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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Apo‐ and holo‐forms of horse liver alcohol dehydrogenase (LADH) in solution were studied by diffuse x‐ray scattering. Experimental scattering curves for apo‐ and holo‐forms coincide both with the curves calculated from the crystal structures of apo‐ and holo‐enzymes, and with each other. Thus the “sliding” of catalytical domains in LADH upon substrate binding, which has been shown by x‐ray analysis, cannot be detected by diffuse x‐ray scattering. Sensitivity of the scattering curves to the domain displacements of sliding and “locking” types has been investigated. It has been shown that the scattering curves of LADH are rather sensitive to the domain “unlocking.” However, these curves change only slightly upon sliding of domains, including the sliding of domains observed in LADH by x‐ray analysis.

A new method is proposed for the study of the structure and arrangement of components in multi‐component particles. In this method the difference between the scattering curve of a solution containing two types of structurally identical particles differing only in degree of deuteration and the scattering curve of a solution containing particles of a third type deuterated to an intermediate degree is considered. This difference scattering curve is distinguished only by a numerical multiplier from the `vacuum' scattering curve of the particle in which the scattering density is equal to the difference between the scattering densities of particles of the first and second types. This means that components protonated (deuterated) to the same degree within particles of the three types considered do not contribute to the difference scattering curve and, consequently, are `invisible' to neutrons. The difference scattering curve depends neither on the solvent isotopic content nor on interparticle interference and particle association. Possible applications of the method to a study of the structure of macromolecules in solution are discussed.

A new method is proposed for studying the structure and mutual arrangement of selected components in multicomponent particles. Here the difference between the scattering curve of a solution containing two types of structurally identical particles (differing only in the degree of deuteration) and the scattering curve of a solution containing particles of a third type (deuterated to an intermediate degree) is considered. This difference scattering curve differs only by a numerical multiplier from the “vacuum” scattering curve of the particle in which the scattering density is equal to the difference between the scattering densities of the particles of the first and second types. This means that any particle component which is deuterated (protonated) to the same degree in the particles of three different types does not contribute to the difference scattering curve and, consequently, is “invisible” for neutrons. The difference scattering curve depends neither on the isotopic content of the solvent nor on the inter‐particle interference and particle association. Possible applications of this method are discussed.