International
Tables for
Crystallography
Volume B
Reciprocal space
Edited by U. Shmueli

International Tables for Crystallography (2006). Vol. B. ch. 2.3, p. 242   | 1 | 2 |

Section 2.3.3.1. Introduction

M. G. Rossmanna* and E. Arnoldb

aDepartment of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, USA, and  bCABM & Rutgers University, 679 Hoes Lane, Piscataway, New Jersey 08854-5638, USA
Correspondence e-mail:  mgr@indiana.bio.purdue.edu

2.3.3.1. Introduction

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One of the initial stages in the application of the isomorphous replacement method is the determination of heavy-atom positions. Indeed, this step of a structure determination can often be the most challenging. Not only may the number of heavy-atom sites be unknown, and have incomplete substitution, but the various isomorphous compounds may also lack isomorphism. To compound these problems, the error in the measurement of the isomorphous difference in structure amplitudes is often comparable to the differences themselves. Clearly, therefore, the ease with which a particular problem can be solved is closely correlated with the quality of the data-measuring procedure.

The isomorphous replacement method was used incidentally by Bragg in the solution of NaCl and KCl. It was later formalized by J. M. Robertson in the analysis of phthalocyanine where the coordination centre could be Pt, Ni and other metals (Robertson, 1935[link], 1936[link]; Robertson & Woodward, 1937[link]). In this and similar cases, there was no difficulty in finding the heavy-atom positions. Not only were the heavy atoms frequently in special positions, but they dominated the total scattering effect. It was not until Perutz and his colleagues (Green et al., 1954[link]; Bragg & Perutz, 1954[link]) applied the technique to the solution of haemoglobin, a protein of 68 000 Da, that it was necessary to consider methods for detecting heavy atoms. The effect of a single heavy atom, even uranium, can only have a very marginal effect on the structure amplitudes of a crystalline macromolecule. Hence, techniques had to be developed which were dependent on the difference of the isomorphous structure amplitudes rather than on the solution of the Patterson of the heavy-atom-derivative compound on its own.

References

First citation Bragg, W. L. & Perutz, M. F. (1954). The structure of haemoglobin. VI. Fourier projections on the 010 plane. Proc. R. Soc. London Ser. A, 225, 315–329.Google Scholar
First citation Green, D. W., Ingram, V. M. & Perutz, M. F. (1954). The structure of haemoglobin. IV. Sign determination by the isomorphous replacement method. Proc. R. Soc. London Ser. A, 225, 287–307.Google Scholar
First citation Robertson, J. M. (1935). An X-ray study of the structure of phthalocyanines. Part I. The metal-free, nickel, copper, and platinum compounds. J. Chem. Soc. pp. 615–621.Google Scholar
First citation Robertson, J. M. (1936). An X-ray study of the phthalocyanines. Part II. Quantitative structure determination of the metal-free compound. J. Chem. Soc. pp. 1195–1209.Google Scholar
First citation Robertson, J. M. & Woodward, I. (1937). An X-ray study of the phthalocyanines. Part III. Quantitative structure determination of nickel phthalocyanine. J. Chem. Soc. pp. 219–230.Google Scholar








































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