On the application of phase relationships to complex structures. XXXV. Some experiments with 2-Zn insulin

The direct-methods program SAYTAN has been applied to the known structure of 2-Zn insulin with 806 atoms, excluding solvent, in the asymmetric unit. Useful sets of phases can be obtained and selected by figures of merit for data resolutions between 1.5 and 2.25 Å and these can be extended by SAYTAN to give mean phase errors of 68° for more than 2000 reflections. A feature of the phases so found is that the phase errors decrease with increasing resolution - which is the opposite of the situation when phases are found by isomorphous-replacement techniques.