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

The drive for yet higher output of protein crystal structures sustains an interest in streamlining data-collection protocols. Highly organized automated robotics attend to one aspect. The data-collection elapsed time also depends on the number of wavelengths used, the data redundancy and the desired diffraction resolution limit. Synchrotron-radiation beamlines offer a centralized data-collection approach with considerable flexibility of choice of wavelength(s) with high beam intensity and fine collimation. The arrival of automated IPs (imaging plates) and CCDs (charge-coupled devices) with sample freezing has widened the scope for ease of data collection and achieving a high redundancy, the latter at the cost of elapsed time per sample. This paper reports a single-wavelength approach where a large optimized f'' signal involving the Xe L1-absorption edge is harnessed. Even with modest data redundancy (7), a high-quality electron-density map was obtained from that single data set when combined with phase improvement methods. Two phasing protocols are compared for the same data set. Resolution extension (to 1.4 Å), in this test, was also done but via a second data set. In future, with a tilted detector geometry, full diffraction resolution will be collectable in one experiment on the one beamline. Moreover, careful minimization of beamline Be window thickness on the new Daresbury SRS Multipole Wiggler Beamline 10 will maximize the 2-3 Å (softer) wavelength intensity performance specification. The work has application to the harnessing of iodine f'' optimized signals and enhanced sulfur f'', as well as xenon f'', in protein crystallography. This approach, softer-SWAT, should `swat' many protein structures when it comes online.