Data collection at short wavelengths in protein crystallography.

The development of high-intensity X-ray sources and the use of insertion devices will make it possible to collect data routinely from protein crystals at very short wavelengths (lambda </= 0.5 A). Possible benefits of using shorter wavelengths can be inferred from the improvement in the quality of the data when using a wavelength lambda approximately 0.9 A instead of one close to the Cu Kalpha emission edge. In addition to fewer absorption errors, two factors might contribute to this improvement. These are an increase in the lifetime of the protein crystal and a better signal-to-background ratio. In this paper we address the second of these. In order to compare the quality of the data and the relative background level in the diffraction patterns at different wavelengths two data sets have been collected at lambda = 0.92 and 0.55 A. The results obtained from data processing and careful measurement of the background in the raw images suggest that, in the absence of absorption errors and radiation damage, data collection at very short wavelengths does not provide higher quality data. There is no improvement in the signal-to-background ratio in the short-wavelength data.