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INTRODUCTION TO X-RAY CRYSTALLOGRAPHY

INTRODUCTION TO X-RAY CRYSTALLOGRAPHY. X-rays: the right tool for the job. Haemoglobin crystals , 1960s.

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INTRODUCTION TO X-RAY CRYSTALLOGRAPHY

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  1. INTRODUCTION TO X-RAY CRYSTALLOGRAPHY X-rays: the right tool for the job

  2. Haemoglobin crystals, 1960s By studying X-ray diffraction photographs of crystals, scientists could calculate the arrangement of atoms within molecules. Some of the materials analysed occur naturally as crystals. Others, such as haemoglobin, have to be crystallised first. Crystallising a substance orders the arrangement of its atoms into a repetitive pattern, which makes it possible to interpret accurately the diffraction patterns it produces. Making a crystal of adequate quality to study is a complex process and can take considerable time.

  3. X-ray diffraction pattern from a haemoglobin crystal, 1962 When X-rays hit a crystal they scatter, producing a regular pattern of spots, which are recorded on a photographic plate. The crystal is rotated and a diffraction pattern is made from each angle. These images provide the information required to solve the structure of the crystal substance. Arriving at this solution involves the application of sophisticated mathematics and a repetitive process of refinement.

  4. Fourier drawing, 1957 This shows the process of mapping contours on an electron density map. The data from all the spots in a diffraction pattern can be combined to represent molecules in the structure of a crystal by applying a mathematical procedure known as a Fourier synthesis.

  5. W H Bragg and his spectrometer, c.1915 Father and son William Henry and William Lawrence Bragg shared the Nobel Prize in 1915 for their services in the analysis of crystal structure by means of X-rays. This image shows the elder Bragg with his spectrometer, a crucial piece of equipment that he had constructed while teaching at the University of Leeds

  6. Reproduction of a page from W L Bragg’s notebook, c.1913 At the top of this page from one of W L Bragg’s early notebooks is a diffraction pattern. The dark spot at the centre is made by the X-ray beam; the diffraction of the beam by the crystal creates the surrounding dots.

  7. Helen Megaw Dr Helen Dick Megaw (1907-2002) was Assistant Director of Research at the Cavendish Laboratory in Cambridge. In this photograph she is shown setting a photogoniometer: a piece of equipment used to take X-ray photographs of crystals. Behind her is a ball-and-spoke model of the structure of afwillite, one of the minerals she researched. One of Megaw’s drawings of afwillite can be seen in the next image.

  8. Afwillite 8.45 diagram Helen Megaw explained the diagram as follows: “The ‘peaks’ with six or seven contours are calcium atoms, those with five contours are silicon, and the others are hydroxyl and water.” Helen Megaw’s electron-density contour map of afwillite inspired both textiles and wallpapers. In the next image is an example of one of these textiles.

  9. Afwillite 8.45 dress fabric The structure of afwillite is seen here on screen-printed spun rayon. Helen Megaw, who discovered this structure, would later play a crucial role as Adviser on Crystal Structure Diagrams in the Festival Pattern Group. It was entirely due to her that so many eminent crystallographers contributed, including Nobel Prize winner and crystallographer Max Perutz, who can be seen in the next image.

  10. But first … some Chemistry jokes Two atoms are walking down the street. Says one atom to the other, "Hey! I think I lost an electron!" The other says, "Are you sure??" "Yes, I'm positive!“ A neutron walks into a restaurant and orders a couple of cokes. As she is about to leave, she asks the waiter how much she owes.  The waiter replies, "For you, No Charge!!!“ A sign outside the chemistry hotel reads "Great Day Rates,  Even Better NO3-'s"

  11. Max Perutz at his diffractometer Max Perutz (1914-2002) insisted on performing the measurements on the diffractometer himself in order to ensure their accuracy. He describes how the diffractometer was kept running “day and night for 15 months while I measured the intensities of some 100 000 reflections…The final electron density map was so beautiful that I soon forgot the tedium of data collection.”

  12. Horse methaemoglobin diagram X-ray photographs of horse methaemoglobin crystals were studied by Max Perutz during his early research on protein structures. Methaemoglobin is a variant of haemoglobin in which the iron atom is unable to carry oxygen. As a result blood is brown rather than red.

  13. Horse methaemoglobin 8.23 dress fabrics Max Perutz’s wife, Gisela, had a dress made from this fabric, which she wore at the International Union of Crystallography conference in Stockholm in 1951. The patterns are on printed acetate rayon crepe and can be seen here in four colour variations.

  14. Correspondence between Helen Megaw and Dorothy Hodgkin Helen Megaw and Dorothy Hodgkin (1910-1994) became friends whilst working as research students in Cambridge between 1932 and 1934. Their close ties are revealed in this letter: Megaw has asked Hodgkin’s permission to use some of her insulin diagrams, an example of which is in the next image; Hodgkin agrees, but refuses to sign an official form or accept a fee on ethical grounds.

  15. Insulin 8.27 diagram This delicate multicoloured drawing dates from the late 1930s when Dorothy Hodgkin first began to publish her research on insulin. Like the other insulin diagrams selected by Helen Megaw, this is a Patterson contour map showing the distances between atoms. As well as being the source for wallpaper (in the next image), this diagram inspired laminates, carpets, linoleum and lace.

  16. Insulin 8.27 wallpaper Although the pattern has been simplified, the motif in this screen-printed wallpaper is essentially the same as Dorothy Hodgkin’s contour map. From a distance the pattern resembles floral rosettes. This wallpaper was used in the Regatta Restaurant at the Festival of Britain, which can be seen in the Festival of Britain section.

  17. John Kendrew and Max Perutz John Kendrew (1917-1997) (left) and Max Perutz (right) are shown with Kendrew’s second structure model of myoglobin, known as ‘the forest of rods’. The two men worked alongside each other from 1947 onwards in a special research group within the Cavendish Laboratory called the Unit for the Study of the Molecular Structure of Biological Systems. A diagram of myoglobin can be seen in the following image.Max Perutz won the Nobel Prize for Chemistry in 1962 for determining the structure of hemoglobin, the substance which carries oxygen in the blood around the body. He also showed how hemoglobin works.

  18. Myoglobin 8.46f diagram John Kendrew began researching myoglobin, the oxygen-storing molecule in muscle tissue, after joining the Cavendish Laboratory. He determined the molecule’s structure in 1957. These early diagrams of horse myoglobin inspired printed leathercloths, which are featured in the next image.

  19. Myoglobin 8.46g vynides This nitrocellulose-coated upholstery fabric features the myoglobin structure. ICI Leathercloth was keen to promote its new range of heavy-duty wall coverings and upholstery fabrics intended for schools, cafes and cinemas.

  20. QUESTIONS • X-rays are ……………..rays with ………. wavelengths, which are comparable to the interatomic distances in solids. • Why must a substance be crystalline in order for this method to work? • When X-rays hit a crystal they ……………… producing a regular pattern of ……. called a …………….. pattern • The dots are connected to form an ………………………….(2 words) map. Atoms of different elements are then identified and bond ………. can be measured. • The first protein to have its structure determined by X-ray crystallography was …………………… The scientist who achieved this was …………………. • The structure and functioning of hemoglobin were worked out by ………………………..

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