Issue Date: August 18, 2008
Recognizing A Pioneer
In the world of scientific discovery, the story behind the story is usually left untold. The basic facts, such as who discovered what and when, tend to be well documented. But the personalities and idiosyncrasies, the private struggles, and the tabloid-type tidbits that humanize the people who make the discoveries rarely find their way into history books. And they almost never make it onto the pages of scholarly journals.
This is the story of a chemist who sought to uncover the origins of his field and ran into surprises at nearly every turn. After digging about in 18th- and 19th-century journals, he realized that a key discovery in the area of chiral crystals—the "who discovered what and when" part—wasn't credited properly after all. Then he unexpectedly found that the field's true pioneer was a really nasty piece of work. That finding led to a chance meeting and collaboration with another scientist who had insider information about Professor Nasty's nasty ways. Eventually the chemist-historians learned that the long-dead professor—nasty as he was—wasn't guilty of some of the most egregious deeds attributed to him.
The story starts at the University of Washington, Seattle, where Bart E. Kahr, a chemistry professor, specializes in the optical properties of crystals. "I have a natural predisposition to history and the history of science," Kahr says. He also has a strong interest in chirality, stereochemistry, and crystals—topics he focused on as a graduate student and postdoc.
One topic that especially caught Kahr's interest early in his career is dyed crystals. Many crystals are endowed with color naturally due to the inclusion of impurities such as metal ions. Examples include ruby and sapphire, which owe their colors to chromic and ferric ions, respectively, in otherwise colorless alumina crystals. In contrast, dyed crystals are deliberately colored by adding small quantities of organic dyes to solution during crystal growth. Questions about these types of crystals, such as how their lattices can accommodate large organic molecules, intrigued generations of researchers, Kahr notes. Not all of the questions have been answered.
"When I learned about dyed crystals from the 19th-century scientific literature, I recognized that this was a very general and beautiful aspect of supramolecular chemistry that had not been brought to the attention of contemporary scientists," Kahr says. Given the growing importance of supramolecular chemistry in 21st-century science, the topic seemed like a good choice for an assistant professor to pursue. So he did.
In 2001, Kahr published a paper in which he set out to compile an exhaustive list of studies on crystal dyeing (Chem. Rev. 2001, 101, 893). While poring over some 500 references for the review, he came across a forgotten paper from 1919. In that study, Eligio Perucca, a physics instructor at the University of Turin, in Italy, and later a professor at the nearby Turin Polytechnic, reported observing optical rotatory dispersion (ORD) as a result of passing linearly polarized light through sodium chlorate crystals that had been colored by growing in an equilibrium racemic mixture of a triarylmethane textile dye called extra China blue (Nuovo Cimento 1919, 18, 112).
As Kahr explains, Perucca was trying to sort out details of an experiment reported in 1860 in which a researcher claimed that amethyst, a form of quartz colored purple due to impurities, exhibits optical activity in the visible part of the spectrum.
To evaluate the 1860 research, Perucca prepared an amethyst substitute. He used readily available sodium chlorate, which like quartz is chiral in the crystalline state, and added the organic dye to the solution from which the crystals grew. The goal of the experiment was to see whether the chiral crystals could induce enhanced, or strongly wavelength-dependent, optical activity (rotation) in the 500–600-nm absorption band of the otherwise optically inactive dye. (At these wavelengths, the undyed crystals are transparent and would not exhibit enhanced optical activity.) According to Perucca, that's exactly what happened. By dyeing the crystals, he had made a material that was strongly optically active in that wavelength region.
With the benefit of 90 years' hindsight, Kahr concluded that what Perucca must have observed was the resolution of the racemic mixture of dye molecules on the growing crystal surfaces. Specifically, as Perucca's
NaClO3 crystals grew from solution, they were dyed by selectively embedding one enantiomer of the organic compound into their lattices. For good measure, Kahr and coworkers Yonghong Bing and Werner Kaminsky repeated the experiment this year, using aniline blue as the dye compound. They confirmed the 1919 results.
Once again drawing on hindsight, today's chemists know that in solution the triarylmethyl compound is an equilibrium racemic mixture of propeller-shaped chiral molecules. Perucca couldn't reasonably have known that bit of stereochemistry, Kahr says, because that idea was first proposed in the 1940s by G. N. Lewis. Nonetheless, Perucca sought to induce a chirality-based optical (chiroptical) response from an achiral dye solution, which unbeknown to him happened to be a racemic mixture, by exposing it to a chiral crystal. He succeeded.
As fate would have it, however, Perucca's contribution was forgotten. Induced chirality of this type eventually came to be known as the Pfeiffer effect in recognition of University of Bonn chemistry professor Paul Pfeiffer's studies, which weren't conducted until 1931, a dozen years after Perucca's pioneering study.
Perucca's work could have been recognized as a seminal study in another context—in connection with enantioselective adsorption of racemic mixtures on inorganic crystals. Interest in that subject rose in the 1970s when many researchers began investigating crystal-induced enantioselection in search of the origin of the homochirality of life. One of the key challenges in that topic is understanding why naturally occurring amino acids exist almost exclusively as L-enantiomers. Here again, Perucca's work isn't cited.
"Perucca's paper should be viewed as a milestone in the chiroptics literature, as well as in the history of enantioselective chemistry," Kahr says. Perhaps thanks to the efforts of Kahr and coworkers, Perucca, who died in 1965, may be recognized posthumously for his discovery. Perhaps not.
For some scientists, the story might have ended there. But not for Kahr. "This Perucca guy became my new stereochemistry hero, so I tried to find out who he was," Kahr explains. He quickly learned that Perucca had a reputation as an outstanding experimentalist and was the author of textbooks. So far, so good. But after a little more digging, Kahr discovered that Perucca was remembered in a written history of the science faculty of the University of Turin as a "personaggio bizarro." "That was an apple I just couldn't leave hanging on the tree," Kahr says. "I had to know what that meant."
Soon, Kahr found himself rereading "The Periodic Table" by the Italian chemist Primo Levi, one of Kahr's all-time favorite authors, on the off chance that Levi had mentioned something about his Turinese contemporary, Perucca. To his surprise, Kahr found that Levi took an interest in optical rotation, but the book makes no mention of Perucca. Levi did note, however, that as a Jew living during the Mussolini regime, he was barred from completing a graduate thesis that required experimental work.
Elaborating on those troubles, Levi pointed out that in 1941, he made "desperate attempts" to be taken on as a student assistant by various professors—but doesn't name them. "Some of them snidely or even arrogantly told me that racial laws prohibited it," Levi wrote. Was Perucca one of those snide anti-Semitic professors who toed the Fascist Party line and refused to let Levi work in his lab because he was a Jew, Kahr wondered?
Yes he was, according to one of Levi's biographies. Intrigued by that finding, Kahr contacted Myriam Anissimov, the book's author, to learn how she knew that Perucca was guilty as charged. Anissimov explained that the information came from interviews she conducted in the past with Levi's personal acquaintances, most of whom were no longer living. According to Kahr, Anissimov said that by chance, she had just spoken with one of Levi's friends who confirmed that Perucca was a "perverse and extremely spiteful man."
The friend went on to say that "cruel" Perucca's legend lives on to this day in Turin and backed up that assertion with a story of how Perucca questioned a student during an oral exam about the reason that washbasins were set 1 meter above the floor. The student tried unsuccessfully to rationalize the sink's position by drawing on principles from fluid mechanics. According to Levi's friend, Perucca failed the already demoralized student and told him the reason: The sinks are 1 meter above the floor so you can wash your hands standing up.
Once again the story could have ended there. But it didn't.
Kahr presented it—washbasin and all—at the 2007 meeting of the American Crystallographic Association (ACA) in Salt Lake City. "Then an older gentleman stood up and asked me to yield the floor," Kahr recalls. "I thought I was sunk. This guy was going to say he was Perucca's son or nephew, and I was about to get an earful."
"My name is Davide Viterbo," the man told the audience, Kahr recounts. It turns out Viterbo wasn't Perucca's relative. But he grew up and lived in Turin and knew all about the man he called "the terrible physics professor." Viterbo, a chemistry professor at the University of Eastern Piedmont, in Alessandria, near Turin, happened to be in Salt Lake City at that time on business with the International Union of Crystallography. Knowing that the ACA meeting was being held at the same time, he attended the conference and upon consulting the program was shocked to see the name Eligio Perucca in the title of a presentation. "I felt I had to attend the talk and say something to the audience," Viterbo tells C&EN.
Viterbo says he was never Perucca's student, but several of his friends were. And from them and others he heard stories of Perucca's mean ways. Sharing Turinese lore with the audience, Viterbo confirmed Kahr's characterization and told of another student who was asked by Perucca during an oral exam to describe the Carnot cycle. The flustered student did poorly. So Perucca instructed the student to draw a circle on the blackboard and then another circle and then a couple of lines. Finally, Perucca informed the confused student that he had sketched not the Carnot cycle, but a Carnot bicycle, and to get on and ride away because he had just failed the exam.
Attendees were stunned. The session chair, Joel Bernstein, a chemistry professor at Ben Gurion University of the Negev, in Israel, describes the scene as "a rare and truly memorable moment. It was the stuff memories are made of."
Yet even as Viterbo confirmed the popular description of the infamous physics professor, he remained unconvinced of Perucca's refusal to accept Levi in his research group. Recalling his reaction to Kahr's presentation, Viterbo says it seemed very unlikely that Levi, a chemistry student at the University of Turin, would have tried to approach Perucca about the possibility of conducting graduate research in his laboratory at the Polytechnic, an entirely separate academic institution.
So Viterbo and Kahr teamed up and kept on digging. Upon returning to Italy, Viterbo contacted Perucca's former assistant, Luigi A. Radicati di Brozolo, now emeritus professor at the University of Pisa; Perucca's nephew, Gianni Perucca; and others who knew Perucca. All agreed that far from being a fascist, Perucca was in fact an antifascist.
The team found further support for that conclusion in an essay about Perucca entitled "A Fascinating and Terrifying Teacher." Written by a Turinese student, Angiola Maria Sassi Perino, the article describes how Perucca received Mussolini during an official visit to Turin in 1939. Rather than donning the black shirt expected of Mussolini supporters, the physics professor made a bold political statement by deliberately showing up for the affair outfitted in fancy ceremonial attire.
Viterbo also consulted his friend Renato Portesi, who for several years worked with Levi at a varnish factory near Turin. Portesi recalled that Levi had once spoken of occasionally attending lectures by Perucca because they were interesting and peppered with witty antifascist remarks.
All in all, on the basis of anecdotal evidence and various interviews, including those with Levi's sister and son, Kahr and Viterbo concluded that Perucca was neither a fascist nor by association an anti-Semite.
And so now the story finally comes to an end. By digging about in long-ago yellowed journals, scientists today rediscovered a forgotten pioneer of chirality, stereochemistry, and optical measurements. And through interviews and anecdotes, they uncovered his nature and personality. A brief paper summarizing their findings has recently been accepted for publication in Angewandte Chemie.
By all accounts, Eligio Perucca was a sharp scientist and a complex man. Thanks to Kahr and coworkers, Perucca's contributions to science may soon come to be recognized broadly. His nastiness may come to be recognized too. Perucca certainly wasn't a nice guy, Kahr says. "But that doesn't mean he should be sentenced to carrying all the burdens of history. It's important to set the record straight."
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