A textbook of biochemistry for students of medicine and science / A.T. Cameron.

  • Alexander Thomas Cameron
Date:
1942
Catalogue details

Licence: In copyright

Credit: A textbook of biochemistry for students of medicine and science / A.T. Cameron. Source: Wellcome Collection.

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    which are not superimposable on one another. Such an arrangement is pictured in Fig. 5. It is found that, provided some one of the four attached radicals is sufficiently massive, there is always the possibility of optical activity. If a compound is found without this property, but possessing asymmetric carbon atoms, then it is usually possible to break it up into equal mixtures of two active forms of the same compound, the activity of the two being opposite in sign, but equal in degree, so that the effect of every molecule of the one kind is exactly neutralised by the cor- responding effect of a molecule of the other kind in the original compound, which is termed a racemic mixture. Lactic acid is one of the simplest examples of an optically active substance. Since we cannot easily use solid figures to illustrate the asymmetry, we may imagine them projected on one plane, and thus get, for lactic acid, CH, fH Substances which rotate the plane of polarisation clockwise, to the right, are called dextro-iptatory, and written, for example, as d-lactic acid. Those which rotate counter-clockwise, to the left, are termed laevo-rotatory, and the corresponding example is written l-lactic acid. The racemic or inactive mixture of lactic acid is written i-lactic acid. The so-called “ sarco-lactic acid,” which can be extracted from muscle, is d-lactic acid. When cane-sugar is fermented by certain bacteria Mactic acid is formed. The ordinary lactic acid from sour milk is i-lactic acid, or dl-lactic acid. The great ma jority of compounds which play a part in living processes are optically active, and the biochemical catalysers, the enzymes, which bring about reactions with these compounds, are themselves optically active ; their chemical activity is affected and controlled by their optical activity. Each optically active substance produces a rotation effect the degree of which depends on the specific effect of the molecules of that substance, and the number of molecules of it that are acting. Consequently we are able to speak of the specific rotatory power, [a]D, (D being the sodium line of the spectrum that is used for accurate measurements), which can be calculated.from the formula. where a is the observed rotation at t° of p grams of substance, dissolved in 1 c.c. of liquid, and l is thedength of tube, containing the solution, in decimetres. For the actual method Of measurement practical textbooks must be consulted. X-ray Studies of Molecular Structure. The X-rays, electromagnetic waves of very short wave-length, liberated from a metal target bombarded by cathode rays in a vacuum tube, through their great power of penetration can be used to produce photographs of homogeneous material such as crystals, and proteins and other compounds of large molecular size, and these photographs exhibit definite diffraction