History of Visualization of Biological Macromolecules

On-line Museum

A. A. Barker's Models of Myoglobin

In 1958, John Kendrew and his team reported the first glimpse ever at the structure of a protein, that of myoglobin. It was the result of a low resolution x-ray crystallographic analysis (6Å). The model they proposed gave a rough outline of the tertiary structure of myoglobin and looked somewhat like a contorted sausage out of a Daliesque nightmare. In 1960, Kendrew reported the result of a 2Å analysis of the structure. Through this analysis, most of the atoms in the polypeptide chain backbone and the heme group could be resolved with a fair amount of confidence, but not that of the side-chains. Improved interpretative techniques finally allowed to identify most of the side chains, and a full atomic-level model of the structure was finally produced. This was by far the most complex structure ever solved crystallographically, and it raised question of how it should be represented for publication purposes. The report of this work in Nature includes the black-and-white picture of a skeletal model of the structure. This photograph is a testimony to the complexity of the structure and most of its details are lost in the apparent entanglement of its components. The spatial relationship between groups is very difficult to evaluate or appreciate. A white cord was used to help identify the course of the main chain and made it the only easily identifiable feature. After learning to solve protein structure, x-ray crystallographers would have to deal with the problem of the graphical representations of these structures.

Model of myoglobin made by A. A. Barker, model maker in Cambridge (UK). This model belongs to Dr. Britton Chance (with many thanks for allowing me to take these pictures).

One particular solution was of course to produce models of the structure of myoglobin. Shortly after the 1961 publication, Kendrew started to receive requests for such models. It is not until 1965 that Kendrew approached A. A. Barker, an employee of the Cambridge University Engineering Laboratories and owner-operator of "a small model making concern as a private venture". At the time, Barker was offering two dozen models of interest to chemists and biochemists, including DNA, vitamin B12, insulin and the alpha helix. These models were of the ball-and-stick type, assembled on a scale of 2,5 cm per Å. Kendrew's plan was to use components (balls and spokes) provided by C. A. Beevers, professor of chemistry at the University of Edinburgh. Beevers had recently devised a machine to drill holes in small perspex balls and had a small outfit producing a variety of crystal models, which is still active today. With a diameter of 6.9 mm (or 9/32"), these balls allowed for models on a much smaller scale (1cm=1Å) than the scales typically found in ball-and-stick models, a scale suitable for such a large structure.

Kendrew started to take orders for the models in May 1966. The price was set at £210 (roughly 600$US), a considerable sum at the time. In all, 29 orders were received between May 1966 and March 1968 for models which were produced at the rate of one per month.

References

Kendrew, J. C., R. E. Dickerson, R. G. Strandberg, R. G. Hart, D. R. Davies, D. C. Phillips, and V. C. Shore. 1960. Structure of Myoglobin. A Three-Dimensional Fourier Synthesis at 2 A. Resolution. Nature 185:422-27. (back)

Kendrew, J. C., H. C. Watson, B. E. Strandberg, R. E. Dickerson, D. C. Phillips, and V. C. Shore. 1961. A Partial Determination by X-Ray Methods, and Its Correlation with Chemical Data. Nature 190:666-670. (back)