The optimal wavelengths for phase determination

Schoenborn (1975) successfully phased the structure of cadmium-myoglobin. 

There is a best combination of wavelengths which will most effectively determine the phase of each reflection. In figure 9.9 we represent 

Between A2 and A3 we exploit some differences in the/ values. With three wavelengths, however, the change in / can be optimised. Hence, Ax is chosen well removed from the absorption edge (/ — 0), A2 at the point of inflection of the edge (/ most negative) and A3 just on the short wavelength side of the edge (/ most positive). 

however, we had measured FpH at three wavelengths on the low energy side of the edge (/ does not reach its minimum and/ is constant and 0), then though the centres are well separated they are collinear and a phase ambiguity results. At first sight the variation of/ at three A s seems to simulate isomorphous replacement with native and two derivatives figure 9.11 shows that it does not. 

The effect of a larger and larger 5A/A would gradually reduce the size of f and / so that the centres of the phase circles would move closer and the unique intersection of the phase circles could not be defined leading to a phasing error. Likewise, in the plot off versus/ (figure 9.2) the area of the loop decreases as 5A/A increases (Ramaseshan, pers. comm.). We 

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