The structure of haemoglobin - V. Imidazole-methaemoglobin: a further check of the signs

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Abstract
The crystals are apparently orthorhombic; their unit-cell dimensions and the intensities of the 00lreflexions suggest a structure closely related to monoclinic methaemoglobin. Each structure appears to contain the same molecular layers parallel to (001), but in the monoclinic form the molecules are tilted the same way in each layer, while in the new form the tilt is alternately left and right in successive layers. Precession pictures show an interesting sequence of sharp and diffuse layer lines, those with certain indiceshcontaining streaks parallel to c*. In other layer lines the spots are broadened in varying degrees. The amplitudes of theh0lreflexions are related to the molecular Fourier transform described in previous papers of this series. Each value ofF(h0l) is compounded from the sum or the difference of the amplitude at two points on the transform having the co-ordinatesha* 0Ic* andha* 0l̄c*. The rules for addition and subtraction of amplitudes follow a simple scheme which is related to the sequence of sharp and diffuse layers. The scheme accounts for all observed values ofF(h0l), a reliability factor of 0. 21 being obtained. This correlation provides an independent check for the sequence of signs along certain layer lines of the transform. A preliminary analysis of the structure is made, and molecules in neighbouring layers are shown to be displaced by 11. 2 Å in theadirection.