286 PRINCIPLES OF GENERAL PHYSIOLOGY 



It is not easy to see what is the advantage to the organism of this undoubted 

 preference for particular optical isomers. We have to remember that the 

 existence of asymmetric carbon atoms is geometrically unavoidable. The enzymes 

 which act on such compounds will probably also be themselves optically active, 

 and the rate of action on one kind of optical isomer will no doubt be greater than 

 that on the opposite one. A certain economy in the number of enzymes necessary 

 is effected by limiting them to those required for one set of optical isomers, but 

 it is scarcely to be supposed that this can be of much consequence, and it would 

 seem indeed that more is lost than gained in the process. The replacement of 

 an enzyme acting on one isomer by that acting on the opposite one, moreover, 

 does not appear to be of much difficulty. Currie (1911) found that, of various 

 pure cultures of Bacillus bulgaricus (the vigorous Bulgarian lactic acid organism), 

 obtained from different sources, some formed d-lactic acid alone, others a mixture 

 of d- and I- forms, and one culture produced /-lactic acid alone. 



We must suppose that the external forces, under which the asymmetric carbon 

 compounds forming the basis of living protoplasm were produced, were in some 

 way or other themselves asymmetric. At that geological period, the synthesis 

 may have received a bias in one direction, which has naturally been adhered to, 

 as more and more elaborate compounds were evolved. On this view, the preference 

 of one isomer over the other is, as it were, a matter of chance as to which happened 

 to be first produced by the particular direction of the asymmetrical force. 



Recent work by Emil Erlenmeyer (1913), however, suggests a possible way of 

 separating the constituents of a racemic mixture without the aid of optically 

 active substances. 



Since the properties of isomers depend only on the relative position and 

 distance from one another of atomic groups in the molecule, it is clear that 

 molecules, which are mirror-images of one another, must be identical in all those 

 properties which depend on molecular dimensions and attractions. So that their 

 unlikeness can only be expressed in the shape of their crystals, their behaviour to 

 polarised light, or to other asymmetrical forces or substances, such as those in living 

 organisms. This fact was pointed out by Pasteur and by van't Hoff (1901, 1'ti-s 

 Heft, p. 98). On the other hand, isomers will have different chemical and 

 physical properties when their atomic structure is neither the same as, nor the 

 mirror-image of, each other. 



Thus, when a compound of a d-acid with a d-base is compared with that of the same acid 

 with an /-base, the two salts are neither of the same structure nor mirror-images, so that they 

 are chemically and physically separable. This is, of course, the usual means adopted for 

 the purpose ; the racemic mixture is caused to form salts with an optically active base or acid 

 and it is found that the salt of one isomer has different solubilities from that of the other, so 

 that they can be separated by fractional crystallisation. The lactic acids, for example, can be 

 isolated by combination with the optically active base, brucine. Similarly the two isomeric 

 forms of glucosides, as pointed out above, are not mirror-images and can be separated l>y 

 crystallisation, etc. 



But, if these considerations were invariably and unconditionally true, it would 

 be for ever impossible to separate the components of a racemic mixture without 

 the aid of another optically active compound. Further, unless experiments of 

 producing asymmetric compounds by the action of asymmetric external forces, 

 such as polarised light, are rewarded with more success than hitherto, we are 

 apparently compelled to assume the intervention of unknown, supernatural forces 

 in the origin of life. 



Now Emil Erlenmeyer (1913, p. 442) points out that van't Hoff himself shows 

 the possibility of the occurrence of a form of isomerism of a different kind, which 

 may be called relative. Thus, when two carbon atoms are united together, there 

 are six free affinities and when these are satisfied by six different univalent groups, 

 twelve different arrangements are possible. But eight of these are derived from 

 the other four by mere rotation, without change of combination. The four 

 different ones are shown diagrammatically in the scheme below, where the two 

 carbon atoms are represented by discs, supposed to be white on one side, black on 

 the other, and the letters, A, B, C, D, E, F, are six different chemical groups. 



