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The application of structure envelopes in structure determination from powder diffraction data
Author(s) -
Brenner Simon,
McCusker Lynne B.,
Baerlocher Christian
Publication year - 2002
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889802001759
Subject(s) - direct methods , electron density , crystallography , subroutine , envelope (radar) , diffraction , data structure , computational physics , algorithm , mathematics , chemistry , molecular physics , physics , computer science , electron , optics , quantum mechanics , telecommunications , radar , operating system , programming language
A structure envelope is a special type of periodic nodal surface that separates regions of high electron density from those of low electron density. Once such a surface has been generated, it can be used in combination with direct‐space methods to facilitate structure solution from powder data. To generate an informative structure envelope, the phases of the structure factors of a few strong low‐order reflections must be determined; an algorithm has been developed for this purpose. The program SayPerm combines ( a ) the use of error‐correcting codes (e.c.c.'s) to sample phase space efficiently, ( b ) a pseudo‐atom approximation of structure fragments to simulate atomic resolution at ca 2.5 Å, and ( c ) phase extension and phase set ranking using the Sayre equation. The effect of using a structure envelope in structure solution was first tested in combination with a subroutine for finding zeolite topologies in the program FOCUS . Then extension to molecular structures in combination with a simulated‐annealing program was explored. This resulted in the development of the program Safe and the subsequent determination of the structure of a tri‐β‐peptide (C 32 N 3 O 6 H 53 ) with 17 variable torsion angles.