Ab initio phasing of proteins with heavy atoms at non-atomic resolution: pushing the size limit of solvable structures up to 7890 non-H atoms in the asymmetric unit

The success of the ab initio phasing process mainly depends on two parameters: data resolution and structural complexity. In agreement with the Sheldrick rule, the presence of heavy atoms can also play a nonnegligible role in the success of direct methods. The increased efficiency of the Patterson methods and the advent of new phasing techniques based on extrapolated reflections have changed the state of the art. In particular, it is not clear how much the resolution limit and the structural complexity may be pushed in the presence of heavy atoms. In this paper, it is shown that the limits fixed by the Sheldrick rule may be relaxed if the structure contains heavy atoms and that ab initio techniques can succeed even when the data resolution is about 2 Å, a limit unthinkable a few years ago. The method is successful in solving a structure with 7890 non-H atoms in the asymmetric unit at a resolution of 1.65 Å, a considerable advance on the previous record of 6319 atoms at atomic resolution.