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أرشيف شهر: نوفمبر 2013

allosteric enzyme

An enzyme that has two structurally distinct forms, one of which is active and the other inactive. In the active form, the quaternary structure of the enzyme is such that a substrate can interact with the enzyme at the active site. The conformation of the substrate-binding site becomes al­tered in …

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allenes

Compounds that contain the group >C=C=C<, in which three carbon atoms are linked by two adja­cent double bonds. The outer carbon atoms are each linked to two other atoms or groups by single bonds. The simplest example is 1,2-propadiene, CH2CCH2 . Allenes are dienes with typical reactions of alkenes. Under …

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alkyd resin

A type of polyester resin used in paints and other sur­face coating. The original alkyd resins were made by copolymerizing phthalic anhydride with glycerol, to give a brittle cross-linked polymer. The properties of such resins can be modified by adding monobasic acids or alcohols during the polymeriza­tion.

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(algin (alginic acid

A complex poly­saccharide occurring in the cell walls of the brown algae (Phaeophyta). Algin strongly absorbs water to form a viscous gel. It is produced commer­cially from a variety of species of Laminaria and from Macrocystis pyrifera in the form of alginates, which are used mainly as a stabilizer and …

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aldoximes

Compounds formed by reaction between hydroxylamine and an aldehyde RCOH + H2NOH → RCH:NOH + H2O  If R is an aliphatic group, the al­doxime is generally a liquid or low ­melting solid. If R is aromatic, the aldoxime is a crystalline solid. Al­doximes have a planar structure and can exist …

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aflatoxin

A poisonous compound, C15H12O6, produced by the fungus As­pergillus flavus. It is extremely toxic to farm animals and can cause liver can­cer in humans. It may occur as a con­taminant of stored cereal crops, cotton seed, and, especially, peanuts. There are four isomeric forms.

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affinity chromatography

affinity chromatography A bio­chemical technique for purifying nat­ural polymers, especially proteins. It functions by attaching a specific lig­and by covalent bonding to an insolu­ble inert support. The ligand has to have a specific affinity for the poly­mer, so that when a solution contain­ing the ligand is passed down a column …

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adsorption isotherm

adsorption isotherm An equation that describes how the amount of a substance adsorbed onto a surface de­pends on its pressure (if a gas) or its concentration (if in a solution), at a constant temperature. Several ad­sorption isotherms are used in sur­face chemistry including the BET isotherm and the Langmuir ad­sorption …

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