Supplement to "Nature" December 23, 1922 



In the year 1850 Pasteur discovered yet a fourth 

 variety of tartaric acid- meso-tartaric acid — which 

 is truly and permanently without action on the plane of 

 polarisation of light, and so is quite unresolvable into 

 two optical antipodes. This further fact threw great 

 light on the subject, and eventually Pasteur showed 

 that the explanation of the whole matter is to be found 

 in the symmetry or dissymmetry of the chemical 

 molecules themselves. Not only is the empirical 

 formula the same for all, but the constitutional 

 formula written in one plane is also identical, namely, 



CHOH-COOH 



1 : but the atoms or groups are differently 



CHOH-COOH 



disposed in space in these different physical isomers. 

 Physical isomerism is possible when the substance 

 possesses what has since been called an asymmetric 

 carbon atom, an atom of carbon of which the four 

 tetrahedrally disposed valency bonds are satisfied by 

 four different elements or groups. Optical activity 

 is usual in all such cases, but it has more recently been 

 shown that it is not the absolutely essential condition 

 for the development of optical activity, the absence 

 of second order symmetry elements (planes of sym- 

 metry or second order axes) and possession of only 

 first order (axial) symmetry being the more truly 

 determinative condition for the development of two 

 enantiomorphous varieties of physical isomerides and 

 of their accompanying opposite optical activities. 

 Tartaric acid, however, does possess this essential 

 condition, and it has also two asymmetric carbon 

 atoms, namely, those in the two CHOH groups (starred) 



. . , r , *CHOH-COOH , , 

 of the constitutional formula 1 , and the 



*CHOH-COOH 

 group of atoms in one half-molecule may be either 

 symmetrically disposed with respect to those of the 

 other half-molecule or differently disposed. When 

 they are symmetrically arranged, internally compen- 

 sated, the whole molecule is optically inactive, this 

 corresponding to the case of the truly inactive tartaric 

 acid ; when they are unsymmetrically arranged they 

 are so in either a right- or a left-handed manner, the 

 crystals being helically constructed (recently confirmed 

 by X-ray analysis) as a right or left screw, giving rise 

 to the two optical enantiomorphous antipodes, as in 

 the case of dextro- and lsevo-tartaric acids. Racemic 

 acid, the molecular combination of the two last 

 mentioned, is the case of external compensation as 

 regards the molecules. 



The work during the present year of Mr. W. T. 

 Astbury in the laboratory of Sir William Bragg, on the 

 X-ray analysis of tartaric acid, the results of which 

 have just, most opportunely, been communicated to 

 the Royal Society, has proved without a shadow of 



doubt that the four atoms of carbon in the molecule 

 of ordinary dextro-tartaric acid arc, in very truth, 

 spirally arranged. This is a most welcome confirmation 

 of Pasteur's great work on the very eve of his centenary. 



The principles on which the whole of these results 

 are based were eventually summarised in what has 

 since become known as Pasteur's Law, which may be 

 briefly stated thus : — " If the atoms of a chemical 

 molecule be dissymmetrically arranged, this molecular 

 dissymmetry implies the possibility of the existence of 

 two oppositely complementary configurations of the 

 molecule. Both varieties have the same chemical 

 properties, and they are endowed always with equal 

 but oppositely directed rotatory power. The presence 

 of molecular dissymmetry therefore reveals itself by 

 this rotatory power of the molecules and is wholly 

 determined by their chemical constitution. When 

 the atoms of a chemical molecule are dissymmetric- 

 ally arranged, the fact is at the same time betrayed by 

 the occurrence of the two varieties in complementary 

 non-superposable crystalline forms, possessing screw 

 axes of opposite winding." 



This purely chemical and crystallographic work of 

 Pasteur was connected with his later bacteriological 

 and zoological work by the further pioneer observation 

 that when the spores of the ferment Penicillium 

 glaucum were added to a solution of racemic acid 

 containing a small quantity of phosphates (which 

 appear to be essential to the life of the organism), the 

 dextro component of the molecular compound of the 

 two varieties of optically active tartaric acid is some- 

 how isolated and eaten up by the organism, leaving 

 the laevo component untouched so long as any dextro 

 acid remains. Why this is so is a mystery still, con- 

 nected with life itself. By arresting the fermentation 

 at the psychological moment, the residual lawo variety 

 can thus be isolated and crystallised out tolerably pure, 

 and its isolation was, as a matter of fact, thus effected 

 for the first time in yet a second manner by Pasteur, 

 and its crystals shown by him to be the mirror-images 

 of those of Scheele's ordinary dextro-tartaric acid. 



This observation by Pasteur has developed into a 

 general method for the fissure and ' separation of the 

 two separable varieties of a racemic compound — for 

 many such compounds have since been discovered — 

 and for the isolation of one of them, the other being 

 chosen in some mysterious way for preferential destruc- 

 tion by the organism, in assimilating it for its own 

 nourishment and reproduction. Thus the great value 

 of the classic work of Pasteur on the tartaric acids lies 

 in the fact that this group of compounds proved to be 

 but a type of a large class of substances, which exhibit 

 physical isomerism in two enantiomorphous varieties 

 that are optically active in opposite directions and 



