On the routine use of soft X-rays in macromolecular crystallography. Part II. Data-collection wavelength and scaling models.

Complete and highly redundant data sets were collected at nine different wavelengths between 0.80 and 2.65 A on a xenon derivative of porcine pancreatic elastase in both air and helium atmospheres. The magnitude of the anomalous signal, as assessed by the xenon-peak height in the anomalous difference Patterson synthesis, is affected by the wavelength of data collection as well as by the scaling model used. For data collected at wavelengths longer than 1.7 A, the use of a three-dimensional scaling protocol is essential in order to obtain the highest possible anomalous signal. Based on the scaling protocols currently available, the optimal wavelength range for data collection appears to be between 2.1 and 2.4 A. Beyond that, any further increase in signal will be compensated for or even superseded by a concomitant increase in noise, which cannot be fully corrected for. Data collection in a helium atmosphere yields higher I/sigma(I) values, but not significantly better anomalous differences, than data collection in air.