Clear as crystal the story of the Braggs: how X-ray crystallography has contributed to science.
X FOR UNKNOWN...
Let us begin this brief history of the Braggs by looking at the scientific discoveries that underpin their work. It all began with the chance discovery of X-rays ('X for unknown) by Professor Wilhelm Roentgen in Germany on 8 November 1895. The X-rays were well named; it took 15 years of controversy before Max von Laue and the Braggs, by measuring the diffraction patterns of crystals, showed what X-rays are: 'light waves' with about a thousand times shorter wavelength than visible light. Like ultraviolet rays, which Roentgen was studying using a discharge tube, they are invisible to the human eye. The special feature of crystals, which enabled the wavelength to be measured, is their regular internal structure with planes of atoms spaced about a wavelength apart. The wavelength can then be used to measure this spacing. However, at the time, Roentgen did not know this. When asked what he thought when he made the discovery, Roentgen replied, "I did not think, I investigated".
Roentgen found he could cast shadows of objects hidden by opaque material, for example a leg bone normally not visible through the skin, onto photographic plates and develop them (in basically the same way we still take X-rays today). Bones, fillings in teeth, metal fragments under the skin, all showed up clearly under the new rays. At this point, there was no thought of crystals or finding their internal structure, that is, crystallography. Indeed the existence of atoms was still controversial. However, the promise of using the newly discovered X-rays in medicine was realised very quickly. In addition, Roentgen also suggested possible future uses such as testing for weaknesses in metals. X-rays (or X-radiation) are still regarded as the scientific discovery with the shortest time between discovery and application to medicine.
THE AUSTRALIAN CONNECTION
On hearing of Roentgen's discovery in the first few months of 1896, William Bragg (in Adelaide) and Thomas Lyle (in Melbourne) produced X-ray images (radiographs) and even made X-ray tubes. Father Thomas Slattery, a Physics teacher at St Stanislaus College in Bathurst, took a radiograph of a young man's hand riddled with shotgun pellets. With the aid of Slattery's laboratory-made X-ray apparatus the hand was saved by the doctor who could now more easily locate the pellets using the radiograph. It was unheard of for anyone to make such equipment in a country town.
Similarly, in Adelaide, six-year-old Lawrence Bragg suffered a broken left elbow. At the time, his father William Bragg, then a professor, was experimenting at the University of Adelaide with one of the new X-ray tubes that had recently arrived in Australia. That night Lawrence was taken into his father's laboratory. In Lawrence's own words:
I well remember my father's first experiments with X-ray tubes ... I must have been one of the first patients to bew X-rayed in South Australia ... My father ... took radiographs of the broken elbow. I was scared stiff by the fizzing sparks and smell of ozone (from the galvanic batteries and large induction coil which supplied the high voltage to the X-ray tube). (Jenkin, 2008)
Professor Bragg gave several well-documented public lectures with demonstrations of these new rays in Adelaide in May and June 1896. A radiograph of his own hand with a coin under a finger, taken at the time is shown in Figure 1. The X-ray tube contained air under low pressure and cold metal electrodes. When a high voltage was placed across the electrodes, the air was ionised and a beam of electrons was attracted to and stopped by the positive plate, releasing X-rays. Bragg was an exquisite experimenter and became skilled in operating the tubes. His son, and collaborator, Lawrence said later, "Few understood what brutes these early X-ray tubes were" (Caroe, 1978).
While Lawrence and his younger brother Robert grew up and attended school and university in Adelaide and a sister Gwendolen was born (Gwendolen was named after her mother, the daughter of Sir Charles Todd FRS, of Overland Telegraph fame), William Bragg's interest shifted to the study of radioactivity. In 1904, William Bragg persuaded a benefactor to give the University of Adelaide [pounds sterling]500 to buy a gram of radium bromide. Very methodically he began to study the properties of the alpha rays (helium nuclei), emanating from the decay of the radium, using his own specially designed apparatus. William had a theory which occurred to him while preparing a talk for an Australian and New Zealand Association for the Advancement of Science (ANZAAS) scientific conference. He believed that given their range in air, which he set out to measure, and the number of air atoms in their way, the alpha particles must pass straight through the air atoms. William's theory came before Rutherford's discovery of the atomic nucleus at the centre of an atom. It was known that alpha particles leave individual tracks in a cloud chamber due to the ions they produce in air. William discovered an ionisation peak at the end of this range, that is most of the ions are produced and energy lost just before they stop moving. This was a significant discovery and earned William a Fellowship of the Royal Society (FRS) and a new professorship back in England, at the centre of things'. This 'Bragg peak' is today used in radiation therapy, particularly the new proton therapy used to significantly minimise damage to tissue adjacent to the cancer being treated.
DRIVEN TO DIFFRACTION
The Bragg story does not end here; this is merely the beginning. Young Lawrence Bragg, a very gifted child, entered the University of Adelaide at age 15. He studied Mathematics and Physics with his father as his teacher and graduated with a BA. The Bragg family returned to England in 1909 when William Bragg took up a Physics professorship at Leeds University and Lawrence enrolled at Cambridge University. However, when war broke out in 1914, both Lawrence and Robert Bragg entered the military. Unfortunately, Robert was killed at Gallipoli just weeks prior to the announcement of the Nobel Prize being awarded to his father and brother in November 1915.
Just before the Great War commenced in 1914, the understanding of the nature of X-rays received a huge boost when a German scientist Max von Laue and his students discovered a diffraction pattern by shining a beam of X-rays through a crystal of zinc blende (ZnS). News soon reached William and Lawrence while on holiday. It appeared to them that there was a problem with the Laue interpretation of the pattern of spots in Figure 2. Back at his studies in Cambridge, Lawrence was out walking when the correct interpretation occurred to him and he wrote down his famous equation as an undergraduate student: d.sine = n[lambad] in which d is the layer spacing, e is the angle between the X-ray beam and the planes of atoms and X is the X-ray wavelength and n is 1, 2, 3 etc.
Lawrence then set out to verify his equation by repeating Laue's experiment. His father, William, designed an improved X-ray spectrometer to measure the spectrum of 'spots' precisely, and together he and Lawrence began to study many different crystals using the new technique. They showed the cubic structure of common salt (NaCI), proving that it was an ionic crystal. They then progressed onto other crystals. As Lawrence was to say later, "It was like finding nuggets in the gold rush. Every day brought new discoveries" (Caroe 1978).
The importance of crystal structure is that it provides an understanding of the properties and function of molecules. For example, the strong bonds and densely packed carbon atoms in the structure of diamond discovered by the Braggs in 1913 (see Reference 3) showed why it is the hardest material known and why it sparkles and flashes when cut. Many years later Crick and Watson and coworkers in Lawrence Bragg's Cambridge laboratory carried the concept through to some of the most complicated structures known today, including those of DNA, haemoglobin (the oxygen carrier in red blood cells) and vitamins.
The vital clue used by Crick and Watson, which confirmed the helix, was the striking 'X' pattern in an X-ray diffraction pattern (Figure 3) obtained by Rosalind Franklin at the University of London. (It took Rosalind Franklin 100 hours of observation to photograph this image due to its faintness.) Rosalind recognised the pattern but not its structural implication (Maddox, 2008). It is worth noting here that X-ray diffraction patterns, while they enable structures to be determined, do not look at all like images of the molecules they represent. The helix diffraction pattern was the start. Crick and Watson then used other information, including a 3D model, to reconstruct the details of the DNA structure to show it is a double helix with cross-links.
A NOBEL DISCOVERY
Physics had been revolutionised by Roentgen's and Laue's discoveries and then carried much further by the work of the Braggs. William and Lawrence's joint work in 1912 was recognised in 1915 when they were jointly awarded the Nobel Prize for Physics, "... for their services in the analysis of crystal structure by means of X-rays" (see Reference 8). Lawrence was the first Australian born Nobel Laureate and is still the youngest person ever to receive the award. Both William and Lawrence were later knighted. William became the president of the Royal Society and Lawrence became director of the Royal Institution of Great Britain (RiGB) in London as well as professor and director of the famous Cavendish Physics Laboratory in Cambridge in 1937 (successor to Lord Rutherford).
As Sir Lawrence Bragg, a world-renowned scientist, Lawrence returned to Adelaide for the first time for a lecture tour of Australia in 1961. Lawrence was an inspiring physicist to all who met and worked with him. He never pretended great ability in mathematics or chemistry, despite his Nobel Prize winning work leading to the Bragg Law and discovery of chemical structures but he had a deep understanding of his work due to his ability to visualise the abstract and draw 3D pictures and diagrams of complicated molecules. (Lawrence's mother was a gifted artist and so was he.)
There is a plaque dedicated to Sir William and Sir Lawrence Bragg on North Terrace in Adelaide near Government House and nearby is a bust of Sir Lawrence Bragg (Figure 4) alongside Sir Howard Florey and Sir Mark Oliphant, two other great South Australian scientists. The bust of Sir Lawrence was unveiled by his granddaughter Claire in December 2012, after a Centenary Symposium on X-Ray Crystallography which was held at the University of Adelaide. A commemorative stamp (Figure 5) was also issued.
Although the Braggs' best work was done in the United Kingdom the foundations were laid in Adelaide and all Australians can take pride in two of our most famous scientists. At the time of their Nobel Award, Professor G. Granqvist, chairman of the Nobel Committee for Physics of the Royal Swedish Academy of Science said at the award ceremony:
Thanks to the methods that the Braggs, father and son, have devised for investigating crystal structures, an entirely new world has been opened ... The significance of these methods, and of the results attained by their means, cannot as yet be gauged in its entirety, however imposing its dimensions already appear to be. (See Reference 6)
The importance of the Braggs' contribution to today's science is best summed up by Baroness Susan Greenfield, past director of the RiGB (and current member of the RiAus Council, a sister institution to the RiGB), an institution at which both William and Lawrence Bragg held important positions:
The Braggs' contribution was the first step towards the mapping of the genome, molecular biology, and all the genetic modifications, for good or ill, that will characterise much of our lives and much of those of our children and grandchildren in the 21st century. (See Reference 7)
The United Nations has declared 2014 as the International Year of Crystallography
The Year of Crystallography promotes widespread access of information about crystallography and activities involving this science. Find out more at http://www.iycr2014.org
As is now well known, the DNA molecules in cell nuclei, which carry our genetic and heredity information from one generation to the next, consist of a double helix, that is, two intertwined coils of thousands of carbon atoms joined by amino acid cross-links (containing nitrogen and oxygen atoms). When a cell divides into two, the chains split down the middle, one chain going into one new cell and the other chain into another new cell, where they immediately reconstruct the opposite half of the chain and carry on as before. The cell multiplication process is conceptually so simple and seemingly obvious now, but it took the genius of Crick and Watson, in Lawrence Bragg's laboratory at Cambridge in 1953 to come up with it (Watson, 1968).
(1.) Jenkin, John. (2008). William and Lawrence Bragg, Father and Son, the most extraordinary collaboration in science. OUP.
(2.) Caroe, G. M. (1978). William Henry Bragg 1862-1942. Cambridge University Press.
(3.) Image of the diamond structure: http://en.wikipedia.org/wiki/Diamond_cubic
(4.) Watson, J. D. (1970). The Double Helix (with forward by W L Bragg). Penguin Books.
(5.) Maddox, B. (2002). Rosalind Franklin biography. Harper Collins.
(6.) Nobel Lectures, Physics 1901-1921, Elsevier Publishing Company, Amsterdam, 1967.
(7.) Cooper, Linda. (2009) Driven to Diffraction (film). Government of South Australia, Premier's Department.
(8.) The Nobel Prize in Physics 1915, Nobelprize.org: http://www.nobelprize.org/nobel_prizes/physics/laureates/1915/index.html
Robert George and John Patterson On behalf of RiAus
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||International year of CRYSTALLOGRAPHY|
|Author:||George, Robert; Patterson, John|
|Date:||Jun 1, 2014|
|Previous Article:||Mark your calendar.|
|Next Article:||Classroom activity ideas and online resources.|