Mixtures of O₃ and OCS were photolyzed at 197°, 228°, 273°, and 299°K with radiation above 4300 Å to produce O(³P) from the photolysis of O₃, and thus study the competition between reaction (3) and From the above value of k₃, k₁ could be computed. When combined with all the previous data, the best espression for k₁ is k₁ = 1.2 × 10^?11 exp (?4300/RT) cm³/particle-sec.     

Semigroups satisfying variable identities

The purpose of this note is to generalize a theorem of Tamura’s [3] providing a self-contained and, we think, more elementary proof than Tamura’s in that it avoids using the theory of contents. Tamura’s result states that a semigroup S satisfies an identify xy=f(x,y) with f(x,y) a word of length greater than 2 which starts with y and ends in x if and only if S is an inflation of a semilattice of groups satisfying the same identity. We investigate semigroups as in Tamura’s Theorem, except that we permit f(x,y) to vary with x and y.     

Kinetics of particle growth VII. NH4NO3from the NH3-O3 reaction in the presence of air and water vapor

Our earlier work on the formation of particulate NH₄NO₃ in the NH₃? O₃ reaction at 25°C is extended to include air as a diluent and H₂O vapor as an additive. More extensive data at different values of [NH₃]/[O₃]₀ were obtained also, where [O₃]₀ is the initial O₃ concentration. In our earlier work we concluded that NH₄NO₃ vapor was dissociated to NH₃ + HNO₃ and that the HNO₃ was removed by diffusion to the walls with a rate coefficient k_diff = 0.4 min^?1 or by condensation on the suspended particles. Particles were nucleated by 8 NH₃? HNO₃ pairs when their concentration product reached 5.8 × 10²⁷ molec²/cm⁶ with a rate coefficient k_nucl of 6.2 × 10^?224 cm⁴⁵/min and removed by coagulation with a rate coefficient k_coag of 1.3 × 10^?7 cm³/min. A corrected calculation modifies the number of pairs required to 6–7 with a correspondingly changed value of k_nucl. With the more extensive data of the present study the indications are that the vapor-phase NH₄NO₃ monomer is not dissociated and that its diffusion constant for loss to the walls varies between 0.3 and 0.9 min^?1 for different reaction conditions. Nucleation occurs when the NH₄NO₃ vapor concentration reaches 1.0 × 10¹² molec/cm³ via. where r is 9 and the nucleation rate coefficient k_nucl is 3 × 10^?108 cm²⁴/min. With 5.0 or 9.5 torr of H₂O vapor present, there is an excess of particles produced over that expected from this rate coefficient, indicating an additional nucleation step in which H₂O vapor participates directly to produce a hydrated salt. The coagulation coefficient of (1.87 ± 0.14) × 10^?7 cm³/min found here is in good agreement with that found previously.     

New method for estimating survival curves based upon subpopulations

The overall Kaplan-Meier estimate of the survival curve underlying a study population does not individually account for the ages at which patients were first seen and thence followed up in the study. Since the number of patients in these age-specific subgroups varies quite arbitrarily for a given study, the overall Kaplan-Meier curve which assumes that all patients were seen from the beginning of the study usually either overestimates or underestimates the survival probability, depending on the varying sample sizes. In this paper, we therefore propose an estimate of the overall survival curve underlying a study population based on the individual age-specific Kaplan-Meier estimates and the probability distribution of the age at which patients were first seen in the study. In the particular case when there is no censoring, it is shown that the usual overall Kaplan-Meier survival curve which assumes that all patients irrespective of their age at entry into the study were followed from the beginning of the study is identical with the proposed estimate. This result does not seem to necessarily hold good in the presence of arbitrary censoring. Further, the estimate is unbiased and consistent, a property which the Kaplan-Meier estimate also enjoys. The efficiency of the proposed estimate relative to the Kaplan-Meier estimate is being investigated and will be reported along with results of its application in a separate communication.     

Structural studies of apatite-type oxide ion conductors doped with cobalt

A series of Co doped lanthanum silicate apatite-type phases, La9.83Si4.5Co1.5O26, La9.66Si5CoO26, La10Si5CoO26.5 and La8BaCoSi6O26, have been synthesised, and neutron diffraction, EXAFS and XANES used to investigate their structures in detail. All compositions were shown to possess the hexagonal apatite structure, and the results confirmed that cobalt can be doped onto both the La and Si sites within the structure depending on the starting composition. The Co doping is shown to cause considerable local distortions within the apatite structure. In the case of Si site doping two compositions showed anisotropic peak broadening, which has been attributed to incommensurate ordering of oxygen within the apatite channels.     

Many triangulated 3-spheres

We construct combinatorial types of triangulated 3-spheres on n vertices. Since by a result of Goodman and Pollack (1986) there are no more than 2^{O(n log n)} combinatorial types of simplicial 4-polytopes, this proves that asymptotically there are far more combinatorial types of triangulated 3-spheres than of simplicial 4-polytopes on n vertices. This complements results of Kalai (1988), who had proved a similar statement about d-spheres and (d+1)-polytopes for fixed d4.Mathematics Subject Classification (1991): 52B11, 52B70, 57Q15Supported by the Deutsche Forschungsgemeinschaft within the European graduate program Combinatorics, Geometry, and Computation (GRK 588/1) and an MSRI post-doctoral fellowship.Partially supported by Deutsche Forschungs-Gemeinschaft (DFG), via the DFG Research Center Mathematics in the Key Technologies (FZT86), the Research Group Algorithms, Structure, Randomness (Project ZI 475/3), and a Leibniz grant (ZI 475/4).     

Large-Scale Production of 3D Bioactive Glass Macroporous Scaffolds for Tissue Engineering

For tissue engineering applications, a scaffold is required that can act as a template and guide for cell proliferation, cell differentiation and tissue growth. Interconnected pores with diameters greater than 100 m are required for tissue ingrowth, vascularisation and nutrient delivery to the centre of the scaffold. 3D bioactive glass scaffolds have been produced, by foaming sol-gel derived bioactive glasses. The method to produce foams with a modal macropore diameter of 100 m, and a handling strength suitable for cell culture, was to foam 50 ml batches of sol with the aid of a surfactant and gelling agent. In vitro and in vivo tests show that the scaffolds have high potential to be used in bone tissue engineering applications. Larger batches are required to produce scaffolds commercially. The aim of this work was to investigate how the process could be up-scaled for commercial use. This study shows that foaming larger aliquots of sol decreased the scaffold porosity and interconnectivity and investigates methods of modifying the process to obtain large quantities of foam scaffolds with pores in excess of 100 m. The optimum method to produce foams of similar pore structure from 200 ml sol to those produced from 50 ml sol comprised of adding 3 ml surfactant and 12 ml dionised water to the sol to start foaming and injecting a gas mixture (70% helium, 30% nitrogen) at 0.2 bar while applying vigorous agitation.     

Effects of ultraviolet B exposure on the expression of proliferating cell nuclear antigen in murine skin

Proliferating cell nuclear antigen (PCNA) is an active nuclear protein involved in DNA replication, recombination and repair. PCNA is found throughout the basal layer in normal skin and in all layers of the epidermis in malignancy. This study evaluates PCNA's expression after acute and chronic UV-B irradiation. Skh-1 hairless mice exposed to 1.5 and 4.5 kJ/m2 of UV-B were sacrificed at 6, 12, 24, 48, 72 and 168 h. Immunohistochemical analysis revealed PCNA expression throughout the basal layer of untreated skin, with diminished expression at 6 h, indicative of immediate UV damage, and evidenced by the observable upregulation in pyrimidine dimer formation early on. Subsequently, PCNA immunoreactivity progressively increased, demonstrating an aberrant upward epidermal migratory pattern in association with chronic exposure. The 4.5 kJ/m2 group exhibited prolonged recovery in staining and also demonstrated this altered migratory pattern with chronic exposure. Progressive reactivation of PCNA expression occurs with repair. PCNA migration to upper layers of the epidermis indicates proliferation and possibly a subsequent increased malignant potential. We conclude that PCNA can serve as a marker of DNA repair and indirectly as an indicator of UV-B-induced damage, expression being time dependent and dose related. Specific immunoreactivity patterns and the observable atypia in keratinocytes are relevant in elucidating malignant potentiation.     

An approach to developing a force field for molecular simulation of martensitic phase transitions between phases with subtle differences in energy and structure

d,l-Norleucine is one of only a few molecules whose crystals exhibit a martensitic or displacive-type phase transformation where the emerging phase shows a topotaxial relationship with the parent phase. The molecular mechanism for such phase transformations, particularly in molecular crystals, is not well understood. Crystalline phases that exhibit displacive phase transitions tend to be very similar in structure and energy. Consequently, the development of a force field for such phases is challenging as the phase behavior is determined by subtle differences in their lattice energies and entropies. We report an approach for developing a force field for such phases with an application to d,l-norleucine. The proposed procedure includes calculation of the phase diagram of the crystalline phases as a function of temperature to identify the best force field. d,l-Norleucine also presents an additional problem since in the solid state it exists as a zwitterion that is unstable in vacuo and therefore cannot be characterized using high-level ab initio calculations in the gas phase. However, a stable zwitterion could be obtained using Onsager's reaction-field continuum model for a solvent (SCRF) using both Hartree-Fock and density functional theory. A number of force fields and the various sets of partial charges obtained from the SCRF calculations were screened for their ability to reproduce the crystal structures of the two known phases, alpha and beta, of d,l-norleucine. Selected parameter sets were then employed in free energy minimizations to identify the best set on the basis of a correct prediction of the alpha-beta phase transition. The Williams' nonbonded parameters combined with partial charges from SCRF-Polarized Continuum Model calculation were found to reproduce the structures of the phases accurately and also maintained their stability in extended molecular dynamics simulations in the Parrinello-Rahman constant stress ensemble. Moreover, we were also able to successfully simulate the phase transformation of the beta- to the alpha-phase. The identified force field should enable detailed studies of the phase transformations exhibited by crystals of d,l-norleucine and hence enhance our understanding of martensitic-type transformations in molecular crystals.