Since many materials can form crystals—such as salts, metals, minerals, semiconductors, as well as various inorganic, organic and biological molecules—X-ray crystallography has been fundamental in the development of many scientific fields. In its first decades of use, this method determined the size of atoms, the lengths and types of chemical bonds, and the atomic-scale differences among various materials, especially minerals and alloys. The method also revealed the structure and function of many biological molecules, including vitamins, drugs, proteins and nucleic acids such as DNA. X-ray crystallography is still the chief method for characterizing the atomic structure of new materials and in discerning materials that appear similar by other experiments.
X-ray crystal structures can also account for unusual electronic or elastic properties of a material, shed light on chemical interactions and processes, or serve as the basis for designing pharmaceuticals against diseases.
X-rays are electromagnetic radiation with wavelengths between about 0.02 Å and 100 Å (1Å = 10-10 meters). They are part of the electromagnetic spectrum that includes wavelengths of electromagnetic radiation called visible light which our eyes are sensitive to (different wavelengths of visible light appear to us as different colors). Because X-rays have wavelengths similar to the size of atoms, they are useful to explore within crystals.
Unfortunately, unlike with visible light, there is no known way to focus x-rays with a lens. This causes an x-ray microscope to be unfeasible unless someone finds a way of focusing x-rays. Until then it is necessary to use crystals to diffract x-rays and create a diffraction pattern which can be interpreted mathematically by a computer. This turns the computer into a virtual lens, so it on a monitor we can look at the structure of a molecule. Crystals are important because by definition they have a repeated unit cell within them. Dental x-ray diffraction from one unit cell would not be significant. Fortunately, the repetition of unit cells within a crystal amplifies the diffraction enough to give results that computers can turn into a picture.
One example of the use of X-ray crystallography has been our understanding of the structure and function of hemoglobin and its close relative, myoglobin. Hemoglobin is the protein that transports oxygen from the lungs to the tissues via red blood cells while myoglobin is its counterpart in muscle that stores oxygen and gives it up to muscle cells as needed.