  The application of powder diffraction methods in two-dimensional
crystallography is regarded as intractable because of the uncertainties
associated with overlapping reflections. Here, we report an approach that
resolves these ambiguities and provides reliable low-resolution phase
information directly from powder diffraction data. We apply our method to the
recovery of the structure of the bacteriorhodopsin (bR) molecule to a
resolution of 7 angstroms using only powder diffraction data obtained from
two-dimensional purple membrane (PM) crystals.
