Editor’s Page: Seeing Molecular Structures

This issue of C&EN celebrates the 100th anniversary of X-ray crystallography. As science magazines go, C&EN is a little bit late to this celebration, but that’s okay. It’s a “sort of” 100th anniversary that could have been celebrated in 2012, 2013, 2014, or 2015. In 1914, Max von Laue received the Nobel Prize in Physics for his demonstration in 1912 that crystals diffract X-rays. Sir William Henry Bragg and his son William Lawrence Bragg received the 1915 Physics Nobel for their use of X-ray diffraction to determine a number of structures from 1913 to 1915.

No matter. As much as or more than any other experimental technique, X-ray crystallography transformed science, especially chemistry and eventually biology. As Greg Petsko points out in his essay on the future of crystallography (see page 42), most experimental science is an attempt to overcome the limitations of the human eye. X-ray crystallography allowed scientists to “see” the structure of matter—whether it was molecular or otherwise—at atomic resolution.

In chemistry, structure often determines, or at least strongly influences, function. This special feature consists of an introductory essay by Assistant Managing Editor Amanda Yarnell, who coordinated C&EN’s overall effort, followed by 11 short essays on compounds whose structures were determined by X-ray diffraction. In planning the feature, Yarnell consulted with a number of prominent X-ray crystallographers and assembled a list of 35 candidate compounds to profile ranging from table salt to penicillin, from a ribosome to a G protein-coupled receptor.

To winnow these possibilities to a manageable number, Yarnell and I hit on the concept of featuring structures that had an impact on the chemical knowledge of that day. Hence the structure of hexamethylbenzene, determined in 1928 by Kathleen Lonsdale, the first woman tenured professor at University College London. It proved that the benzene ring was flat with an evenly distributed cloud of electrons—the three double bonds were shared.

It seems impossible to us today that the structure of water ice was a mystery less than 100 years ago. But it wasn’t until 1929 that William H. Barnes, working in Bragg the elder’s lab at London’s Royal Institution, showed that ice formed under natural conditions had a structure with the oxygens arranged in a planar hexagon and the hydrogens located between the oxygens. This work predated our understanding of hydrogen bonding!

With each essay, we provide the number of atoms in the molecule whose structure was revealed by X-ray crystallography. The numbers climb precipitously, from 30 for hexamethylbenzene and three for ice to 181 in 1956 for vitamin B-12 to about 1,950 in 1965 for lysozyme and about 2,400 in 1973 for transfer RNA. This trend was interrupted in 1994 with nitroprusside ion (13 atoms), thought at the time to be the first X-ray crystal structure of a short-lived excited state.

The number of atoms for my contribution to the feature, the Watson-Crick structure of DNA, is a range: 77 to >109. The fibers that provided Watson and Crick the X-ray diffraction pattern that led to their discovery were almost certainly in the upper range. The “77” is the number of atoms in the dinucleotide Alexander Rich determined the X-ray crystal structure of in 1973.

A number of names pop up more than once during the century of crystallography and in our feature. Rich, for example, nailed the X-ray crystal structures of both DNA and transfer RNA in 1973.

A remarkable figure in the history of X-ray crystallography is Dorothy Crowfoot Hodgkin. She appears twice in our feature, having determined the structure of penicillin in 1945 and vitamin B-12 in 1956. She appears four times in the list of 35 seminal structures Yarnell assembled, having also solved the crystal structures of insulin in 1969 and thiostrepton, a cyclic oligopeptide antibiotic, in 1970. She was awarded the Nobel Prize in Chemistry in 1964 “for her determinations by X-ray techniques of the structures of important biochemical substances.”

Please also visit C&EN’s online crystallography feature (cen.xraycrystals.org), where you can tell us your favorite X-ray crystal structure and read about other readers’ favorites.

Thanks for reading.

Rudy Baum

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