Lawrence Bragg, Cambridge circa 1913

In 1912 a young graduate working in Cambridge University’s Cavendish Laboratory made a breakthrough that represents the birth of x-ray crystallography. Professor Sir John Meurig Thomas tells the remarkable story of the career of Lawrence Bragg, youngest-ever winner of a Nobel prize.

The method of structure determination developed by Bragg and his father in 1912 is still at the heart of x-ray crystallography.

Professor Sir John Meurig Thomas

This month marks the centenary of the scientific breakthrough that had a profound influence on the evolution of modern science. At the centre of it all was a 22-year-old graduate at the Cavendish Laboratory, working under the supervision of JJ Thomson, the discoverer of the electron. The breakthrough constitutes the birth of x-ray crystallography, a technique that can resolve in atomic detail the invisible architecture of all crystalline solids.

The student responsible for this major advance, Lawrence Bragg was an Australian, born in Adelaide, where his father WH Bragg was a professor of mathematics and physics. In 1912 the young Lawrence was unhappy in the Cavendish, which he had joined as a graduate of Trinity College in natural sciences. But in the summer of that same year his world was transformed.  During a family holiday on the Yorkshire coast, he learnt from his father (who was by then professor of physics at the University of Leeds) that a remarkable observation had been made at the University of Munich by the theoretical physicist Max von Laue and his colleagues.

The German scientists had reported to the Bavarian Academy the observation that x-rays underwent a process known as diffraction, a kind of interference, on penetrating a crystal of the mineral zinc blende (ZnS). This proved without doubt that x-rays were wave-like, just like ordinary light, but of much shorter wavelength. Working intensely in Leeds later that summer, young Bragg and his father repeated and extended the experiments of Laue et al. 

On returning to Cambridge Lawrence Bragg had a brilliant idea. While walking along the Backs, not far from St John’s College, he reasoned that Laue’s results could be interpreted simply as arising from the reflection of x-rays by planes of atoms in the crystal. He realised that x-ray observations, of the kind initiated by Laue, provided evidence from which the arrangement of atoms in the crystal could be inferred. Moreover, he was able to re-interpret and correct the results of x-ray photographs published by Laue and colleagues. And in a paper read by his supervisor – being a student he was not permitted to do so himself – to the Cambridge Philosophical Society on 11 November 1912, Bragg made two important proposals.

First, he suggested that Laue’s results arose from the reflection of a continuous range of x-ray wavelength by planes of atoms within the crystal. This interpretation yielded Bragg’s law of x-ray diffraction  where θ is the angle of incidence of x-rays of wavelength λ, d is the separation if the reflecting planes and n is an integer. Second, he proposed that Laue’s diffraction pattern from ZnS was characteristic of atoms located not only at the corners of the three-dimensional array of cubes, but also at the centre of the faces of each cube – a face-centered lattice.

In 1915 this work won Bragg and his father a Nobel prize. So far, some two dozen Nobel prizes have been awarded for work related to x-ray crystallography, the technique that Bragg set in train with his paper and used for his pioneering work on the structures of minerals, metals, their oxides and alloys. His colleagues were the first to use x-ray crystallography to determine the structures of a protein and an enzyme, and to formulate the model for the DNA double helix. This technique is still the single most powerful analytical tool for scientists in physics, biology, medicine, materials and Earth sciences, as well as many breeds of engineer.

In 1938 Bragg became Cavendish professor at the University of Cambridge. Here, after the Second World War, he encouraged his protégés Max Perutz and John Kendrew in their fiendishly difficult x-ray crystallographic determination of the proteins haemoglobin and myoglobin. Later he gave free rein to Francis Crick and James Watson’s work on DNA.

In 1953 Bragg became director of, and Fullerian professor at, the Royal Institution of Great Britain (RI) in London, where he appointed David Phillips, Tony North and others to investigate biological structures, with Perutz and Kendrew as honorary readers. A special automated linear x-ray diffractometer built at the RI enabled Kendrew, Phillips and others to produce the first structure of a protein – myoglobin. It also helped Phillips and Louise Johnson to establish the structure and mode of action of lysozyme, the first enzyme to yield to Bragg’s x-ray technique. While at the RI, Bragg had the satisfaction of hearing in 1962 of the award of Nobel prizes to Perutz, Kendrew, Crick and Watson.

The method of structure determination developed by Bragg and his father in 1912 is still at the heart of modern x-ray crystallography. It is now almost completely automated by advanced, ultra-sensitive x-ray detectors and associated algorithms for data analysis of hundreds of thousands of diffraction intensities.

Meanwhile, the advent of accessible synchrotron radiation sources and rapid read-out detectors is especially well suited to charting structural changes that take place of sub-picosecond timescales in biological macromolecules such as the photoactive yellow protein PYP. A striking example is the work of an international team of researchers, almost a century after the pioneering papers by Laue and Bragg. The team aimed femtosecond synchrotron pulses at a stream of droplets containing biologically significant macromolecules such as photosystem I, which is central to photosynthesis. The x-ray pulses are short enough to avert radiation damage but sufficiently intense to produce high-quality diffraction data.

The seminal work begun in Yorkshire in the summer of 1912 still resonates worldwide.  Earlier this month Venki Ramakrishan – who in 2009 shared the Nobel prize in chemistry for unravelling the structure of the ribosome which catalyses protein synthesis – gave a lecture to the Cambridge Philosophical Society. His theme? “Seeing is believing: how a century after its discovery, Bragg’s law allows us to peer into molecules that read the information in our genes.”

At 25, Lawrence Bragg is still the youngest-ever recipient of the Nobel prize, shared with his father for “their services in the analysis of crystal structure by means of x-ray”. He kept working at a prodigious pace for some 50 further years. Laue was awarded the Nobel prize in 1914 for the discovery of x-ray diffraction by crystals but unlike Bragg he largely ceased to work on its consequences, turning instead to relativity and other pastures in theoretical physics.

Professor Sir John Meurig Thomas was Lawrence Bragg’s successor-but-one as director and Fullerian professor at the Royal Institution. From 1978 to 1986 he was head of Physical Chemistry at Cambridge and from 1993 to 2003 he was Master on Peterhouse. He is now in Cambridge’s Department of Materials Science and Metallurgy.

The Cambridge Philosophical Society will hold a one-day meeting to mark the centenary of Bragg’s Law on Friday 11 January 2013. The event which will take place at the Cavendish Laboratory is open to all who are interested.


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