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The Protein Data Bank is a computer-based archival file for macromolecular structures. The Bank stores in a uniform format atomic co-ordinates and partial bond connectivities, as derived from crystallographic studies. Text included in each data entry gives pertinent information for the structure at hand (e.g. species from which the molecule has been obtained, resolution of diffraction data, literature citations and specifications of secondary structure). In addition to atomic co-ordinates and connectivities, the Protein Data Bank stores structure factors and phases, although these latter data are not placed in any uniform format. Input of data to the Bank and general maintenance functions are carried out at Brookhaven National Laboratory. All data stored in the Bank are available on magnetic tape for public distribution, from Brookhaven (to laboratories in the Americas), Tokyo (Japan), and Cambridge (Europe and worldwide). A master file is maintained at Brookhaven and duplicate copies are stored in Cambridge and Tokyo. In the future, it is hoped to expand the scope of the Protein Data Bank to make available co-ordinates for standard structural types (e.g. a-helix, RNA double-stranded helix) and representative computer programs of utility in the study and interpretation of macromolecular structures.The Protein Data Bank' [1,2] was established in 1971 as a computer-based archival file for macromolecular structures. The purpose of the Bank is to collect, standardize, and distribute atomic co-ordinates and other data from crystallographic studies. As the number of solved protein and nucleic-acid structures has grown to the point where some lo7 characters are necessary to represent the co-ordinate information currently held, the need for such a computer-readable file has become very clear, and demands for the Bank's services have increased accordingly. The Protein Data Bank is one of several data base activities in the field of crystallography, e.g. the Bibliographic

The molecular structure of Zeise's salt, KPt(C2H4)Cl3-H20, has been studied by single-crystal neutron diffraction techniques.The PtCb moiety forms a nearly planar group with the platinum atom 0.03 A from the least-squares plane through the PtCb group. The Pt-Cl bond trans to the ethylene group [2.340 (2) A] is significantly longer than the cis Pt-Cl bonds [2.302 (2) and 2.303 (2) A], This difference, which is 19 times its standard deviation, is probably attributable to the trans-directing influence of the ethylene ligand. The carbon atoms are approximately equidistant from the platinum atom [2.128 (3) and 2.135 (3) A], and the distance from the midpoint of the C-C bond to the platinum atom is 2.022 A. The C-C distance, 1.375 (4) A, is 0.038 A longer than the value found in free ethylene, indicating some dx-px* back-bonding from the platinum atom to C2H4. Back-bonding is also indicated by a bending of the four hydrogen atoms away from the platinum atom: the carbon atoms are at an average distance of 0.164 A from the plane of the four hydrogen atoms, and the angle between the normals to the methylene planes (the a angle) is 32.5°. Both the magnitude of a and the C-C bond lengthening are considerably smaller in Zeise's salt than in metal complexes of C2F4 and C2(CN)4, suggesting that the amount of metal-ligand back-bonding may be greater in these complexes than in those involving ethylene.

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