THE ESTIMATION OF 4-PHASE STRUCTURE-INVARIANTS USING THE SINGLE DIFFERENCE OF ISOMORPHOUS STRUCTURE FACTORS

The recently introduced approach of using the difference between isomo rphous structure factors as a random variable in the derivation of joi nt probability distributions of isomorphous structure factors has been extended to secure the conditional joint probability distribution of the quartet phase sums present among isomorphous structure factors. It is shown for calculated data sets (native and heavy-atom derivative) of the proteins avian pancreatic polypeptide and cytochrome c that, wi th single-wavelength anomalous-scattering data, reliable estimates can be obtained for the quartet phase sums, even if the estimates are bas ed on the structure-factor differences of the four quartet main-term r eflections only.