Louis Pasteur

Researches on the Molecular Asymmetry of Natural Organic Products

III.

When I began to devote myself to special work, I sought to strengthen myself in the knowledge of crystals, foreseeing the help that I should draw from this in my chemical researches. It seemed to me to be the simplest course, to take, as a guide, some rather extensive work on crystalline forms; to repeat all the measurements, and to compare my determinations with those of the author. In 1841, M. DE LA PROVOSTAYE, whose accuracy is well known, had published a beautiful piece of work on the crystalline forms of tartaric and paratartaric acids and of their salts. I made a study of this memoir. I crystallised tartaric acid and its salts, and investigated the forms of the crystals. But, as the work proceeded, I noticed that a very interesting fact had escaped the learned physicist. All the tartrates which I examined gave undoubted evidence of hemihedral faces.

This peculiarity in the forms of tartrates was not very obvious. This will be readily conceived, seeing that it had not been observed before. But when, in a species, its presence was doubtful, I always suceeded in making it manifest by repeating the crystallisation and slightly modifying the conditions. Sometimes the crystals bore all the faces demanded by the law of symmetry, but the hemihedry was still betrayed by an unequal development of one half of the faces. This is seen, for example in tartar emetic. It must be admitted that a circumstance which adds greatly to the difficulty in recognising hemihedry is the frequent irregularities of the crystals, which never develop quite freely. From this cause there arise deformations, arrestments of development in one direction or another, faces suppressed by accident, etc. Unless in circumstances of an almost exceptional character, the recognition of hemihedry, particularly in laboratory crystals, demands very attentive study. To this we must add the fact that, although hemihedry may be possible in a given form, and although it is a function of the internal structure of the substance, it may not be indicated externally, any more than one finds on every crystal of a cubic species all the forms compatible with the cube.

But however these things may be, I repeat that I found the tartrates hemihedral.

This observation would probably have remained sterile, without the following one.

Let a, b, c, be the parameters of the crystal form of any tartrate, and α, β, γ, the angles of the crystallographic axes. The latter are ordinarily perpendicular, or slightly oblique. In addition, the ratio of two parameters, such as a and b, is almost the same in the various tartrates, whatever may be their composition, their quantity of water of crystallisation, or the nature of the bases; c alone shows sensible variations. There is a kind of semi-isomorphism among all the tartrates. One would imagine that the tartaric group dominated and stamped with similarity the forms of all the various substances in spite of the difference in the other constituent elements.

The results of this are, a resemblance in the forms of all tartrates, and the possibility of parallel orientation, taking, for example, as basis of orientation the position of the axes a and b.

Now if we compare the disposition of the hemihedral faces on all the prisms of the primitive forms of the tartrates, when they are oriented in the same manner, this disposition is found to be the same.

These results, which have been the foundation of all my later work, may be summed up in two words: the tartrates are hemihedral, and that in the same sense.

Guided then on the one hand by the fact of the existence of molecular rotary polarisation, discovered by Brat in tartaric acid and all its compounds, and on the other by HERSCHEL’S ingenious correlation, and yet again by the sagacious views of M. DELAFOSSE, with whom hemihedry has always been a law of structure and not an accident of crystallisation, I believed that there might be a relation between the hemihedry of the tartrates and their property of deviating the plane of polarised light.

It is important thoroughly to grasp the development of the conceptions:—HAÜY and WEISS observe that quartz possesses hemihedral faces and that these faces incline to the right on some specimens and to the left on others. Brat on his part finds that quartz crystals likewise separate themselves into two sets, in relation to their optical properties, the one set deviating the plane of polarised light to the right, the other to the left, according to the same laws. HERSCHEL in his turn supplies to these hitherto isolated facts the bond of union, and says:—Plagihedra of one kind deviate in the same sense; plagihedra of the other kind deviate in the opposite sense.

For my part I find that all tartrates are plagihedral, if I may so express myself, and that in the same sense; so that I might presume that here, as in the case of quartz, there was a relation between the hemihedry and the circular polarisation. At the same time the essential differences to which I have just referred between circular polarisation in quartz and in tartaric acid must not be neglected.

Thanks to the above discoveries, and to the relations which I have just enumerated, we are now in possession of a preconceived notion (for it is still nothing more than that) as to the possible inter-relation of the hemi-hedry and the rotative power of the tartrates.

Being very anxious to find by experiment some support for this still purely speculative view, my first thought was to see whether the very numerous crystallisable organic products which possess the molecular rotative property, have hemihedral crystalline forms, an idea which had not previously occurred to any one in spite of Herschel’s correlation. This investigation met with the success which I anticipated.

I also occupied myself with the examination of the crystalline forms of paratartaric acid and its salts. These substances are isomeric with the tartaric compounds, but had all been found by BIOT to be inactive towards polarised light. None of them exhibited hemihedry.

Thus the idea of the inter-relation of the hemihedry and the molecular rotatory power of natural organic products gained ground.

I was soon enabled to establish it clearly by a wholly unexpected discovery.

After discussing Mitscherlich’s work Pasteur continues.

IV.

You will now understand why, being preoccupied, for the reasons already given, with a possible relation between the hemihedry of the tartrates and their rotative property, MITSCHERLICH’S note of 1844 should recur to my memory. I thought at once that MITSCHERLICH was mistaken on one point. He had not observed that his double tartrate was hemihedral while his paratartrate was not. If this is so, the results in his note are no longer extraordinary; and further, I should have, in this, the best test of my preconceived idea as to the interrelation of hemihedry and the rotatory phenomenon.

I hastened therefore to re-investigate the crystalline form of MITSCHERLICH’S two salts. I found, as a matter of fact, that the tartrate was hemihedral, like all the other tartrates which I had previously studied, but, strange to say, the paratartrate was hemihedral also. Only, the hemihedral faces which in the tartrate were all turned the same way, were, in the paratartrate inclined sometimes to the right and sometimes to the left. In spite of the unexpected character of this result, I continued to follow up my idea. I carefully separated the crystals which were hemihedral to the right from those hemihedral to the left, and examined their solutions separately in the polarising apparatus. I then saw with no less surprise than pleasure that the crystals hemihedral to the right deviated the plane of polarisation to the right, and that those hemihedral to the left deviated it to the left; and when I took an equal weight of each of the two kinds of crystals, the mixed solution was indifferent towards the light in consequence of the neutralisation of the two equal and opposite individual deviations.

Thus, I start with paratartaric acid; I obtain in the usual way the double paratartrate of soda and ammonia; and the solution of this deposits, after some days, crystals all possessing exactly the same angles and the same aspect. To such a degree is this the case that MITSCHERLICH, the celebrated crystallographer, in spite of the most minute and severe study possible, was not able to recognize the smallest difference. And yet the molecular arrangement in one set is entirely different from that in the other. The rotatory power proves this, as does also the mode of asymmetry of the crystals. The two kinds of crystals are isomorphous, and isomorphous with the corresponding tartrate. But the isomorphism presents itself with a hitherto unobserved peculiarity; it is the isomorphism of an asymmetric crystal with its mirror image. This comparison expresses the fact very exactly. Indeed, if, in a crystal of each kind, I imagine the hemihedral facets produced till they meet, I obtain two symmetrical tetrahedra, inverse, and which cannot be superposed, in spite of the perfect identity of all their respective parts. From this I was justified in concluding that, by the crystallisation of the double paratartrate of soda and ammonia, I had separated two symmetrically isomorphous atomic groups, which are intimately united in paratartartic acid. Nothing is easier than to show that these two species of crystals represent two distinct salts from which two different acids can be extracted.

Using the treatment always employed in such cases, the purpose is served by precipitating each salt with a salt of lead or baryta, and then isolating the acids by means of sulphuric acid. . . .

VI.

Let us return to the two acids furnished by the two sorts of crystals deposited in so unexpected a manner in the crystallisation of the double paratartrate of soda and ammonia. I have already remarked that nothing could be more interesting than the investigation of these acids.

One of them, that which comes from crystals of the double salt hemihedral to the right, deviates to the right, and is identical with ordinary tartaric acid. The other deviates to the left, like the salt which furnishes it. The deviation of the plane of polarisation produced by these two acids is rigorously the same in absolute value. The right acid follows special laws in its deviation, which no other active substance had exhibited. The left acid exhibits them, in the opposite sense, in the most faithful manner, leaving no suspicion of the slightest difference.

That paratartaric acid is really the combination, equivalent for equivalent, of these two acids, is proved by the fact that, if somewhat concentrated solutions of equal weights of each of them are mixed, as I shall do before you, their combination takes place with disengagement of heat, and the liquid solidifies immediately on account of the abundant crystallisation of paratartaric acid, identical with the natural product.³

In accord with their chemical and crystallographic properties, all that can be done with one acid can he repeated with the other under the same conditions, and in each case we get identical, but not superposable products; products which resemble each other like the right and left hands. The same forms, the same faces, the same angles, hemihedry in both cases. The sole dissimilarity is in the inclination to right or left of the hemihedral facets and in the sense of the rotatory power. . . .

¹ Compt. rend., 26: 535 (1848); Ann. chim., 24: 442 (1848).

² "Leçons de chimie professées en 1860," Paris, 1861; translated, in part, in: Alembic Club Reprints, No. 14, "Researches on the Molecular Asymmetry of Natural Organic Products by Louis Pasteur, (1860)," Edinburgh, 1905; Ostwald’s Klassiker, No. 28, "L. Pasteur., Ueber die Asymmetrie bei natürlich vorkommenden organischen Verbingungen. (1860). Uebersetzt und herausgegeben von M. und A. Ladenburg," Leipzig, 1891; the collected works of Pasteur, "Oeuvres. Réunies par Pasteur Vallery-Radot . . . ," Paris, 1922–1939, 7 vols. Volume 1 contains his publications on molecular asymmetry.

³ [This beautiful experiment called forth applause from the audience.]