© Corbin O’Grady Studio/Science Photo Library
Dorothy Crowfoot Hodgkin (1910–1994).
Dorothy Crowfoot Hodgkin (1910–1994).
The application of X-ray crystallography in experiments to determine the structure of biological molecules, in particular proteins, was developed by Dorothy Crowfoot Hodgkin, following her early work with J. D. Bernal (see here). She made so many contributions that it is hard to pick out one experiment as more important than the others – indeed, the Nobel committee didn’t try, awarding her the Nobel Prize in Chemistry in 1964 ‘for her determinations by X-ray techniques of the structures of important biochemical substances’. This made her only the third woman to receive the chemistry prize, following Marie Curie and her daughter Irène. As the citation suggests, what really mattered was the experimental technique that Hodgkin applied to all the problems she tackled.
A key step in the analysis of the X-ray diffraction patterns, which were recorded photographically, was to use chemical techniques to insert heavy atoms at known locations in the molecules of the crystals being investigated. These extra atoms produce a distinctive ‘signature’ in the X-ray patterns, and by mapping out their locations it is possible to get an insight into the overall structure of the crystal. This is a long and tedious process. The photographs reveal the way electrons are distributed in the crystal, but the pattern for the whole crystal is too complicated to be worked out in one go. One region has to be analysed first, to provide a local map of the electron density, then this information helps to map out a neighbouring region, and so on. This involved a lot of mathematical analysis, initially without the aid of electronic computers.
In the 1940s, Hodgkin was working in Oxford, where she learned about the breakthrough research on penicillin from Ernst Chain (see here). By 1943, chemical analysis had established that the penicillin molecule contains 11 hydrogen atoms, 9 carbon atoms, 4 oxygen atoms, 2 nitrogen atoms, and 1 sulphur atom. But this combination of atoms could be arranged in either of two different ways, and it was essential to know which structure was correct if penicillin was to be manufactured in large quantities and similar antibiotics were to be synthesized. Hodgkin took on the task of finding out.
Fortunately, the single sulphur atom in these molecules was heavy enough to provide key information about the structure without the need to introduce another heavy atom, and, after a great deal of analysis, Hodgkin was able to work out which of the structures corresponded to biologically active penicillin. After she had worked this out, the results were checked using a machine which is sometimes described as a computer but was really a glorified calculator, not a proper Turing machine (see here). The results were published in 1945.
Hodgkin then turned her attention to vitamin B12. This had recently been identified as an essential substance used by the body to make red blood cells. A lack of vitamin B12 causes a debilitating illness known as pernicious anaemia. Vitamin B12 is present in foods derived from animals but not in vegetables, so vegans, who do not eat dairy products, meat, fish, or eggs, may need supplements of the vitamin. Analysing its structure, though, proved much harder than analysing penicillin, because it is a much larger and more complicated molecule. Although Hodgkin started work on the project in 1948, it was only completed in 1956, with the results being published a year later, and represents her greatest achievement as a crystallographer. This time, however, she did have the help of a genuine computer, although it was not in Oxford, or even in England. She collaborated with Kenneth Trueblood, the head of a team of American crystallographers based at the University of California in Los Angeles, who had access to a computer. Hodgkin provided the data obtained by crystallography for analysis, and the UCLA team ran the calculations. Long before the advent of email and the Internet, they exchanged the information by post.
In her Nobel lecture, after describing her successes up to that point, Hodgkin said: ‘I should not like to leave an impression that all structural problems can be settled by X-ray analysis or that all crystal structures are easy to solve. I seem to have spent much more of my life not solving structures than solving them. I will illustrate some of the difficulties to be overcome by considering our efforts to achieve the X-ray analysis of insulin.’47
She then described how she had been tackling the analysis of insulin, the drug used to treat diabetes, off and on since the mid1930s, with only limited success. The molecule contains 777 atoms (penicillin has just 27 atoms), and has an extremely complex structure. But Hodgkin was still not finished with insulin, and, aided by improved computer technology, she did solve the structure. This work was completed in 1969, five years after she received her Nobel Prize.