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The preparation of single crystals suitable for X-ray analysis is frequently the most difficult step in structural studies of proteins.With the aid of two examples, it is shown that de novo solution of the crystallographic phase problem can be achieved at low resolution using microcrystalline powder samples via the single isomorphous replacement method. With synchrotron radiation and optimized instrumentation, high-quality powder patterns have been recorded, from which it was possible to generate phase information for structure factors up to 6 A resolution. pH- and radiation-induced anisotropic lattice changes were exploited to reduce the problem of overlapping reflections, which is a major challenge in protein powder diffraction. The resulting data were of sufficient quality to compute molecular envelopes of the protein molecule and to map out the solvent channels in the crystals. The results show that protein powder diffraction can yield low-resolution data that are potentially useful for the characterization of microcrystalline proteins as novel micro- and mesoporous materials as well as for structural studies of biologically important macromolecules.