The use of wavelet transforms in low-resolution phase extension

A method to extend low-resolution phases has been developed using histogram matching not only of the electron density itself but also of histograms ob tained from the different levels of detail provided by the wavelet transfor m of the electron density. It is shown that the method can extend phases fr om 10 Angstrom to around 6-7 Angstrom on a wide range of trial structures d iffering in size, space group and solvent content. This level of phase exte nsion can improve the electron-density map from little more than a molecula r envelope to one in which secondary structure can often be identified.