International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 19.1, p. 419   | 1 | 2 |

Section 19.1.1. Introduction

A. A. Kossiakoffa*

aDepartment of Biochemistry and Molecular Biology, CLSC 161A, University of Chicago, Chicago, IL 60637, USA
Correspondence e-mail: koss@cummings.uchicago.edu

19.1.1. Introduction

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Neutron and X-ray crystallography are similar in both their experimental methodologies and in the resulting information content. The principal difference between the two methods is brought about by the characteristic scattering potential of the atom types. The scattering of neutrons by material is not proportional to the atomic number, as is the case in X-ray scattering, but rather depends on the individual nuclear characteristics of each atom type. As seen in Table 19.1.1.1[link], these characteristics show considerably less deviation and systematic trend among the different atom types. For instance, the heavy atoms in biological material – carbon, oxygen and nitrogen – scatter with about the same magnitude as a lead or uranium atom. In addition, neutrons are scattered by the atomic nuclei, which are essentially point sources, producing diffracted intensity not attenuated by a form-factor fall-off at increasingly higher scattering angles, as is the case in X-ray diffraction (Bacon, 1975[link]).

Table 19.1.1.1| top | pdf |
Scattering lengths for atom types

Element Atomic No. Scattering length (f; 1 f = 10−13 cm)
H 1 −3.7
D 1 6.7
C 6 6.6
N 7 9.4
O 8 5.8
Mg 12 5.2
S 16 2.8
Ca 20 4.7
Hg 80 12.7
Pb 82 9.4
U 92 8.5

There are a few atomic nuclei that induce a phase change of 180° in the scattered neutron, which results in negative peaks in a neutron density map. An extremely important example of this is the hydrogen nucleus, with a scattering length of −3.7 f (1 f = 10−13 cm). Its isotope, deuterium, on the other hand, scatters to give positive peaks (+6.7 f). The fact that H and D atoms can be so clearly distinguished from one another has very important implications for assessing biophysical parameters, as will be discussed below.

The application of the neutron-diffraction technique, which assigns H-atom positions in proteins and differentiates between H and D atoms, has been mainly focused on structural issues in three research areas: (1) protein reaction mechanisms; (2) protein dynamics; and (3) protein–water interactions (Kossiakoff, 1985[link], and references therein). It must be pointed out that recent advances in nuclear magnetic resonance have made protein dynamics investigations using H/D exchange procedures much easier than similar experiments by neutron diffraction. Additionally, the advances in ultra-high-resolution X-ray crystallography, which have allowed some level of experimental determination of hydrogen atoms in proteins, have further limited the uniqueness of the neutron method. Nevertheless, a number of important structural issues that are best approached by neutron crystallography remain.

References

Bacon, G. E. (1975). Neutron diffraction, pp. 155–188. Oxford: Clarendon Press.Google Scholar
Kossiakoff, A. A. (1985). The application of neutron crystallography to the study of dynamic and hydration properties of proteins. Annu. Rev. Biochem. 54, 1195–1227.Google Scholar








































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