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A difference of seven conservative amino-acid substitutions between two single-chain antibodies (scFvs) specific for chlamydial lipopolysaccharide does not significantly affect their molecular structures or packing contacts, but dramatically affects their crystallization. The structure of the variable domain (Fv) of SAG173-04 was solved to 1.86 Å resolution and an Rcryst of 18.9% in space group P212121. Crystals of the homologous SAG506-01 diffracted to 1.95 Å resolution and appeared at first to have Patterson symmetry I4/m or P4/mmm; however, no solution could be found in space groups belonging to the former and refinement in the only solution corresponding to the latter (in space group P43212) stalled at Rfree = 30.0%. Detailed examination of the diffraction data revealed that the crystal was likely to be twinned and that the correct space group was P212121. Both translational pseudo-symmetry and pseudo-merohedral twinning were observed in one crystal of SAG506-01 and pseudo-merohedral twinning was observed for a second crystal. The final R factor for SAG506-01 after refinement in P212121 was 20.5%.

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