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![[Figure 1]](dw5226fig1mag.jpg)
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Figure 1
(a) Precession photographs of the two main planes of the reciprocal lattice (left, hk0; right, h0l) of crystals of C-phycocyanin, a blue-colored protein extracted from the green alga Agmenellum quadruplicatum. The precession angle is μ = 9°. These diffraction patterns were obtained with an overnight exposure on a precession camera model from the 1970s manufactured by the Charles Supper Co., Boston, Massachusetts, USA mounted on an Elliot rotating-anode generator. Note the 6mm symmetry of the diffraction pattern along the unique c trigonal axis (left) and the centrosymmetric pattern for the h0l plane (right), with the X-rays perpendicular to the main symmetry axis (threefold). A very small beam stop was used to visualize the lowest resolution reflections to understand the packing of the 32 molecular aggregates (Hackert et al., 1977  ). In the h0l reciprocal-lattice plane (right) the main axis (00l reflections) is horizontal. (b) Oscillation diffraction pattern (Δφ = 1.5–3.5°) of the same crystal form as in (a), with the c axis of the crystal oriented perpendicular to the X-ray beam (crystal-to-film distance 7.5 cm). The central lune corresponds to the h0l plane shown on the right in (a). The crystallographic parameters of the unit cell were a = b = 184.5, c = 60.5 Å (trigonal space group P321). |
![[Figure 1]](dw5226fig1.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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