Revealing the structure of matter
For centuries, crystals have been desired for their alleged magical healing and mystical powers. While there is absolutely no scientific evidence that crystals can be used to predict the future or provide protection or healing, they play a critical role in establishing the structure of matter. Knowledge of the structure of matter underpins most science today, yet the majority of the general public is completely unaware of how scientists determine structure.
Modern techniques for determining structure were developed in the 20th century after it was discovered that X-rays could be used to ‘see’ the structure of matter in a non-intrusive manner and that, with their repeatable lattice structure, crystals made great candidates for investigation. Since this discovery, X-ray crystallography has become the leading technique for studying the structure of matter at the atomic or molecular level.
Crystallography permeates our daily lives and forms the backbone of industries which are increasingly reliant on knowledge generation to develop new products. The science has many applications in widely diverse fields such as agro-food, aeronautics, automobiles, cosmetics and computers, as well as the electromechanical, pharmaceutical and mining industries.
The importance of crystallography to scientific and technological progress, combined with the lack of community knowledge and understanding of the science, induced the United Nations General Assembly to proclaim 2014 as the International Year of Crystallography (IYCr2014).
So far, 29 Nobel Prizes for scientific achievements have related to, or involved the use of, crystallography. The first of these was 100 years ago with Max von Laue’s discovery of the diffraction of X-rays by crystals.
The following year, father-and-son team William Henry and William Lawrence Bragg won the Nobel Prize for Physics for their work in determining the structure of sodium chloride using X-ray diffraction. Although the father and son went on to lead the Cavendish Laboratory team at Cambridge University, Sir William Bragg started his work on X-rays and crystal structure when he was Elder Professor of Mathematics and Physics at the University of Adelaide, and his Australian-born son Lawrence was a graduate of the university. Lawrence’s equation to translate the diffraction into a structure, Bragg’s Law, is still in use today.
The Bragg Institute at the Australian Nuclear Science and Technology Organisation (ANSTO) is named in tribute to the Nobel Prize winners. This institute leads Australia in the use of neutron scattering used in crystallography. At the centre of the research institute is the OPAL research reactor, along with state-of-the-art neutron beam instruments affectionately named Kowari, Wombat, Echidna and Koala, after Australian fauna.
How X-ray crystallography works
Modern crystallographic methods depend on analysis of the diffraction patterns of a sample targeted by a beam of some type. When a beam hits an object, the object’s atoms scatter the beam. This is facilitated by the wave properties of the particles making up the beam. Crystallographers discovered that crystals, because of their regular arrangement of atoms, scatter the rays in just a few specific directions. By measuring these directions and the intensity of the scattered beams, scientists produce a three-dimensional picture of the crystal’s atomic structure.
Beams of X-rays are most commonly used; other beams include electrons or neutrons. Crystallographers often explicitly state the type of beam used: X-ray diffraction, neutron diffraction or electron diffraction. These three types of radiation interact with the specimen in different ways.
X-rays interact with the spatial distribution of electrons in the sample, while electrons are charged particles and therefore feel the total charge distribution of both the atomic nuclei and electrons of the sample.
Neutrons are scattered by the atomic nuclei through the strong nuclear forces but, in addition, the magnetic moment of neutrons is non-zero. They are therefore also scattered by magnetic fields. When neutrons are scattered from hydrogen-containing materials, they produce diffraction patterns with high noise levels. However, the material can sometimes be treated to substitute deuterium for hydrogen. Because of these different forms of interaction, the three types of radiation are suitable for different crystallographic studies.
Crystallography and the life sciences
Most bio-macromolecules do not form crystals in their native environment, but by the middle of the 20th century, protein crystallisation techniques had evolved sufficiently to enable their analysis by X-ray crystallography. In 1964, Dorothy Crowfoot Hodgkin was awarded a Nobel Prize for her work on establishing the structure of vitamin B12 and penicillin.
Hodgkin was one of the first to use X-ray crystallography to look at biological molecules. Her first big breakthrough was with penicillin. Chemists didn’t quite know what its structure was, but she solved that in 1945 and then she went on to solve the structure of the protein insulin.
Another female crystallographer, Rosalind Franklin, made a huge contribution to Francis Crick and James Watson’s work in establishing the double helix structure of DNA.
Other notable crystallography-related Nobel prizes include Johan Deisenhofer, Robert Huber and Hartmut Michel - for revealing the structure of the first membrane-bound protein (1988); and Venkatraman Ramakrishnan, Thomas Steitz and Ada Yonath - solving the structure of the ribosome (2009).
In the past 50 years, the structures of more than 90,000 biological molecules have been revealed by crystallographers, with great ramifications for health care.
Crystallography is now commonly used in drug discovery. The detailed analysis of protein-ligand complexes allows scientists to design drugs to target certain molecules. The influenza drug, Relenza, is the first drug developed in Australia based on a protein crystal structure. The protease inhibitor that fights the Human Immunodeficiency Virus (HIV) is another significant finding, which was developed using diffraction. It is regarded as a major success of structure-based drug design.
In exciting news, a new version of Relenza that could stop the flu virus in its tracks has been developed by Australian and international researchers at the Australian Synchrotron. The drug prevents flu virus particles from detaching themselves from the surface of a cell and spreading to infect other cells, buying time for vaccines to be developed. The team of researchers used the Australian Synchrotron to obtain finely detailed information about how the drug interacts with the virus and were then able to use the information to improve the drug’s effectiveness.
In addition, ANSTO now operates the Australian Synchrotron, which produces extremely intense X-ray beams. Synchrotron light sources have revolutionised X-ray crystallography since the 1980s, and the Australian Synchrotron is now the centrepiece of crystallography in Australia, with five of its nine beamlines dedicated to crystallography and atomic structure determination.
The beauty of crystals
Crystals were found to be ideal subjects for studying the structure of matter at the atomic or molecular level, on account of three common characteristics: they are solids, three-dimensional and built from very regular and often highly symmetrical arrangements of atoms.
These crystals are often extremely beautiful and in some cases quite awe inspiring.
In 2000, miners excavating a new tunnel for the Industrias Peñoles mining company were 300 metres below the surface at Naica, Chihuahua, Mexico when they discovered the Cueva de los Cristales (Cave of the Crystals) and in it some of the largest natural crystals ever found.
The giant selenite crystals (gypsum, CaSO4·2 H2O) are up to 12 m in length, 4 m diameter and weigh a massive 55 tons. The cave is usually flooded but the mining company had pumped out the water to facilitate their tunnelling.
Sadly, the cave is on private property and totally unsuitable for tourism as the air temperature reaches up to 58°C and the humidity is always 99-100%. Scientists working in the cave have to limit the time they spend in there to half-hour shifts and wear specially constructed refrigerated suits and cold-breathing systems.
The cave was explored in detail in 2006 by a scientific team coordinated by Paolo Forti. Forti is a cave minerals specialist and crystallographer at the University of Bologna (Italy). Besides mineralogical and crystallographic studies, biogeochemical and microbial characterisation of the gypsum giant crystals were also performed.
Stein-Erik Lauritzen from the University of Bergen, Norway performed uranium-thorium dating on the crystals. He determined that the maximum age of the giant crystals was about 500,000 years.
Crystallography without the crystals
University of Adelaide researchers Associate Professors Christian Doonan and Christopher Sumby and their team in the School of Chemistry and Physics, have developed a new material for examining structures using X-rays without first having to crystallise the substance.
“Today, crystallography is an area of science that’s still providing new insights into the structures of materials - our new research is a prime example of that. It allows us to study chemical reactions that have just happened, or potentially even while they are still happening, which we can’t do using normal crystallography,” explained Associate Professor Sumby.
The researchers are using a new nanomaterial - called a metal-organic framework - to bind the metal complex catalyst and its chemical reactants in place.
“We can then examine the structures of the reaction products using X-rays without having to isolate the product or grow crystals,” said Associate Professor Doonan.
“We are effectively taking snapshots of the chemistry, enabling us to study the reaction products in their native state. In this way we can provide structural evidence for the chemical transformations that are taking place.”
The research, being undertaken in the Centre for Advanced Nanomaterials, is supported by the Australian Research Council and the Science and Industry Endowment Fund.
The work is being carried out in the Bragg Crystallography Facility at the university’s North Terrace campus.
Conclusion
Today, crystallography underpins all the sciences. It forms the backbone of a wide range of industries, including pharmaceuticals, agri-foodstuffs, aeronautics, computing, mining and space sciences. It is essential for the development of almost all new materials and deserves to be more understood and appreciated by the whole community
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