Tuesday, 13 March 2012

X raying Crystals

In 1912, Max von Laue had an idea for an experiment that would answer a question in physics. The results inspired scientists to make discoveries that have influenced chemistry and biology and provide evidence for many of the principles taught at GCSE and A level.
X rays were discovered by Wilhelm Rontgen in 1895 but physicists could not agree on whether they were particles (like alpha and beta rays) or waves (like light). If they were a form of light they must have wavelengths less than a nanometre. Max von Laue was teaching at the University of Munich in 1912 when he had his big idea.

Von Laue knew that the distance between atoms in a crystal was less than a nanometre. Diffraction happens when waves pass through narrow gaps and form a distinctive pattern. Von Laue predicted that the gaps between the rows of atoms in a crystal should produce a diffraction pattern with X rays. Two of his students, Friedrich and Kipping, tried out the experiment. They put a copper sulfate crystal surrounded by a sheet of photographic paper in a lead box and shone a narrow beam of X rays at it. A very complicated pattern was formed so they tried zinc sulfide instead. This gave a simple, regular pattern of spots. Von Laue’s method worked and proved that X rays were waves. Von Laue won the Nobel Prize in 1914 but that was just the start of the story of X ray crystallography.

As soon as von Laue published the results of his idea in 1912, the father and son team of W.H. Bragg and W.L.Bragg realised that not only was it a means of investigating X rays it was a method for exploring crystals. The Braggs worked out the mathematical formula for analysing the diffraction patterns and set to work. The method was very difficult because the crystals had to be quite large and perfectly formed but soon they had discovered the pattern of carbon atoms in diamond and the structure of sodium chloride.

At the time, most chemists thought that sodium chloride was made up of molecules of a sodium atom joined to a chlorine atom. The Braggs showed that in fact it consisted of sodium ions surrounded by six chloride ions, and vice versa, arranged in a cube. There were no molecules with the formula NaCl. Their discoveries transformed chemists’ ideas about the importance of the structure of substances and the bonds between atoms. The Braggs were awarded the Nobel Prize in 1915. In the 1920s Linus Pauling used results from X ray crystallography to develop the ideas about ionic and covalent bonding taught in GCSE and A level chemistry today.

In 1924, while at the Royal Institution in London with W H Bragg, a young scientist, John Bernal worked out the arrangement of carbon atoms in graphite. He also realised that X ray crystallography would be useful to find the structure of organic compounds. In 1929, Bernal’s student, Kathleen Lonsdale, proved that benzene was a flat hexagonal molecule. Lonsdale was a Quaker who went to prison in 1943 for refusing to do war work but in 1945 she became the first woman to be made a Fellow of the Royal Society.

X ray crystallography attracted a number of female scientists. Bernal built up a team at the University of Cambridge. They studied the patterns produced by crystals of vitamins and viruses. Dorothy Hodgkin studied with Bernal in 1934 before setting up her own X ray unit at the University of Oxford. She worked with large biological molecules. In 1949 she worked out the structure of penicillin, just as it was becoming famous as an antibiotic. In 1956 she found the structure of vitamin B12 for which she won the Nobel Prize in 1964, and in 1969, after thirty years’ work, she completed the structure of insulin. It had taken so long because thousands and thousands of calculations were needed for each structure. Computers can now do in minutes what used to take years and many thousands of complex molecules have had their structure worked out using X ray crystallography.

Back in 1953 however, Rosalind Franklin did not have the use of a computer. Kings College, London had given her the task of collecting X ray pictures of DNA. DNA was very difficult to crystallise but Franklin succeeded and produced patterns that convinced Crick and Watson that DNA has a double helix shape. They leapt to the structure with model building while Franklin worked towards the same answer by doing the long-winded calculations. It was her work that confirmed Crick’s and Watson’s insight.


1 Explore the lives of the scientists mentioned in the articles – Max von Laue, W.H and W.L Bragg, J.D. Bernal, Kathleen Lonsdale, Dorothy Hodgkin, Rosalind Franklin.

2 Find out the arrangement of the atoms or ions in diamond, graphite, sodium chloride, benzene and zinc sulfide.

3 Why do you think the X ray diffraction pattern of copper sulfate crystals (CuSO4.5H2O) proved to be more complicated than that of zinc sulfide (ZnS)?

4 Why is it important to know the structure of biological molecules such as insulin?
5 Why do you think that many of the early X ray crystallographers were women?
6 Discuss whether biology, chemistry and physics should be treated as separate subjects or as one subject – science.

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