Robert West
University of Wisconsin Madison
Siegfried Ruhemann and the Discovery of Ninhydrin
The discovery of ninhydrin by Siegfried Ruhemann in 1911 is a well documented but little known example of "chance" discovery, made by a man whose name is almost unknown to chemists today. In the course of a turbulent career, Ruhemann was the first to observe the characteristic color reaction of ninhydrin (1,2,3-triketohydrindene hydrate, I) with amino acids and peptides, which makes it one of the most valuable of all biochemical reagents. 0
Ruhemann was born in Johannesburg in East Prussia in 1859, hut after his father's death in 1866 his family moved to Berlin where he obtained most of his education. He worked his way through the University a t Berlin, obtaining his degree in 1882, and then studied briefly with Wichelhaus before transferring to Hofmann's laboratories. In 1888 a t Hofmann's recommendation, he accepted a position as assistant to Sir James Dewar, then Jacksonian Professor of Organic Chemistry a t Cambridge University. Ruhemann rapidly became known as a colorful and popular lecturer. He was also a dedicated laboratory worker, who took up living quarters in the laboratory in order to have more time to spend on research. Trouble soon developed between Ruhemann and Dewar, and as a result Ruhemann moved in 1891 to Caius College, where laboratories were provided for him and his growing group of collaborators. The next 17 years were highly productive ones; Rnhemann and his students published about 80 papers during this time, on a variety of topics including nitrogen heterocycles, cyclic carbonyl compounds, beta-ketoacids, and others. In 1908, however, the College Laboratories
Preeented at the Annual Meeting of the Midwest Section of the History of Science Society, Madison, Wisconsin, April 1964.
386
/
Journal o f Chemicol Education
were closed, and Ruhemann returned to the University Chemical Laboratories, but with a much smaller number of co-workers than before. Here, with his own hands, he carried out the work which culminated in the discovery of ninhydrin. Ruhemann's research leading to ninhydrin stemmed from an interest in the enolization of cyclic dicarbonyl compounds. The compound I1 had been obtained in his group as a product of a condensation reaction, and had been shown to exist mainly in the en01 form (1). Ruhemann reasoned that the enolization of I1 might have resulted a t least in part from the presence of the
exocyclic acetyl group. To check on this possibility he sought to prepare a five-ring compound containing the structural elements shown in 111: a double bond, two carbonyl groups, and a methylene group. A convenient starting material was available in a-indanone, IV, which Ruhemann planned to oxidize with pnitrosodimethylaniline to the imine V. Hydrolysis of the latter, he thought, would give the desired diketone structure (8). 0
0
The attempted synthesis of V was a failure in that it did not produce any of the desired imine. However, the experiment led to a much more significant result. Both methylene groups in I V proved reactive toward pnitrosodimethylaniline, and so the product was the
diimine VI.
Hydrolysis gave the triketone monohy-
drate to which Ruhemann assigned the correct structure I, reasoning the because the compound was colorless it could not be a 1,2-diketone (@.' We may never know why Ruhemann treated ninhydrin with ammonia. However, his original paper on the compound contains the following key passage: "of especial interest is the action of ammonia on the triketone. If the aqueous solution of the mixture of both substances is kept for a short time it turns a deep reddish-violet, and no longer precipitates AgN03. . ." (8). This striking observation was followed up by a second paper only a few months later, in which the first mention is made of the color reaction of ninhydrin with ol-amino acids, peptides, and proteins (4). And the next year, Ruhemann writes: "It has been found that it (I)is a most valuable reagent for proteins and their hydrolytic products, since by means of it mere traces of these substances can be recognized" (5).
I n a classic series of papers (4-8) all following within two years of the original discovery, Ruhemann set forth most of what is now known of ninhydrin chemi ~ t r y . I~n his systen~aticexploration of this field, Ruhemann elucidated the alkaline degradation of ninhydrin to o-carboxymandelate (4), and discovered the facile bimolecular reduction of I to the pinacol, hydrindantin (6). He also demonstrated the relationship of ninhydrin to analogous monocyclic compounds (5),and synthesized a related, more complex compound which also showed the ninhydrin color test (8). Ruhemann first proposed an incorrect formula for the red condensation product of ninhydrin with ammonia (s), but he soon amended this to the essentially correct structure VII (7). The only change we might make today would he to represent the anion of this red dye in delocalized form, as VIII.
signed his University appointment in 1914, and after the war returned to Germany, where he served for two years as an assistant in Fischer's laboratory. I n 1921, a position was found for him as head of a small industrial research institute, the Braunkohle und Mineralolforschungsinstitut in Charlottenburg. Thus in his sixties Ruhemann embarked on a second career in peat and lignite chemistry. His modest institute gradually expanded, and during his 10 years there Ruhemann published 11 papers, mostly on applied chemistry. During this period, he made the lirst discovery of an azulene from a "mineral" source, peat tar (11,I,!?). Ruheman retired in 1933 and lived quietly in Zehlendorf for several years; but fate continued to hound him in his old age. He was bitterly opposed to the Nazi regime, and considered leaving Germany for some time. Finally, just before the outbreak of the war in 1939, he returned again to England where he spent his last few years living quietly with his wife and son. His death in 1943 was unmarked in German publications, but occasioned a biographical tribute by R. Morrell in the Journal of the Chemical Society (13).3 Most of Ruhemann's papers, though constituting solid and substantial bricks built into the masonary of chemistry, are not in themselves remarkable and so have been largely forgotten. His discovery of ninhydrin is a brilliant exception. If he had been merely a skilled and careful chemist, he might have isolated and identified the unexpected triketone, and then nught have taken another approach to his original objective. The mark of his genius is that he realized the sign& cance of his unexpected finding, and so pursued the chemistry of ninhydrin until he discovered and rationalized the reaction with amino acids which makes it so useful. It is ironic that his studies in this area were barely completed when the war ended forever Ruhemann's r e searches in fundamental organic chemistry. In these days of teamwork, it is interesting to note that Ruhemann's finest research was done when he was forced by circumstances to return to individual laboratory work. Since age spares no one, it may he heartening to some to realize that Ruhemann's most important discovery was made when he was 51 years old. Finally, the discovery of ninhydrin provides an example of a highly original and important contribution made by a man best remembered as a gifted and stimulating teacher. Literature Cited
VII
VIII
As this work was being completed, Britain was entering into a period of anti-German hysteria which ended Ruhemann's scientific life in that country. He re-
' The synthesis of ninhydrin was very nearly anticipated two decades before Ruhemann's work by Wislicenus, who p r e pared a monoxime of ninhydrin and some related derivatives in 1888 (5). However, no mention of the free triketone or its hydrate appears in the earlier literature. However, Ruhemann did not coin the trivial name "ninhydrin," nor did he ever use it in his writings. The name first appears in print in a publicstion by Abderhalden and Schmidt in 1913 (9). Shortly after the color reaction was discovered, Ruhemann sent semples of ninhydrin to Abderhalden (IO),who played an important part in pop~ilarieingthe use of the reagent on the continent.
(1)RUHEMANN, S., Trans. Chem. Soc., 98, 1438,2025(1910) S., T~an.9.Chem. Soc., 97, 1438 (1910). (2)RUEEMANN, (3)WISLICENUS. W.. Ann. Chem.. 246. 306 (1888).
(7) RUHEMANN, S., ibid., ! (8)RUKEMANN, S., ibid., 101, 780 (1912). (9)ABDERHALDEN, E..AND SCHMIDT. phwiol. Chem. . H.,. Z. . . 85,143 (1913). (10)ABDERHALDEN, E., AND SCHMIDT, H., Z. phygiol. Chem., 72.37(1911). , (11)HERZENBERG, J., AND RUREMANN, S., Ber., 58B, 2249
. .
(1925). ~-.-.,.
(12)RUHEMANN, S., AND LEWY,K., Be?., 60B, 2459 (1927). R. S., J . Chem. Soc., 46 (1944). (13)MORRELL,
' Most of the biographical detail8 in the present paper were drawn from this source. Volume 42, Number 7, July 1965
/ 387
