SIR88 is an integrated package of computer programs for the solution of crystal structures. The package is based on the estimation of one- and two-phase structure seminvariants and three- and four-phase structure invariants according to the theory of representations [Giacovazzo (1977). Acta Cryst. A33, 933–944; (1980). Acta Cryst. A36, 362–372]. The program works in all the space groups and in most cases it is able to provide the correct solution without user intervention. Some prior information like the availability of a partial structure or of pseudotranslational symmetry is easily exploited to obtain the structure solution.

The crystalline State and isometric Operations 1 Symmetry elements 3 Axes of rotational symmetry 3 Axes of rototranslation or screw axes 5 Axes of inversion 5 Axes of rotorefiection 5 Refiection planes with translational component (glide planes) 6 Lattices 6 The rational properties of lattices 7 Crystallographic directions 7 Crystallographic planes 8 Symmetry restrictions due to the lattice periodicity and vice versa 9 Point groups and symmetry classes 11 Point groups in one and two dimensions 16 The Laue classes 17 The seven crystal Systems 17 The Bravais lattices 18 Plane lattices 18 Space lattices 19 The space groups 22 The plane and line groups 30 On the matrix representation of symmetry Operators 32 Appendices: LA The isometric transformations 35 l.B Some combinations of movements 37 l.C Wigner-Seitz cells 41 l.D The space-group rotation matrices l.E Symmetry groups l.F Symmetry generalization References Crystallographic Computing Carmelo Giacovazzo

The quality of an E map is usually affected by systematic and/or random phase errors, by amplitude truncation effects in the series representation of the electron density, by the experimental uncertainty in the estimation of I EI and by the intrinsic nature of the Fourier coefficients use~t (i.e. the E's). It is shown that simple supplemental calculations can improve the quality of an E map. Large molecular fragments can often be localized in the new map even when the original one is not easily interpretable.

MOLDRAW is a simple and easy to learn molecular graphics program running on personal computers, which allows drawing and manipulation of any molecular structure. In this article the program functions and examples of its application are presented. The user-friendly interface and the simple but powerful underlying logic are the main reasons for the widespread diffusion of MOLDRAW in the crystallographic and chemical community.

Siliceous sponges, one of the few animal groups involved in a biosilicification process, deposit hydrated silica in discrete skeletal elements called spicules. A multidisciplinary analysis of the structural features of the protein axial filaments inside the spicules of a number of marine sponges, belonging to two different classes (Demospongiae and Hexactinellida), is presented, together with a preliminary analysis of the biosilicification process. The study was carried out by a unique combination of techniques: fiber diffraction using synchrotron radiation, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetric (DSC), Fourier transform infrared spectroscopy (FTIR), and molecular modeling. From a phylogenetic point of view, the main result is the structural difference between the dimension and packing of the protein units in the spicule filaments of the Demospongiae and the Hexactinellida species. Models of the protein organization in the spicule axial filaments, consistent with the various experimental evidences, are given. The three different species of demosponges analyzed have similar general structural features, but they differ in the degree of order. The structural information on the spicule axial filaments can help shed some light on the still unknown molecular mechanisms controlling biosilicification.