Describing a conjugated molecule in which the atoms can be divided into two sets of alternate atoms such that no atom has a direct link to another atom in the same set. Naphthalene, for example, has an al­ternant conjugated system.

The most common form of secondary structure in pro­teins, in which the polypeptide chain is coiled into a helix. The helical structure is held in place by weak hy­drogen bonds between the N-H and C=O groups in successive turns of the helix.

أي من مجموعة من الأدوية المشتقة من حمض الباربيتوريك التي لها تأثير مثبط للجهاز العصبي المركزي. كانت الباربيتورات تستخدم في الأصل كمهدئات وحبوب منومة ولكن استخدامها السريري محدود الآن بسبب آثارها الجانبية السامة ؛ حيث يمكن أن يؤدي الاستخدام المطول إلى الإدمان. فالباربيتورات المحددة في الاستخدام السريري تشمل الفينوباربيتون ، لعلاج الصرع ، وميثوهيكسيتان الصوديوم ، المستخدم كمخدر. Any one …

The stepwise degradation of a car­boxylic acid to the next lower homo­logue. First the ester is converted into a tertiary alcohol using a Grig­nard reagent (PhMgX) and acid (HX): RCH2COOCH3 → RCH2C(OH)Ph2 The secondary alcohol is then dehy­drated using ethanoic anhydride (CH3COOCOCH3) to give an alkene: RCH2C(OH)Ph2 → RCH=CPh2 The alkene is oxidized with chromic acid: RCH=CPh2 → RCOOH + …

baking powder A mixture of pow­dered compounds added to dough or cake mixture to make it rise in cook­ing. It is used as a substitute for yeast in bread-making. Baking powders consist of a source of carbon dioxide, such as sodium hydrogencarbonate or ammonium hydrogencarbonate, and an acidic substance such as cal­cium hydrogenphosphate, potassium hydrogentartrate (cream of tartar), or sodium …

A re­arrangement reaction, sometimes known as the Dakin reaction, com­monly used in organic synthesis in which a ketone reacts with a peroxy acid to form an ester. For example, R-CO-R → R-CO-O-R The reaction is equivalent to the in­sertion of an oxygen atom next to the ketone’s carbonyl (>C=O) group. Meta-chloroperbenzoic acid (m-CPBA; ClC6H4.CO.O.OH) and trifluoro­perethanoic acid (CF3.CO.O.OH) are typical …

A test for unsaturated compounds in which potassium per­manganate is used. Alkenes, for ex­ample, are oxidised to glycols, and the permanganate loses its colour: 3R2C=CR2 + 2KMnO4 + 4H2O → 2MnO2 + 2KOH + 3R2COHR2COH

A membrane­ bound protein of the halophilic (salt­resistant) bacterium Halobacterium halobium. When activated by light, it pumps protons out of the cell; this creates a concentration gradient, which enables ATP to be synthesized. Bacteriorhodopsin is composed of seven a-helix segments, which span the membrane and are joined to­gether by short amino-acid chains. It contains the prosthetic group retinal, which is …

back titration : A technique in vol­umetric analysis in which a known excess amount of a reagent is added to the solution to be estimated. The unreacted amount of the added reagent is then determined by titra­tion, allowing the amount of sub­stance in the original test solution to be calculated.   بوسترات (لوحات) كيميائية بدقة عالية (أكثر من 25 لوحة) …

Low inten­sity ionizing radiation present on the surface of the earth and in the atmosphere as a result of cosmic radiation and the presence of radio­isotopes in the earth’s rocks, soil, and atmosphere. The radioisotopes are either natural or the result of nu­clear fallout or waste gas from power stations. Background counts must be taken into account when measuring the …