Phase determination for membrane diffraction by anomalous dispersion

The phase problem of membrane diffraction is usually solved by the swelling method; however, this method does not always resolve the phases unambiguously. An alternative method of phase determination using anomalous dispersion is illustrated by the multiple-wavelength diffraction of membranes containing gramicidin ion channels. The anomalously scattering atoms are thallium ions bound to the channel. The result determines the location of the ion-binding sites in the gramicidin channel and the electron-density profile of the membrane. The applicability and limitation of the anomalous-dispersion method are discussed.