PHASING PROTEINS AT LOW-RESOLUTION

A method for obtaining phases of low-order reflections is presented. I t is based on four observations: (1) the electron density inside prote ins is smooth and uniform at low resolution. (2) Since all proteins ha ve almost the same density, the total volume of the protein is known i f the molecular weight is known. (3) The overall shape of many protein s is fairly spherical. (4) The total scattering from a sphere of unifo rm density is in phase with a point scatterer at its centre of gravity , up to a well defined cross-over. After the first cross-over the tota l protein molecule scatters out of phase with its centre. If the centr e of the protein can be found, the phases of typically the ten lowest resolution reflections can be very accurately determined. The method w orks, provided low-order reflections can be measured accurately and th e centre of gravity can be well positioned from these data. The correc tly phased low-resolution reflections may be used as a starting set fo r phase extension. By combining the measured amplitudes with these pha ses we believe that the size and low-resolution shape of an unknown pr otein, i.e. the envelope of the molecule, can be obtained.