S-SWAT (softer single-wavelength anomalous technique): potential in high-throughput protein crystallography

Acta Cryst. (2003). A59, 327–334

Overview of apocrustacyanin A1 dimer and anomalous difference Fourier electrondensity map. Xe atoms are coloured in yellow; disulfide bridge sulfurs are in orange.

An example of the quality of the electron-density portions (phases from MLPHARE/DM, 2F_o–F_c, 2 r.m.s.) overlaid on a fragment of the main chain of apocrustacyanin A1.

The drive for higher output of protein crystal structures sustains an interest in streamlining protocols. Synchrotron radiation beamlines offer automated data collection with faster and larger detectors, choice of wavelength(s), intense beams and fine collimation. Presented here is a single-wavelength approach 'Softer-SWAT' exploiting the large optimized f'' signal of the Xe L1 absorption edge, and producing de novo phases. A high-quality electron-density map was obtained when phase improvement methods were used. This approach may also harness the iodine optimized f'' and enhanced sulfur f'' signals. This case study contributed to the design of the new Daresbury SRS Multipole Wiggler Beamline 10. It also led to the development of a new formula to evaluate the anomalous signal of multiple anomalous scatterers in protein crystals.