06413_12 - L labelling The process of replacing a stable atom in a compound with a radioisotope of the same element to enable its path through a

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L labelling The process of replacing a sta- ble atom in a compound with a radioiso- tope of the same element to enable its path through a biological or mechanical system to be traced by the radiation it emits. In some cases a different stable iso- tope is used and the path is detected by means of a mass spectrometer. A com- pound containing either a radioactive or stable isotope is called a labelled com- pound and the atom used is a label . If a hydrogen atom in each molecule of the compound has been replaced by a tritium atom, the compound is called a tritiated compound . A radioactive labelled com- pound will behave chemically and physi- cally in the same way as an otherwise identical stable compound, and its pres- ence can easily be detected using a * Geiger counter. This process of radio- active tracing is widely used in chemistry, biology, medicine, and engineering. For example, it can be used to follow the course of the reaction of a carboxylic acid with an alcohol to give an ester, e.g. CH 3 COOH + C 2 H 5 OH C 2 H 5 COOCH 3 + H 2 O To determine whether the noncarbonyl oxygen in the ester comes from the acid or the alcohol, the reaction is performed with the labelled compound CH 3 CO 18 OH, in which the oxygen in the hydroxyl group of the acid has been ‘labelled’ by using the 18 O isotope. It is then found that the water product is H 2 18 O; i.e. the oxygen in the ester comes from the alco- hol, not the acid. labia See labium . labile Describing a chemical compound in which certain atoms or groups can eas- ily be replaced by other atoms or groups. The term is applied to coordination com- plexes in which ligands can easily be re- placed by other ligands in an equilibrium reaction. labium 1. The lower lip in the * mouth- parts of an insect, which is used in feed- ing and is formed by the fusion of a pair of appendages (the second * maxillae). 2. Either member of two pairs of Û eshy folds that form part of the * vulva. The outer and larger pair, the labia majora , are covered by pubic hair and contain adi- pose tissue; the smaller labia minora lack adipose tissue and pubic hair. Both pairs of labia contain sebaceous glands. labrum The upper lip in the * mouth- parts of an insect. It is formed from a plate of cuticle hinged to the head above the mouth and is used in feeding. labyrinth The system of cavities and tubes that comprises the * inner ear of vertebrates. It consists of a system of membranous structures ( membranous labyrinth ) housed in a similar shaped bony cavity ( bony labyrinth ). lac operon The * operon that regulates lactose metabolism in the bacterium Es- cherichia coli . Its form was Ü rst postulated in 1961 by François Jacob (1920– ) and Jacques Monod (1910–76) to explain con- trol of β -galactosidase synthesis, and it is used as a model for the structure of all other operons. See jacob monod hypoth- esis . lacrimal gland
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This note was uploaded on 05/29/2009 for the course CIVIL ENG 12345 taught by Professor Xhfxfh during the Spring '09 term at Amirkabir University of Technology.

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06413_12 - L labelling The process of replacing a stable atom in a compound with a radioisotope of the same element to enable its path through a

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