A Connection between Oriental Studies and Crystallography

Close-up of the mounting of the protein crystal inside a sealed glass capillary to prevent drying during the exposure. This early method was devised by J.D. Bernal. Nowadays protein crystals are typically frozen using a cryoprotectant.

Fig. 1. Close-up of the mounting of a protein crystal inside a sealed glass capillary to prevent drying during the exposure. This early method was devised by J.D. Bernal. Nowadays protein crystals are typically frozen using a cryoprotectant.

An obituary in the New York Times published June 22, 2013 had a brief review of the life and societal contributions of Martin Gardiner Bernal (1937-2013) (http://www.nytimes.com/2013/06/23/arts/martin-bernal-black-athena-scholar-dies-at-76.html?_r=0), an Oriental studies scholar who taught Chinese history at Cornell University for thirty years. Two things drew my attention to the passing of Prof. Martin G. Bernal. Firstly, that I had the privilege of knowing him personally and of interviewing him in relation to a play and documentary that Jill Campbell, Gregory Gerhard and I were working on entitled Bernal’s Picasso. Second, that he was the ‘illegitimate’ child of John Desmond Bernal (a prominent British scientist, according to the newspaper) and Margaret Gardiner, a writer. Illegitimate in the sense of having been born out of wedlock, something rather unusual –or perhaps not so- in the England of the late 1930’s.

Fig. 1. Photograph of the original pepsin crystals exposed to the X-rays by Bernal and Crowfoot in 1934. Photo courtesy of Judith A.K. Howard (University of Durham, UK) and Georgina Ferry (Oxford).

Fig. 2. Photograph of the original crystals of pepsin  exposed to the X-rays by Bernal and Crowfoot in 1934. Photo courtesy of Judith A.K. Howard (University of Durham, UK) and Georgina Ferry (Oxford).

This latter fact alone provides an important clue to the character of his father, a charismatic political activist and according to Linus Pauling ‘… one of the greatest intellectuals of the 20th century’.  A biography of J. D. Bernal, The Sage of Science was published in 2005 by A. Brown devoting well over five hundred pages to present the remarkable person that “Sage” –as he was named because he knew everything- was.

Born in Ireland and educated in Cambridge, Desmond Bernal was one of the most influential crystallographers of the 20th century. He trained and inspired the most brilliant young minds in the early days of structural and molecular biology. Max Perutz, Dorothy Hodgkin, Aaron Klug, Francis Crick, James Watson, Rosalind E. Franklin, Lawrence Bragg and so many others have recognized the uniqueness of Bernal’s mind, his encyclopedic knowledge and his commitment to the social and political events of his time.

In this brief note, I would like to highlight a unique insight that Desmond Bernal had in 1934, which gave birth to the discipline of protein crystallography and revolutionized the study of biological macromolecules.  The events that led to this insight and to the first successful X-ray diffraction images of crystals of pepsin are worth retelling to illustrate the often quoted aphorism from Louis Pasteur: ‘Fortune favors only the prepared mind’.

John Howard Northrop obtained the first crystals of pepsin, the enzyme extracted from gastric juices, in 1930 at the Rockefeller institute, using a method that was later used to crystallize proteins such as trypsin, chymotrypsin and others. He and his coworkers correlated the enzymatic activity of their preparations with the amount of protein in the sample and established unambiguously that enzymes were indeed proteins. However, the crystals that appeared in Bernal’s laboratory in 1934 were grown by John Philpot in Uppsala. According to his personal account, he went to Uppsala officially to work with the new instrument of biochemical research named the ultracentrifuge; but unofficially, because of the presence of young lady in whom he had special interest.

A friend of Bernal’s, Glen Millikan, visited the laboratory of Svedberg (the father of the ultracentrifuge) in Sweden on his way back to England in the Spring of 1934. He was the son of the American physicist Robert Millikan (famous for having determined the charge of the electron) and an itinerant scientist based at Trinity College in the U.K.  The story goes that Philpot had left a beaker of concentrated pepsin in solution standing while he went on a skiing excursion. Upon his return, he found that some amazingly large (near 2 mm in length, Fig.1) crystals of pepsin had been formed in the beaker solution. Upon seeing these crystals, Millikan thought instantly of Bernal and exclaimed:  ‘I know a man in Cambridge who would give his eyes for those crystals”.

Millikan carried in his pocket the tube with the pepsin crystals bathed in their acidic mother solution back to Cambridge. Bernal immediately began studying the recently arrived pepsin crystals that were the best he had seen so far.  He extracted one from the tube, mounted it in front of the X-rays and exposed it with the intention of getting a diffraction pattern. To his dismay, there was none. Then, he took another one and carefully examined the striking birefringence under the polarizing light microscope of the hexagonal crystals. This was known from well-formed crystals but the crucial observation came next. As the pepsin crystal dried out, it lost its birefringence!  This crucial observation led to Bernal’s insight: as the crystal lost the internal water, it lost its order. The crystals needed to be kept moist with their mother solution.

Helen Megaw in the laboratory was working on ice crystals and for this work she used glass capillaries.  Bernal sucked a single crystal into a thin-walled glass capillary and included a few drops of the mother solution, sealing the glass capillary afterwards (Figs. 1,2).  The resulting diffraction pattern was spectacular. There were diffraction spots all over the film, implying that the pepsin molecules inside the crystal were ordered to the atomic level. Conceptually, at least, the door was open to unravel the atomic structure of proteins in a similar way as the first diffraction patterns of common salt (NaCl) revealed their atomic structure two decades before. “Sage” was jubilant and was known to have wandered about the streets of Cambridge that evening, ruminating on the implications of this finding for the future unraveling of the structures of proteins.

Dorothy Crowfoot (later Hodgkin) had been experiencing her first symptoms of rheumatoid arthritis and was away from the lab on that day.  Upon her return, she reproduced and expanded on Bernal’s observations and together they could derive the basic parameters of the hexagonal unit cell  (a=b=67, c=154 Å) of the crystal and the implications for the structure of pepsin. The evidence for proteins being entities having a well defined globular, ordered structure was sound.  Bernal and Crowfoot sent a letter to Nature, announcing the first diffraction photographs of a soluble protein.  After this momentous insight, Bernal sent a letter to William Thomas Astbury who had firstly exposed small pepsin crystals (grown by J. H Northrop) to X-rays. His results had been rather meager and amounted to ‘two broad rings’ but no distinct spots.

This account of the events connected to the first successful X-ray diffraction of protein crystals reveal the uniqueness of J.D. Bernal’s mind, whose influence together with W.L. Bragg’s was critical for the eventual full development of macromolecular crystallography as we know it today.  Aaron Klug, one of his numerous and brilliant followers, used to emphasize the rareness of Bernal’s intellectual qualities always stressing that ‘crystallographers must learn not to be crystallographers’. An aphorism that is now truer than ever before given the complexity of the biological roles that biological macromolecules play in the cell and in pluricellular organisms.


Further reading

Brown, A.  J. D. Bernal. The Sage of Science. 2005. Oxford University Press.

Dobson, G and C. Chothia. Nature (1984). 309, 309. Fifty years of pepsin crystals.

H.F. Judson. The Eight Day of Creation. The Makers of the Revolution in Biology. (1979). Jonathan Cape, Thirty Bedford Square, London.

Bernal, J.D. and Crowfoot, D. Nature (1934) 133, 794-795. X-ray Photographs of Crystalline Pepsin.