"The treatment of errors in the isomporphous replacement method"

Date:
[1958]
Reference:
PP/CRI/H/2/28
Catalogue details

Licence: In copyright

Credit: "The treatment of errors in the isomporphous replacement method". Source: Wellcome Collection.

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    THE TREATMENT OF ERRORS IN THE ISOMORPHOUS REPLACEMENT METHOD There are two steps in the application of the isomporphous replacement method. The first is the determination of the position of the outstanding features (usually a small number of heavy atoms) which differentiate a pair structure to the structure factors may then be calculated. The second step is the use of these calculated contributions to determine the phases of the reflexions. This is done by comparing them with the observed intensity differences. In this way the structure may be determined. This paper is concerned with the second step. There will be many reflexions for which ^ is very small. The determination of phase will be correspondingly — c poor. How should these reflexions be treated? In non-centrosymmetric structures, as is well known, jtaxBxabEfcx unambiguous phase determinations are possible only if at least three isomorphous compounds are available. How should the results from the two pairs be combined?With simple structures where atoms are resolved, a trial structure can be obtained, and refinement made, for instance by the least squares method. With a large protein, there is no prospect of resolving the individual atoms and no way of refinement from a trial structure. The accuracy of the final Fourier is dependent on the best choice of weights and phases during the second step of the calculation. There will be cases of intermediate complexity where the right trial structure will be found only if the second step is done accurately enough. We will describe a method for treating this question as rigorously as possible. A structural study where the method has been put to practical use is to be published elsewhere. (Blow, 1958) of isomorphous structures. The contribution