Giants of the Past
Giants of the past: Wilhelm Heintz (1817-1880)
Although nearly unknown today, Wilhelm Heintz was one of the most productive scientists of the 19th century, who made significant contributions to fatty acid chemistry. At the time of his death he had authored over 200 publications mainly in the area of physiological chemistry. Other areas included mineral analysis, improved techniques for elemental analysis, and organic chemistry. His relative obscurity was in part due to his position as Halle University, a small and less prestigious German University. As such, Heintz lacked the resources to lead a large research group. His only student of renown was Johannes Wislicenus (1833-1902). Heintz relied upon empirical investigations and published little on the theoretical aspects of chemistry. One exception centered around a dispute with Vladimir Markovikov on the structure of ethyl glycol amides. Heintz was one of the first to apply the new theory of chemical structure, introduced by Aleksandr Butlerov in the 1860’s, to ethyl glycol amides. Virtually all of Heintz’s published work was found in German journals of the day and he published just one book, a textbook on animal chemistry.
Heintz was born in Berlin, the son of a business man. His introduction to science came when he was apprenticed to a pharmacist but decided to pursue chemistry. Since he had not completed studies for entrance to the University, Heintz prepared for and passed the matriculation exam and enrolled at the University of Berlin in 1840. He completed his doctoral dissertation in February 1844 and by 1845 had published 13 papers. In 1846 he was appointed to the medical faculty as a lecturer in physiological and animal chemistry. Between 1845 and 1850, he described novel methods for isolating and identifying components of animal fluids and human urine. This work resulted in 26 publications. By 1851 Heintz took a position at the University of Halle, where remained until his death in 1880.
At Halle, Heintz turned his attention to the chemistry of animal fats which he referred to as the “fat kingdom”. The basic work in fat chemistry had been completed with the publication of Chevreuls “Chemical Research on the fatty bodies of animal origin.” (1823). When Heintz entered the field, other had identified many new fats and fatty acids all defined by their melting point and chemical composition. The first synthesis of a triglyceride was reported by Berthelot in 1853 by combining glycerol with combinations of three fatty acids.
Heintz became interested in fatty acids through his friendship with the physiologist Ersnt Brucke who had worked in his laboratory during the 1840’s on the composition of human fat. Brucke had assumed that human fat consisted of margaric and olein and therefore should produce margaric acid on saponification, but repeatedly obtained a fatty acid with a melting point below that of “magaric acid.” Although true magaric acid (C:17:0) was found subsequently in trace amounts in fats and oils, Heintz was able to show that the compound is a mixtuture of stearic and palmitic acids. The project was abandoned in 1848 when Brucke left for the University of Konigsberg. However, Heintz continued the research and between 1851 and 1857 a number of publications appeared describing the difficulties Brucke had encountered. Heintz concluded that human fat must contain an additional solid fatty acid that could not be separated from magaric acid by simple crystallization. By slowly crystallizing a sample of human fat below 0ºC and separating any solid fat from the remaining liquid, Heintz concluded that human fat consisted of a mixture of at least 6 different fats including margarin, palmitin, olein and a new fat he named anthropin, but the composition and melting points suggested they were still impure. Heintz struggled on how to seperate fatty acids until 1848 when Liebig reported that short chain fatty acids could be seaparated by conversion to their salts followed by distillation. Futhermore, Leibig found that both butyric and Valeric acid distilled first, leaving acetic acid behind, despite it having the lowest boiling point. Although Liebig offered no explanation, Heintz suggested that the longer chain acids had a lesser affinity for the sodium, thus allowing the separation to occur. In his own words, Heintz said “This train of thought led me to test the idea if it would not be simply possible to separate such substances from one another by their degree of affinity when their properties are so similar both in a pure state and when combined with other substances.” The long chain fatty acids could not be easily distilled, but Heintz settled on treating the fatty acids with lead acetate in hot alcohol. Upon cooling, acids with the greatest affinity for lead precipitated as the lead salt. The precipitate was filtered and acidified to obtain the acid, the filtrate again treated with lead. The process was repeated until the melting points of the acids remained constant. He was able to separate 3 or more acids by this technique. Analysis of mutton tallow and spermaceti lead him to believe they were more complex than earlier assumed. Heintz isolated a new fatty acid from anthropin he named anthropic acid.
In 1844 Lerch identified 4 fatty acids in butter including butyric, caproic, caprylic and capric. Heintz identified 4 more including myristic, palmitic, stearic and arachidic acids He noted that the composition of these acids seemed to follow a general law” that the saponification products of fats contain only those acids whose number of carbon atoms is divisible by 4 implying that any known fatty acid that did not conform to the law would be a mixture of fatty acids that did.
By 1854 Heintz reported compositional data on spermaceti. He found just 4 fatty acids including stearic, palmitic, myristic and lauric. However, a fraction with a melting point below that of lauric suggested a fifth component. In order to explain his results, he prepared mixtures of different fatty acids according to their carbon numbers and determined their melting points in his own words. If composition is plotted against melting point, Heintz wrote” in all cases the curve will sink below (the value of the lowest melting compound, and the turn upwards cutting the abscissa anf the slowly climbing above it. The curve remains constant for every two acids that differ by 4 or 8 carbon atoms. But the greater difference of composition of the 2 acids, the sooner the curve reaches its lowest point.”
However, these results did not explain the fraction melting at 32oC. Heintz suspected the fraction could be a mixture of 3 acids that increased by 4 carbon atoms. Again, mixtures of palmitic/myristic, combined with stearic and palmitic /lauric acid were prepared and their melting points determined. The data fully confirmed that one of the mixtures melted at 32.2oC.
By 1855 Heintz concluded that fats consisted of a small number of fatty acids and these acids always contained a multiple of four carbon atoms. And that mixtures of fatty acids produced a lowered melting point that sometimes remained quite sharp giving a false sense of purity. By 1857 he synthesized the true margaric acid of 34 carbon atoms and found its properties very different from the original margaric acid. Thus the acid is a mixture of stearic (C18) and palmitic (C16 ) acids. Heintz’s work illustrates a number of issues in 19th century chemistry. How could chemists identify a unique chemical species and what is the criterion for purity. Heintz believed the long accepted method of repeated crystallization until a constant melting point was reached was no longer adequate. The origins of melting point determinations is clouded in antiquity. Chevreul used melting points as a criterion of purity but did not describe the experimental details. Heintz used an especially constructed thermometer allowing melting points to be measured within 1/20th degC. He was apparently the first to describe fat melting points in detail and it would appear that the capillary tube method used by him is very similar to the official AOCS method.
Notes and further reading
Much of the material given here can be found in Wilhelm Heintz (1817-1880) and the chemistry of the fatty acids. Peter Ramsberg, Bull.Hist.Chem. 38 pp 19-28 (2013). The article is well written and informative.
Chevreul’s classic work on the nature of fats and oils is summarized in a recent translation by Dr. Albert Dijkstra (A Chemical study of oils and fats of animal origin) and is available from AOCS Press. A full biography of Chevreul was published by Albert Costa, Michel Chevreul : A pioneer of organic chemistry. Wisconsin State Historical Society (1962.)
Heintz’s work was described by Johannes Wislicensus in 1883, Berichte 16 pp 3121-3140. The author was a student of Heintz at Halle University in Germany.
The reader is referred to AJ Dijkstra “How Chevreul (1786-1889) based his conclusions on his analytical results”, Oleagineux Corps Gras. Lipides 16 pp 8-13 (2009)