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One Plane (one + plane)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Compensating for die swell in the design of profile dies

POLYMER ENGINEERING & SCIENCE, Issue 10 2003
W. A. Gifford
Because of the effects of die swell, the final shape of an extrudate is often substantially different from that of the exit opening of the die. As a result, the design of profile dies producing complex shapes often involves more than just "balancing" the die but also compensating for the effects of die swell. Typically, a successful design of such dies is achieved only through much "cut and try," However, with the use of a fully three-dimensional finite element flow algorithm along with quick mesh generating capabilities, the usual cut and try involved in the design of many profile dies can be greatly reduced, if not eliminated. This paper demonstrates how the effects of die swell can be compensated for in the design of profile dies. For profiles with one plane of symmetry, this includes compensating for the sideways translation of the extrudate as well as the change in shape that the extrudate experiences. Completely asymmetric profiles undergo a "twisting" downstream of the die. This twisting, which appears not to have been reported in the literature (at least for isothermal extrusion), is also accounted for here, along with the change in shape that the extrudate undergoes. The translation or twisting of profiles downstream of a die is often attributed to non-Newtonian or non-isothermal effects. Only isothermal Newtonian examples are considered here. These results clearly show that asymmetry of the profile will result in a translation and twisting of the extrudate even in the isothermal Newtonian case. [source]

Features of homotetrameric molecular association in protein crystals

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 1 2009
Uma V. Katre
The crystal structures of proteins showing homotetrameric association, a common feature observed in many lectins, have been analyzed in order to understand the characteristics of tetrameric association in terms of the arrangement of subunits and their biological significance. The analysis could group the tetramer units into the following four categories. (i) Tetrahedral molecules, in which the four monomers form a nearly perfect tetrahedral arrangement. The angle between the axes of any two monomers is ,109°. (ii) Molecules that form a sandwiched dimer of dimers in which the two dimers are arranged perpendicular to each other, one upon the other. (iii) Planar molecules, in which the four monomers lie in one plane and the corresponding sides of adjacent monomers face in opposite directions. This can be considered as a flattened tetrahedral shape. (iv) Planar closed molecules, in which all four monomers lie in one plane arranged in a head-to-tail fashion in a square. The first group and its variant, the third group, are the most commonly found arrangements in crystal structures. Each arrangement has its own importance for biological function. Some tetrameric assemblies that deviate from the majority described above also have relevance to their biological function. [source]

Racemization of the gastrointestinal antisecretory chiral drug esomeprazole magnesium via the pyramidal inversion mechanism: A theoretical study

CHIRALITY, Issue 9 2010
Hili Marom
Abstract The pyramidal inversion mechanisms of the 6-methoxy and the 5-methoxy tautomers of (S)-omeprazole were studied, employing ab initio and DFT methods. The conformational space of the model molecule (S)-2-[(3-methyl-2-pyridinyl)methyl]sulfinyl-1H -benzimidazole was calculated, with respect to rotations around single bonds, at the B3LYP/6-311G(d,p) level. All of the resulting conformations were used as starting points for full optimizations of (S)-omeprazole, at B3LYP/6-31G(d), B3LYP/6-311G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-311G(2df,2pd), MP2/6-31G(d), and MP2/6-311G(d,p) levels. Four distinct pathways were found for enantiomerization via the pyramidal inversion mechanism for each of the tautomers of (S)-omeprazole. Each transition state, in which the sulfur, the oxygen and the two carbon atoms connected directly to the sulfur are in one plane, connects two diastereomeric minima. The enantiomerization is completed by free rotation around the sulfur,methylene bond, and around the methylene,pyridine ring bond. The effective Gibbs' free energy barrier for racemization ,G of the two tautomers of (S)-omeprazole are 39.8 kcal/mol (5-methoxy tautomer) and 40.0 kcal/mol (6-methoxy tautomer), indicating that the enantiomers of omeprazole are stable at room temperature (in the gas phase). The 5-methoxy tautomer of (S)-omeprazole was found to be slightly more stable than the 6-methoxy tautomer, in the gas phase. The energy barrier (,G,) for the(S,M) (S,P) diastereomerization of (S)-omeprazole due to the rotation around the pyridine chiral axis was very low, 5.8 kcal/mole at B3LYP/6-311G(d,p). Chirality 2010. © 2010 Wiley-Liss, Inc. [source]

Mohs micrographic surgery for facial skin cancer

CLINICAL OTOLARYNGOLOGY, Issue 4 2001
H.D. Vuyk
vuyk h.d. &lohuis p.j.f.m. (2001) Clin. Otolaryngol.26, 265,273 Mohs micrographic surgery for facial skin cancer Although it is well established that conventional treatment modalities generally result in high cure rates for non-melanoma skin cancer, it has been demonstrated over recent decades that the highest overall cure rates are achieved using Mohs micrographic surgery. The key to Mohs surgery is the excision and control of complete peripheral and deep resection margins in one plane, allowing orientation, mapping and re-excision of microscopic tumour extension. These extensions can be followed without sacrificing inappropriate amounts of normal tissue, yielding high cure rates and maximum preservation of tissue. These qualities make Mohs surgery an important and reliable treatment for skin cancer of the face, in particular when it concerns large, aggressive or recurrent carcinoma in cosmetic and functionally important areas. In an 8-year study period, 369 basal cell carcinomas (BCCs) and 56 squamous cell carcinomas (SCCs) of the face were treated in our department using Mohs surgery. With a follow-up ranging from 3 months to 99 months (mean 33 months), none of the BCCs recurred and only one (2%) of the SCCs recurred a few months postoperatively. These favourable cure rates using the modality of Mohs surgery are the reason for highlighting this technique in the current review. [source]