Solving proteins at non-atomic resolution by direct methods

A new probabilistic formula estimating triplet invariants and capable of exploiting a model electron-density map gradually created during the ab initio phasing process has been tested on a set of protein structures with data at non-atomic resolution. All the structures contain heavy atoms larger than Ca, and show a structural complexity which may attain the level of about 8000 non-hydrogen atoms in the asymmetric unit. It is shown that the majority of such structures may be solved by our direct methods procedure, which, in combination with a number of ancillary phasing tools, weakens the traditional expectation that atomic resolution is a necessary ingredient for the success of a direct ab initio phasing.