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 the inactive form and interaction with the substrate is not possible. Allosteric enzymes tend to catalyse the initial…
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 basic conditions. they often convert to alkynes. In an allene, the two double bonds lie in planes that…
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.
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 texturing agent in the food industry.
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 in two isomeric forms. In the syn-form, the OH group is on the same side of the…
A hormone produced by the adrenal glands that controls excretion of sodium by the kidneys and thereby maintains the balance of salt and water in the body fluids.
A poisonous compound, C15H1206, 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.
كلمات دليلية: aflatoxin
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 of the material it is specifically retarded and thus separated from any contaminating molecules. An example of a…
A low-density porous transparent material that consists of more than 90% air. Usually based on metal oxides or silica, aerogels are used as drying agents and insulators.
كلمات دليلية: aerogel
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 isotherm. The different isotherms correspond to different assumptions about the surface and the adsorbed molecules.