Tetartohedral twinning could happen to you too

Abstract

Tetartohedral crystal twinning is discussed as a particular case of (pseudo)merohedral twinning when the number of twinned domains is four. Tetartohedrally twinned crystals often possess pseudosymmetry, with the rotational part of the pseudosymmetry operators coinciding with the twinning operators. Tetartohedrally twinned structures from the literature are reviewed and the recent structure determination of tetartohedrally twinned triclinic crystals of human complement factor I is discussed.

Figure 1

Triclinic human complement factor I: κ = 180° section of the self-rotation function in the resolution interval 52–2.7 Å. Contour levels: 1–6σ in steps of 1σ. Three peaks are visible at ω = 90.7°, ϕ = 90.6°, κ = 180° (6.6σ), ω = 89.7°, ϕ = 0.0°, κ = 180° (6.6σ) and ω = 0°, ϕ = 0°, κ = 180° (6.2σ). Computed with the program POLARRFN.

Figure 2

Triclinic human complement factor I: detail of one of the diffraction frames collected on beamline I02 at Diamond in November 2009. Indexing and prediction was performed in MOSFLM. The yellow boxes show the predicted location of the spots, with their hkl indices in black. The reflection 050 appears next to the much stronger 060. After integration, I ₀₅₀/σ(I ₀₅₀) = 294/24, i.e. the intensity of the reflection is weak but still ten times its σ.