SyncpsisAn improved cube method has been developed for calculating the intensity of diffuse x-ray scattering of macromolecules in solution using a certain set of their atomic coordinates. The technique is based on the ideas of B. Lee and F. M. Richards [(1971) J. Mol. Biol. 5 5 , 3 7 4 4 0 1 and Richards [(1977) Annu. Reu. Biophys. Bioeng. 6,151-176) on the possibility of estimating the molecular and accessible surface of a particle by "rolling" a sphere, simulating a water molecule, on its molecular surface. It is shown that this technique is more advantageous than earlier versions of the cube methods. The improved technique for calculating scattering curves was utilized for several globular proteins, and for the first time, reliable scattering curves were obtained for protein-''bound'' water complexes. In the case of globular proteins and tRNA, this technique has permitted a strict evahation of their accessible surfaces, their volumes, and, apparently for the first time, their complete molecular surfaces.

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A comparative analysis of the sperm-whale myoglobin structure in the crystal and in solution has been carried out with the technique previously formulated by the authors which uses the large-angle X-ray diffuse scattering for investigating the protein structure in solution. A 'modified cube method', correctly taking into account the cavities within the protein molecule accessible to the solvent, is proposed for an accurate estimate of the solvent influence and for calculation of the scattering intensities. A comparison of the theoretical myoglobin scattering curves with the experimental scattering curve of this protein obtained by Stuhrmann shows rather noticeable quantitative divergences, which can be eliminated by a small increase in the distance between the 'hairpin' GH and the other part of the protein molecule. It is also shown that several other physically reasonable shifts of helices (or groups of helices) do not lead to an agreement between the experimental and theoretical scattering indicatrices.

Thyroglobulin is a protein involved in the synthesis, storage and secretion of thyroid hormones, reported here are some small-angle X-I ay measurements primarily concet ned with determination of its shape parameters in solution. The protein was fractionated on Sephadex G-200, and the portions corresponding to 'native' protein were collected and re-chromatographed on Sepharose 6-B; the protein samples were examined in phosphate buffer at pH 6"9. A radius of gyration of 64 A, a particle volume of ~ 1"7 x 106 ,~3, and an anisotropy factor of 1.12 were found. Assuming a (reasonable) prolate ellipsoidal shape, one obtains an axial ratio of 2: 1, in good agreement with recent electron micrographic results, and implying a high degree of solvation. Some details and problems connected with the SAXS measurements, in particular of the molecular weight, will be discussed.

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