The idea of the close packing of atoms in crystals can be traced back to Kepler,
and by the end of the nineteenth century, Barlow and Pope had developed its
consequences for the periodic table. Barlow explored not only the packing of
identical spheres but also combinations of spheres of different sizes, and his studies
proved useful when the initial structures of simple salts were solved. Pope and
Barlow’s work interrelating crystal morphologies and crystal packing proved to be
very helpful for W.L. Bragg and W.H. Bragg’s analysis of the structures of alkali
metal halides, zinc blende and calcite in the early days of X-ray crystallography
[28, 33–36].
Finally, in order to emphasise the importance of symmetry in the first 50 years of
X-ray diffraction, it is useful to digress to make a few comments concerning the
proposal of the structure of DNA in 1952 [4]. The X-ray photographs on DNA taken
in 1952 and particularly Rosalind Franklin’s infamous photo 51 of hydrated β-DNA
showed that it belonged to the monoclinic C2 space group and it had a diffraction
pattern characteristic of a helical structure. The symmetry properties of the C2 space
group implied that the DNA strand had a twofold symmetry axis. This piece of
symmetry information implied an intertwined pair of helices proceeding in opposite
directions. Model building based on putting together the base pairs through hydrogen bonds whilst maintaining the twofold symmetry axis contributed significantly to
the proposed structure by Francis Crick and James Watson (Table 1 gives details of
their Nobel Prize). The progress made in the subsequent 50 years takes us to the
2009 Nobel Prize which was awarded to Venkataraman Ramakrishna, Thomas Steitz
and Ada Yonath for “studies of the structure and function of ribosome”. A ribosome
consists of the related RNA molecule and associated proteins and is found in the
cytoplasm of living cells. Its biological function is to bind messenger RNA and
transfer RNA to synthesise polypeptides and proteins. Yonath achieved the important tasks of producing the crystals, which are prerequisites of all diffraction
experiments, and Steitz managed to solve the structure of these important molecules,
and Ramakrishnan developed techniques which enabled the positions of single
atoms to be identified (Table 1 gives details of their Nobel Prize).
4 Early Development of X-Ray Crystallography [19–27]
Von Laue’s success depended more on intuition than careful analysis of the underlying physics because he had no equations to enable him to predict the possible
appearance of the interference pattern [18, 21]. Based on traditional optics, he did
not think that an interference pattern could result from white X-ray radiation,
because of the wide range of frequencies generated by the X-ray source. He hoped
that a crystalline sample with heavy atoms would produce a narrow band of
fluorescence radiation which would produce an interference pattern via a secondary
effect. William Lawrence Bragg the son of W.H. Bragg, a Professor of Physics at
Leeds and who had passed onto him an interest in X-ray phenomena, recognised this
flaw in his logic. The recent graduate in Natural Sciences at Cambridge speculated
14
D. M. P. Mingos
and by the end of the nineteenth century, Barlow and Pope had developed its
consequences for the periodic table. Barlow explored not only the packing of
identical spheres but also combinations of spheres of different sizes, and his studies
proved useful when the initial structures of simple salts were solved. Pope and
Barlow’s work interrelating crystal morphologies and crystal packing proved to be
very helpful for W.L. Bragg and W.H. Bragg’s analysis of the structures of alkali
metal halides, zinc blende and calcite in the early days of X-ray crystallography
[28, 33–36].
Finally, in order to emphasise the importance of symmetry in the first 50 years of
X-ray diffraction, it is useful to digress to make a few comments concerning the
proposal of the structure of DNA in 1952 [4]. The X-ray photographs on DNA taken
in 1952 and particularly Rosalind Franklin’s infamous photo 51 of hydrated β-DNA
showed that it belonged to the monoclinic C2 space group and it had a diffraction
pattern characteristic of a helical structure. The symmetry properties of the C2 space
group implied that the DNA strand had a twofold symmetry axis. This piece of
symmetry information implied an intertwined pair of helices proceeding in opposite
directions. Model building based on putting together the base pairs through hydrogen bonds whilst maintaining the twofold symmetry axis contributed significantly to
the proposed structure by Francis Crick and James Watson (Table 1 gives details of
their Nobel Prize). The progress made in the subsequent 50 years takes us to the
2009 Nobel Prize which was awarded to Venkataraman Ramakrishna, Thomas Steitz
and Ada Yonath for “studies of the structure and function of ribosome”. A ribosome
consists of the related RNA molecule and associated proteins and is found in the
cytoplasm of living cells. Its biological function is to bind messenger RNA and
transfer RNA to synthesise polypeptides and proteins. Yonath achieved the important tasks of producing the crystals, which are prerequisites of all diffraction
experiments, and Steitz managed to solve the structure of these important molecules,
and Ramakrishnan developed techniques which enabled the positions of single
atoms to be identified (Table 1 gives details of their Nobel Prize).
4 Early Development of X-Ray Crystallography [19–27]
Von Laue’s success depended more on intuition than careful analysis of the underlying physics because he had no equations to enable him to predict the possible
appearance of the interference pattern [18, 21]. Based on traditional optics, he did
not think that an interference pattern could result from white X-ray radiation,
because of the wide range of frequencies generated by the X-ray source. He hoped
that a crystalline sample with heavy atoms would produce a narrow band of
fluorescence radiation which would produce an interference pattern via a secondary
effect. William Lawrence Bragg the son of W.H. Bragg, a Professor of Physics at
Leeds and who had passed onto him an interest in X-ray phenomena, recognised this
flaw in his logic. The recent graduate in Natural Sciences at Cambridge speculated
14
D. M. P. Mingos