THE AB-INITIO CRYSTAL-STRUCTURE SOLUTION OF PROTEINS BY DIRECT-METHODS .1. FEASIBILITY

Traditional direct methods based on the tangent formula and/or on Sayr e's equation cannot solve ab initio the large majority of protein crys tal structures [Giacovazzo, Guagliardi, Ravelli & Siliqi (1994). Z. Kr istallogr. 209, 136-142]. Indeed, the amount of information available leads to a signal-to-noise ratio close to unity; consequently, the cor rect solution, even if attained, cannot be recognized among the trial solutions. Attention is here focused onto the case in which diffractio n data of one isomorphous derivative are additionally available. It is shown that in such a case direct ab initio solution of protein struct ures is feasible. Tests based on calculated diffraction data suggest t he procedure to follow for a possible success.