Estimation of twinning parameter for twins with exactly superimposed reciprocal lattices

Britton, Doyle

doi:10.1107/s0567739472000786

Cited by 84 publications

(67 citation statements)

References 5 publications

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“…Since detwinning the Xe-1 and Pt-1 derivatives with their high twin fractions introduced large errors in the data, we Britton plot (Britton, 1972;Fisher & Sweet, 1980), showing the number of negative intensities as a function of the assumed twin fraction. An estimation of the twin fraction that is higher than its real value will result in several detwinned intensities with a negative value.…”

Section: Structure Determination By Mir Methodsmentioning

confidence: 99%

“…We estimated the twin fraction for the selenomethionine data sets using Britton plots (Britton, 1972;Fisher & Sweet, 1980) and used this value to detwin the data. From the dispersive differences between data collected at the in¯ection point and the remote wavelength (Table 1) a Patterson map was calculated, which showed the positions of two heavy-atom scatterers (Fig.…”

Section: Structure Determination By Mir Methodsmentioning

confidence: 99%

“…Instead, the method ®rst proposed by Britton (1972) and further elaborated by Fisher & Sweet (1980) was used. (1a) and (1b) can be rewritten as…”

Section: Determination Of the Twin Law And Twin Fractionsmentioning

confidence: 99%

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Multiple isomorphous replacement on merohedral twins: structure determination of deacetoxycephalosporin C synthase

Scheltinga

Valegård

Ramaswamy

et al. 2001

Acta Crystallogr D Biol Cryst

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Merohedral twinning is a packing anomaly that seriously impairs the determination of macromolecular crystal structures. Crystals of deacetoxycephalosporin C synthase (DAOCS), an enzyme involved in the expansion of the penicillin nucleus to form the core structure of the cephalosporin antibiotics, were found to be merohedrally twinned by many diagnostic criteria. Here, the structure determination of DAOCS from twinned crystals based on a combination of isomorphous replacement and the use of a multiple‐wavelength diffraction data set is described.

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Section: Structure Determination By Mir Methodsmentioning

confidence: 99%

Section: Structure Determination By Mir Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Multiple isomorphous replacement on merohedral twins: structure determination of deacetoxycephalosporin C synthase

Scheltinga

Valegård

Ramaswamy

et al. 2001

Acta Crystallogr D Biol Cryst

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show abstract

“…Since both J 1 and J 2 are positive, neither numerator nor denominator should be negative, as pointed out by Britton (1972), and these two conditions can be expressed as < I 1 /(I 1 + I 2 ) and < I 2 /(I 1 + I 2 ) or, as one inequality,…”

Section: Britton Plot Of`negative Intensities'mentioning

confidence: 99%

“…Data collected from merohedrally twinned crystals can very conveniently be tested and detwinned using the Yeates & Fam twinning server at UCLA (http://www.doe-mbi.ucla.edu/ Services/Twinning/). Britton (1972) pointed out, and earlier Zalkin et al (1964) implicitly mentioned, that the assumption of too large a value of results in an estimation of negative true intensities. Based on this principle, Fisher & Sweet (1980) proposed a practical method for the estimation of the twin fraction .…”

Section: Britton Plot Of`negative Intensities'mentioning

confidence: 99%

Twinned crystals and anomalous phasing

Dauter

2003

Acta Crystallogr D Biol Cryst

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Merohedral or pseudomerohedral twinning of crystals cannot be identi®ed from inspection of the diffraction patterns. Several methods for the identi®cation of twinning and the estimation of the twin fraction are suitable for macromolecular crystals and all are based on the statistical properties of the measured diffraction intensities. If the crystal twin fraction is estimated and is not too close to 0.5, the diffraction data can be detwinned; that is, related to the individual crystal specimen. However, the detwinning procedure invariably introduces additional inaccuracies to the estimated intensities, which substantially increase when the twin fraction approaches 0.5. In some cases, a crystal structure can be solved with the original twinned data by standard techniques such as molecular replacement, multiple isomorphous replacement or multiwavelength anomalous diffraction. Test calculations on data collected from a twinned crystal of gpD, the bacteriophage ! capsid protein, show that the singlewavelength anomalous diffraction (SAD) method can be used to solve its structure even if the data set corresponds to a perfectly twinned crystal with a twin fraction of 0.5.

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High‐Temperature Experimental and Theoretical Study of Magnetic Interactions in Diamond and Pseudo‐Diamond Frameworks Built up from Hexanuclear Tantalum Clusters

Perić

Cordier

Cuny

et al. 2011

Chemistry A European J

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Magnetic interactions in solid-state tantalum cluster compounds have been evidenced by using magnetic susceptibility measurements and corroborated by broken-symmetry DFT calculations. The three selected compounds are based on [Ta(6)X(12)(H(2)O)(6)](3+) (X=Cl or/and Br) units with edge-bridged Ta(6) octahedral clusters. Although two of them crystallise in the tetragonal space group I4(1)/a, all compounds exhibit a similar arrangement of paramagnetic clusters related to the diamond structural framework (Fd ̅3m space group). Magnetic parameters were fitted by using the [5,4] Padé approximant of high-temperature series expansion of susceptibility for the Heisenberg model (S=1/2) in the diamond framework, assuming only nearest-neighbouring interactions. Such a model appropriately describes magnetic-susceptibility measurements at temperatures T>0.7|J|/k. The magnetic interaction parameter J between two [Ta(6)Cl(12)(H(2)O)(6)](3+) clusters is estimated to be -64.28(7) cm(-1) ; it has been enhanced by replacing several chlorine inner ligands with bromine ones (J=-123(3) cm(-1) for two [Ta(6)Br(7.7(1))Cl(4.3(1))(H(2)O)(6)](3+) clusters) and is strongest between two bromine [Ta(6)Br(12)(H(2)O)(6)](3+) clusters with a value of -155(1) cm(-1) . Broken-symmetry DFT calculations within spin-dimer analysis confirmed this trend. Those interactions can be explained on the basis of the overlap between singly occupied a(2u) orbitals localised on neighbouring clusters.

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Estimation of twinning parameter for twins with exactly superimposed reciprocal lattices

Cited by 84 publications

References 5 publications

Multiple isomorphous replacement on merohedral twins: structure determination of deacetoxycephalosporin C synthase

Multiple isomorphous replacement on merohedral twins: structure determination of deacetoxycephalosporin C synthase

Twinned crystals and anomalous phasing

High‐Temperature Experimental and Theoretical Study of Magnetic Interactions in Diamond and Pseudo‐Diamond Frameworks Built up from Hexanuclear Tantalum Clusters

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