Protein crystallography in the soft X-ray region: crystal lifetime and diffraction efficiency.

Multiwavelength anomalous diffraction on the sulfur (5.01 A) and phosphorus (5.77 A) absorption K-edges seems to offer a solution to one of the most appealing problems of protein crystallography - the determination of the phases of structure amplitudes. However, a strong increase in absorption of X-rays in this region may impede the development of this method of structure solution. Analytical expressions for diffraction efficiency and normalized diffraction efficiency have been derived in the present paper. It is shown that the crystal lifetime in the soft X-ray region will be significantly shorter than the lifetime of a macromolecular crystal exposed to 1-1.5 A X-rays during diffraction data collection. An optimum crystal size has been estimated.