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In the summer of 1912, a 22-year-old

graduate student went on holiday withhis parents to Britains Yorkshire coast.

There, his father, the physicist William H.Bragg, received a letter describing a dramaticlecture given by the German theoreticalphysicist Max Laue.

Laues lecture reported the first observa-tion by his colleagues Walter Friedrich andPaul Knipping1 of the diffraction of X-raysby a crystal the mineral zinc sulphide(ZnS). This proved that X-rays were waves,settling a controversy that had lasted the17 years since their discovery. Bragg and hisson Lawrence worked feverishly on X-ray

diffraction all that summer at the University

of Leeds, UK, where William was professor

of physics.On returning to the University of Cam-

bridge at the end of the holiday, Lawrencehad a revolutionary idea. Laues results,he reasoned, could be interpreted simplyas arising from the reflection of X-rays byplanes of atoms in the crystal. He realizedthat X-ray observations, of the kind initiatedby Laue, provide evidence from which thearrangement of atoms in the crystal couldbe inferred.

To explain the patterns they saw (pic-tured), Laue and his colleagues had assumedthat their X-ray source was polychro-

matic comprising six or seven distinct

wavelengths and that the structure of ZnSwas a three-dimensional array of tiny cubes,with the zinc and sulphur atoms occupyingeach alternate corner.

Lawrence examined the X-ray photo-graphs and noted that some of the diffrac-tion spots were elliptical and that some haddifferent intensities. In a paper read by his

supervisor, J. J. Thomson, to the CambridgePhilosophical Society on 11 November 1912,Lawrence made two important proposals2 toaccount for these features.

First, he suggested that Laues results arosefrom the reflection of a continuous range ofX-ray wavelengths by planes of atoms withinthe crystal. This interpretation yieldedBraggs law of X-ray diffraction: n= 2dsin,where is the angle of incidence of X-raysof wavelength , dis the separation of thereflecting planes and n is an integer. Second,he proposed that Laues diffraction patternfrom ZnS was characteristic of atoms located

not only at the corners of the three-dimen-sional array of cubes, but also at the centreof the faces of each cube a face-centredlattice.

So far, some two dozen Nobel prizes havebeen awarded for work related to X-ray crys-tallography, the technique Bragg set in trainwith his paper and used for his pioneeringwork on the structures of minerals, metals,their oxides and alloys. His colleagues werethe first to use X-ray crystallography todetermine the structures of a protein andan enzyme, and to formulate the modelfor the DNA double helix3. In my view, thetechnique is still the single most powerfulanalytical tool for scientists in physics, biol-ogy, medicine, materials and Earth sciences,as well as for many breeds of engineer.

VISIONARIES

In the audience in Cambridge in Novem-ber 1912 was the physicist C. T. R. Wilson,whose work using cloud chambers to trackcosmic rays earned him the Nobel prize in1927. Wilson suggested that X-rays shouldalso reflect from the external faces of crys-tals, provided the surfaces were sufficientlysmooth. So Lawrence tested whether X-raysthat reflected from the cleavage face of mica

known for its supposed f latness at theatomic scale could be photographed. InDecember 1912,Naturepublished his paperon The Specular Reflection of X-rays4.

William Bragg quickly demonstratedthat his X-ray spectrometer could detectdiffracted monochromatic X-rays noton photographic plate, but with a gas ioni-zation detector. The power of Braggs law,together with Williams spectrometer forrecording the intensities of reflected X-raysof fixed wavelength, was spectacularly dem-onstrated in two 1913 papers. Lawrencepublished one on the structures of crystals

of sodium chloride, potassium chloride,

The birth of X-raycrystallographyA century ago this week, physicist Lawrence Bragg

announced an equation that revolutionized fields frommineralogy to biology, writes John Meurig Thomas.

Lawrence Bragg remains the youngest ever winner of a Nobel prize, aged just 25.

COURTESY

S.

L.

BRAGG

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potassium bromide and potassium iodide5,and another with his father6 on diamond.

The Braggs approach provided a reliableway to determine the internal architectureof all crystalline solids, and thus to explaintheir properties. Once the structure of dia-mond was discovered with its infinitearray of carbon atoms bonded strongly

to others in three dimensions its hard-ness could be understood. Likewise, whenX-ray crystallography revealed the struc-ture of graphite in the 1930s, its softnessmade sense. Diamond and graphite havethe same composition, but their structuresmake them mechanically, chemically andelectronically very different.

Not until after the First World War didshock and exhilaration greet the publica-tion of these papers, when their con-tent filtered through to the textbooks.Shock, because Bragg had incontro-vertibly established, contrary to what

all chemists thought at the time, thatthere was no molecule of sodiumchloride inside rock salt simply anextended alternation of sodium andchloride ions. A particularly intemper-ate attack was mounted by Henry Arm-strong, former president of the ChemicalSociety of London. Writing in Nature in1927, he described the chess-board patternof atoms in sodium chloride as repugnantto common sense and absurd to the nth degree7. Others were exhilarated becausethe structure of diamond confirmed the tet-rahedral coordination of carbon as envisagedby J. H. vant Hoff and others 40 years earlier.

For the next several decades, the Braggsequation and spectrometer became the cor-nerstones of X-ray crystallography, largelysupplanting Laues polychromatic X-ray dif-fraction procedure. (Some experimentalistsused both methods notably, Linus Paul-ing in his determination of the structures ofhaematite and corundum in 1925.)

NOBEL HAUL

At 25, Lawrence Bragg is still the youngestever recipient of the Nobel prize, which heshared with his father in 1915 for their ser-vices in the analysis of crystal structure by

means of X-rays. He kept working at a pro-digious pace for some 50 more years8. Lauewas awarded the Nobel prize in 1914 for thediscovery of X-ray diffraction by crystals but,unlike Lawrence, he largely ceased to workon its consequences, turning to relativity andother pastures in theoretical physics.

In 1919, Lawrence succeeded ErnestRutherford as chair of physics at the Uni-versity of Manchester, UK. Using tech-niques that flowed from his 1912 and1913 papers, he accounted for the chemi-cal and physical properties of silicates, thedominant members of the mineral king-

dom, which traditional chemistry had not

explained in satisfactory atomic terms. Heexplained, for example, why mica and talcare so soft, but beryl is tough.

He showed that many minerals, especiallysilicates, are dominated by essentially space-filling, negatively charged oxygen atoms. Hefound that other, smaller (cationic) atomsare lodged in the interstices,and discovereda constant tetrahedral coordination of fouroxygen atoms, whatever the ratio of silicon

to oxygen. At Manchester he also solved thestructures of -brass, magnetic alloys andmany others fundamental to the develop-ment of the modern theory of metals.

In 1938, Bragg again replaced Ruther-ford, this time as Cavendish professor atthe University of Cambridge. Here, afterthe Second World War, he encouraged hisprotgs Max Perutz and John Kendrewin their fiendishly difficult X-ray crystal-lographic determination of the proteinshaemoglobin and myoglobin. Later, hegave free reign to Francis Crick and JamesWatsons X-ray work on DNA.

SEEING IS BELIEVING

In 1953, Bragg became director of, andFullerian professor at, the Royal Institutionof Great Britain (RI) in London, where heappointed David Phillips, Tony North andothers to investigate biological structures,with Perutz and Kendrew as honoraryreaders. A special automated, linear X-raydiffractometer built at the RI enabled Ken-drew, Phillips and others to produce thefirst structure of a protein myoglobin. Italso helped Phillips and Louise Johnson toestablish the structure and mode of action oflysozyme, the first enzyme to yield to Braggs

X-ray technique. While at the RI, Bragg had

the satisfaction of hearing, in 1962, of theaward of Nobel prizes to his acolytes Perutz,Kendrew, Crick and Watson.

The Braggs method of structure determi-nation is still at the heart of modern X-raycrystallography. It is now almost completelyautomated by sophisticated, ultra-sensitiveX-ray detectors and associated algorithms

for data analysis of hundreds of thousandsof diffraction intensities.Meanwhile, the advent of accessible syn-

chrotron radiation sources and rapid read-out detectors is especially well suited tocharting structural changes that take placeon sub-picosecond timescales in biologicalmacromolecules such as the photoactive

yellow protein PYP9. A striking exampleis the work of an international team ofresearchers10, almost exactly a centuryafter the pioneering papers by Braggand Laue. The team aimed femtosec-ond synchrotron pulses at a stream

of droplets containing biologicallysignificant macromolecules such asphotosystem I, which is central to

photosynthesis. The X-ray pulses areshort enough to avert radiation dam-

age, but sufficiently intense to producehigh-quality diffraction data.

The seminal work begun in Yorkshirethat summer of 1912 still resonates world-wide. Just this week, Venki Ramakrishnan who shared the Nobel Prize in Chemistryin 2009 for unravelling the structure of theribosome, which catalyses protein synthesis was scheduled to lecture the CambridgePhilosophical Society. His theme? Seeing isbelieving: how a century after its discovery,Braggs law allows us to peer into moleculesthat read the information in our genes.

John Meurig Thomas was LawrenceBraggs successor-but-one as director andFullerian professor at the Royal Institution.He is now in the Department of MaterialsScience and Metallurgy, University ofCambridge, Cambridge CB2 3QZ, UK.e-mail: jmt2@cam.ac.uk

1. Friedrich, W., Knipping, P. & Laue, M. InSitzungsberichte der Math. Phys. Klasse (Kgl.)

Bayerische Akademie der Wissenschaften303322 (1912).2. Bragg, W. L. Proc. Cambr. Phil. Soc.17, 4357

(1913).3. Bragg, W. L. inFifty Years of X-ray Diffraction (ed.

Ewald, P. P.) 531539 (International Union ofCrystallography, 1962).

4. Bragg, W. L. Nature90, 410 (1912).5. Bragg, W. L. Proc. R. Soc. Lond. A89, 248277

(1913).6. Bragg, W. H. & Bragg, W. L. Proc. R. Soc. Lond. A

89, 277291 (1913).7. Armstrong, H. E. Nature120, 478479 (1927).8. Thomas, J. M. & Phillips, D. C. (eds) Selections

and Reflections: The Legacy of Sir Lawrence Bragg(Science Reviews, 1990).

9. Rubinstenn, G. et al. Nature Struct. Biol.5,568570 (1998).

10. Spence, J. C. H., Weierstall, U. & Chapman, H. N.

Rep. Prog. Phys. 75, 102601 (2012).

Max Laues photo of X-ray diffraction from ZnS

revealed spots of varying shape and intensity.

SOURCE:REF.

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