Away from the edge: SAD phasing from the sulfur anomalous signal measured in-house with chromium radiation.

Abstract

Anomalous scattering with soft X-ray radiation opens new possibilities in phasing for macromolecular crystallography. Anomalous scattering from S atoms collected on an in-house chromium radiation source (lambda = 2.29 A) was used to phase the X-ray diffraction data of thaumatin (22 kDa) and trypsin (24 kDa) crystals. The contribution to the anomalous term, Deltaf" = 1.14 e(-), from sulfur for Cr Kalpha radiation is doubled compared with that for Cu Kalpha radiation, Deltaf" = 0.56 e(-). The direct-methods programs RANTAN or SHELXD successfully found sulfur positions using data sets with resolution limited to 3.5 A. The statistical phasing program SHARP was used to produce the electron-density maps using the sulfur anomalous signal alone at low resolution ( approximately 3.5 A). An interpretable electron-density map for each structure was obtained solely from the phases derived from single-wavelength anomalous dispersion (SAD) data obtained using Cr Kalpha radiation. Much fewer data (that is, lower redundancy) are required for this sulfur SAD phasing procedure compared with the highly redundant data reported in the sulfur SAD phasing procedure with Cu Kalpha radiation. Cr Kalpha radiation can also improve the strength of anomalous scattering of many other intrinsic elements in macromolecules, such as calcium, zinc and phosphorus, because of the increased Deltaf". Furthermore, the anomalous scattering of selenium is increased substantially from 1.14 e(-) with Cu Kalpha radiation to 2.28 e(-) with Cr Kalpha radiation. In order to measure the small Bijvoet differences accurately, several devices were developed for the experiment, including an Osmic Confocal MaxFlux optic optimized for Cr Kalpha radiation, a helium path and a beam stop. In the cases studied here, radiation damage to the samples and reduction of anomalous signal were observed in some long exposure time data sets. Therefore, an adequate data-collection strategy to maximize the completeness in a short scan range was used in subsequent data collections. The results show that the anomalous signal of S atoms can be collected quickly. Since the absorption of solvent and the loop may no longer be negligible with Cr Kalpha radiation, the orientation of the crystal and exposure time were taken into account in order to minimize the effects of radiation damage and absorption. This experimental study shows that using Cr Kalpha radiation from an in-house rotating-anode X-ray generator can provide sufficient phasing power from sulfur anomalous signals to routinely phase protein diffraction data.