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Temperature Excursions (temperature + excursion)

Distribution by Scientific Domains

Chemistry	100%

Selected Abstracts

The Effect of Rapid High Temperature Excursions on the Moisture Absorption and Dynamic Mechanical Properties of Carbon Fibre Epoxy Composite Materials

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2004
G. M. Mcnally
The effect of elevated temperature excursions (thermal spiking) on the moisture absorption characteristics and dynamic mechanical properties of Cycom 8 HS carbon fibre epoxy laminates was investigated. Cured laminate samples were preconditioned (65d,C, 95%R.H.) and these samples were exposed to various thermal spiking (150d,C/2min) programmes. Dynamic mechanical thermal analysis (DMTA) techniques measured the changes in glass transition temperature (Tg) storage modulus (log E') and damping (Tan , max) of the laminates as a result of exposure to these environments. The thermal spiking programme was shown to cause an increase in both the amount and rate of moisture absorption of the laminates. These increments were accompanied by a significant decrease in Tg, log E', and Tan , max. Scanning Electron Microscopy (SEM) analysis also showed the progressive growth of both interlaminar and translaminar micro-cracks as a result of thermal spiking. [source]

Use of a `caged' analogue to study the traffic of choline within acetylcholinesterase by kinetic crystallography

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 11 2007
Jacques-Philippe Colletier
Acetylcholinesterase plays a crucial role in nerve-impulse transmission at cholinergic synapses. The apparent paradox that it displays high turnover despite its active site being buried raises cogent questions as to how the traffic of substrates and products to and from the active site can occur so rapidly in such circumstances. Here, a kinetic crystallography strategy aimed at structurally addressing the issue of product traffic in acetylcholinesterase is presented, in which UV-laser-induced cleavage of a photolabile precursor of the enzymatic product analogue arsenocholine, `caged' arsenocholine, is performed in a temperature-controlled X-ray crystallography regime. The `caged' arsenocholine was shown to bind at both the active and peripheral sites of acetylcholinesterase. UV irradiation of a complex with acetylcholinesterase during a brief temperature excursion from 100,K to room temperature is most likely to have resulted in a decrease in occupancy by the caged compound. Microspectrophotometric experiments showed that the caged compound had indeed been photocleaved. It is proposed that a fraction of the arsenocholine molecules released within the crystal had been expelled from both the active and the peripheral sites. Partial q -weighted difference refinement revealed a relative movement of the two domains in acetylcholinesterase after photolysis and the room-temperature excursion, resulting in an increase in the active-site gorge volume of 30% and 35% in monomers A and B of the asymmetric unit, respectively. Moreover, an alternative route to the active-site gorge of the enzyme appeared to open. This structural characterization of acetylcholinesterase `at work' is consistent with the idea that choline exits from the enzyme after catalysis either via the gorge or via an alternative `backdoor' trajectory. [source]

Prediction of the relaxation behavior of amorphous pharmaceutical compounds.

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 7 2003

Abstract Variability in the time to crystallization is a major technical and economic hurdle in using amorphous solids in dosage forms. It is hypothesized that amorphous solids "age", and that the older they are, the more relaxed they are and the higher the probability of crystallization. At present, there is no method that allows the "effective age" of an amorphous raw material to be assessed relative to its unrelaxed initial condition. A method has been developed that may satisfy this unmet need and provide a first step in subsequent investigation of the crystallization "event". This method consists of using master curves to enable the determination of the effective age (,aging') of an amorphous compound given normal excursions in storage conditions. The present study shows that master curves can be prepared for different storage conditions and subsequently be used to predict the relaxation or aging behavior of amorphous compounds with expected variations in storage conditions. Given the constraint that the system remain within the area enclosed by the equilibrium supercooled liquid line and the glass on the enthalpy,temperature diagram, experimental results using indomethacin and salicin as model compounds show that master curves can be used to predict aging behavior under nonisothermal conditions, with temperature excursions as large as 10°C. The nonisothermal relaxation behavior can be modeled by combining the Kohlrausch,Williams,Watts (KWW) stretched exponential function, the relaxation function, and a shift factor. In addition, a model was developed that extends the range of applicability to time/temperature regions in which partial crystallization occurs. © 2003 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 92:1464,1472, 2003 [source]