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The product of the human major histocompatibility (HLA) class I allele HLA-B*1402 only differs from that of allele HLA-B*1403 at amino-acid position 156 of the heavy chain (Leu in HLA-B*1402 and Arg in HLA-B*1403). However, both subtypes are known to be differentially associated with the inflammatory rheumatic disease ankylosing spondylitis (AS) in black populations in Cameroon and Togo. HLA-B*1402 is not associated with AS, in contrast to HLA-B*1403, which is associated with this disease in the Togolese population. The products of these alleles can present peptides with Arg at position 2, a feature shared by a small group of other HLA-B antigens, including HLA-B*2705, the prototypical AS-associated subtype. Complexes of HLA-B*1402 with a viral peptide (RRRWRRLTV, termed pLMP2) and a self-peptide (IRAAPPPLF, termed pCatA) were prepared and were crystallized using polyethylene glycol as precipitant. The complexes crystallized in space groups P2(1) (pLMP2) and P2(1)2(1)2(1) (pCatA) and diffracted synchrotron radiation to 2.55 and 1.86 A resolution, respectively. Unambiguous solutions for both data sets were obtained by molecular replacement using a peptide-complexed HLA-B*2705 molecule (PDB code 1jge) as a search model.

The calf spleen purine nucleoside phosphorylase (PNP) ternary complex with an N(7)-acycloguanosine inhibitor and a phosphate ion has been crystallized in the cubic space group P2(1)3, with unit-cell parameter a = 94.11 Angstrom and one monomer per asymmetric unit. X-ray diffraction data were collected using synchrotron radiation (Station X31, EMBL Outstation, DESY, Hamburg). The crystal structure was refined to a resolution of 2.2 Angstrom and R and R-free values of 17.5 and 24.5%, respectively. The acyclonucleoside inhibitor is bound in the active site in an inverted ('upside- down') orientation of the purine base compared with natural substrates. The side chain of Asp243 forms two hydrogen bonds with the base ring: N-delta donates a hydrogen to N(3) and O-delta accepts a hydrogen from the guanine N(2)-amino group. N(1)-H of the base is hydrogen bonded to O-delta of Glu201, while N(9) accepts a hydrogen bond from Thr242 O-gamma. In addition, a water molecule (W417) bridges the N(2)-amino group of the base and O-epsilon of Glu201. In the phosphate-binding site, a phosphate ion is bound to Ser33, His64, Arg84, His86, Ala116 and Ser220. The acyclic chain of the N(7)-acycloguanosine inhibitor is in a folded conformation and together with a water molecule (W388) occupies the pentose- binding site, with possible hydrogen bonds to Tyr88 O-eta and His257 N-delta1. This new binding mode fully accounts for the previously observed substrate properties of 7-beta -D- ribofuranosides of hypoxanthine and guanine. It also provides a new starting point for the design of inhibitors of PNP for therapeutic and other applications.

Static and dynamic light scattering have been employed to investigate the behaviour of nucleating lysozyme solutions in the range between 0.34 and 3.08 mM. Preselected concentrations of NaC1 and (NH(4))(2)SO(4) have been used to screen the repulsive Coulombic interactions and trigger aggregation. Initially, mass-fractals undergoing diffusion limited-like aggregation coexist with monomers or small lysozyme oligomers. The growth kinetics of the fractals deliver observables that exhibit distinct tendencies when examined as a function of lysozyme concentration. The behaviour of the observables changes drastically around 2.0 mM lysozyme. Static light scattering experiments revealed progressive restructuring or growth of compact structures at later stages of the aggregation. Based on the correlations between the observables an attempt is made to predict whether the examined solutions will crystallize or not. A tentative scheme, involving the most prominent structures observed in nucleating lysozyme solutions, is discussed.

Static and dynamic light scattering have been employed to investigate the behaviour of lysozyme solutions when varying the concentration of (NH(4))(2)SO(4) and NaCl for screening the repulsive forces between the monomers. At the initial aggregation stages clusters, which can be classified as mass-fractals undergoing diffusion limited-like aggregation, coexist with monomers or small lysozyme oligomers. The kinetics of fractal growth deliver observables that exhibit distinct tendencies when examined as a function of the concentration and nature of the electrolyte. The behaviour of the observables changes drastically above 0.84 M (NH(4))(2)SO(4) and 0.60 M NaCl. Static light scattering revealed a progressive restructuring of the fractals to compact structures at the latter stages of the reaction. Based on the correlations between the various observables an attempt is made to predict the long-term fate of the nucleating solutions.

Helicases are ATP-driven enzymes essential for DNA unwinding. The broad host range plasmid RSFI010 harbours a gene (repA) encoding for one of the smallest known oligomeric helicases, RepA, a homo-hexamer with 30 kDa subunits. Electron micrographs indicate that the overall shape of RepA resembles a hexagon with globular monomers at the corners, diameter 140 A, and a central channel. Below pH 6, the molecules aggregate into tubular structures. The enzyme has been purified and crystallized using the hanging-drop vapour-diffusion method with polyethyleneglycol monomethylether as precipitating agent. The crystals exhibit the monoclinic space group P2(1) with unit-cell parameters a = 105.8, b = 180.3, c = 115.4 A, beta = 95.2 degrees, and diffract to 3.5 A resolution using rotating-anode Cu Kalpha radiation. Assuming two 180 kDa molecules per asymmetric unit, the volume per unit weight is V(m) = 3.06 A Da(-1), equivalent to a solvent content of 60%. A self-rotation search indicates that the sixfold axis of the hexamer is parallel to the ac plane and inclined at about 2 degrees to the c axis. The two hexamers are oriented head-to-head with point-group symmetry D(6).

The human leukocyte antigen (HLA) alleles HLA-B*2704 and HLA-B*2706 show an ethnically restricted distribution and are differentially associated with ankylosing spondylitis, with HLA-B*2706 lacking association with this autoimmune disease. However, the products of the two alleles differ by only two amino acids, at heavy-chain residues 114 (His in HLA-B*2704; Asp in HLA-B*2706) and 116 (Asp in HLA-B*2704; Tyr in HLA-B*2706). Both residues could be involved in contacting amino acids of a bound peptide, suggesting that peptides presented by these subtypes play a role in disease pathogenesis. Two HLA-B*2706-peptide complexes were crystallized using the hanging-drop vapour-diffusion method with PEG as precipitant. Data sets were collected to resolutions of 2.70 A (viral peptide pLMP2, RRRWRRLTV; space group P2(1)2(1)2(1)) and 1.83 A (self-peptide pVIPR, RRKWRRWHL; space group P2(1)). Using HLA-B*2705 complexed with the pGR peptide (RRRWHRWRL) as a search model, unambiguous molecular-replacement solutions were found for both HLA-B*2706 complexes.

(±)-3,4-Di-O-acetyl-,2,5,6-tetra-O-benzyl-myo-inositol, C 38 H 40 O 8 , M r =624.7 («jumping crystal»). Three monoclinic crystal forms. Form I, T= 18 o C, P2 1 /n, a=28.883 (3), b=15.189 (2), c= 15.566 (2) A, β=100.37 (1) o , V=6717 (1) A 3 , Z= 8, R=0.090 for 5788 reflections [F>σ(F)]. Form II, T=18 o C [T=60 o C], P2 1 /c, a=14.269 (2) [14.464 (6)], b=14.862 (2) [14.923 (5)], c= 16.506 (3) A [16.385 (6) A], β=103.21 (1) o [102.73 (3) o ], V=3407 (1) A 3 [3450 (2) A 3 ], Z=4, R=0.086 [0.093] for 3505 [2338] reflections [F>σ(F)]

An analysis is given of C—H bond lengths and their apparent shortening by thermal vibrations in neutron crystal structures. D atoms were excluded. Mean bond lengths and the spread of observed values were compiled for six different chemical connectivities in three temperature ranges (room temperature, 80 ≤ T < 125 and T < 30 K). At low temperatures (T < 30 K), the mean values agree very well with spectroscopic data. Owing to thermal vibrations, the average observed C—H bond length in methyl groups at room temperature (1.057 A) is shortened by 0.03 A compared with that at T < 30 K (1.088 A). The shortening reduces only slowly upon cooling and is still significant at temperatures around 100 K. For the other connectivities, in which the C–H orientations are more rigidly confined, the average shortening at room temperature is 0.01 to 0.02 A, and at T ~ 100 K the mean observed bond lengths have already (or almost) reached the spectroscopic values. The distributions of the observed bond lengths are asymmetric with the maxima at distances greater than the average. The positions of the maxima are less affected by temperature changes than the average values, implying that at room temperature many bond lengths are only slightly affected by thermal motions.

The transcriptional repressor, ω protein, from the Streptococcus pyogenes broad-host-range plasmid pSM19035 was crystallized at pH 7.5 and 8.5 by the vapour-diffusion method using PEG 4000 as precipitant. Two crystal forms were obtained; the first belongs to the tetragonal space group P41212 or P43212 and the second to the hexagonal space group P61 or P65. The crystals are most likely to contain one ω protein in the asymmetric unit, with Vm values of 3.2 and 3.5 Å3 Da−1, respectively. The crystals diffract X-rays to 2.4 and 2.9 Å resolution for the tetragonal and hexagonal systems, respectively.

There has been much recent discussion on the issue of the deposition and release of macromolecular structural data [see editorials in Nature, 391, 617 (1998) and Nature Structural Biology, 5, 83±84 and 165±166 (1998) and the letter from Wlodawer et al. in Science, 16 January 1998]. These editorials and letters rightly point out the critical value that macromolecular structural data now have for biology and medicine and the need to make these data available to the wider scienti®c community. The crystallographic community has in fact been at the forefront of efforts to ensure that these data should be deposited, for the general good. Some ten years ago, extensive consultation by the International Union of Crystallography (IUCr), through its Commission on Biological Macromolecules, led to a set of guidelines that were intended to encourage deposition of structural data (both atomic coordinates and structure-factor amplitudes) with the Protein Data Bank (PDB). This action has been manifestly successful. Prior to that time, few journals made any requirements and deposition was largely voluntary. Now almost all journals require that at least the coordinates are deposited, and there is widespread acceptance that this should be so. The original IUCr guidelines included a provision that release of the data might be delayed for a period after deposition (coordinates for 1 year, structure-factor amplitudes for 4 years). This was essentially a compromise, acknowledging the risky long-term nature of the research and its often `un®nished' nature at the time of initial publication. It is this temporary `hold' provision that has been the focus of current attention. The issue is much wider than this, however, involving deposition practice as well as the question of release. The Commission on Biological Macromolecules, on behalf of the IUCr, has therefore engaged in extensive discussion aimed at reviewing the original guidelines. Below, we summarize some of the issues, and set out proposals for the future.

Three crystal forms of the repressor protein TetR class D in complex with the palindromic 17 bp operator sequence containing T overhangs on both sides were obtained by hanging-drop vapor-diffusion methods using PEG 4000 and PEG monomethylether 5000 as precipitants. Although the crystallization conditions were very similar, up to three different crystal forms were observed in the same drop. The space groups are monoclinic C2, P21 and hexagonal P6122. The asymmetric units of the latter two crystal forms contain one repressor–operator complex. The crystal structures of these forms were solved by molecular replacement using the Tet-repressor molecule of the complex with tetracycline as a search model.

The hydrogen-bonding interactions of the ordered water cluster in the pocket region of vitamin B(12) coenzyme at 15 K are analyzed from published atomic coordinates. Several of these water molecules accept C-H.O hydrogen bonds from the pocket wall, most of which have the function to complete tetrahedral hydrogen-bond coordination. This is the largest water assembly for which the stabilizing function of C-H.O interactions has been described.

CI1H15NS3.AgNO3, Mr=427"3, orthorhombic, Pca2~, a=8.616(1) , b = 10.303(2), c = 16.839 (2) A, V= 1494.8/~3, Z = 4, D,,, = 1.91, Dx = 1.898 M~m-3, F(000) = 856, a(Mo Ka) = 0"71073 A, /z = 1.621 mm-~, T = 293 K. The structure was solved by the heavy-atom method and refined to R = 0-033 and wR = 0.039 for 2094 unique reflections. The Ag + cation sits in the cavity of the macrocycle and coordinates to three S atoms [S(4), S(7) and S(10)] and one N atom, N(1) of the pyridine ring. Ag + also coordinates to an S atom, S(7') ( 0 .5 + x, y , z), from an adjacent ligand. The intermolecular Ag--S interaction [2.498 (1) A] is stronger than the intramolecular Ag--S interactions [2.701 (2), 2-710(2) and 2-856(1)A]. Ag--S distances suggest covalent character of the bonds. The nitrate anion does not interact with the Ag + cation. Introduction. Substitution of N or S for O in cyclic polyethers greatly influences the complexing properties. When S replaces O as a donor atom in the macrocycle the stabilities of Ag ÷ and Hg + complexes increase while those of alkali complexes decrease (Frensdorff, 1971; Izatt, Terry, Hansen, Avondet, Bradshaw, Dalley, Jensen, Christensen & Haymore, 1978). The decrease in stability of alkalimetal complexes of polyether sulfides is probably due to the lower electronegativity, the larger size, and the different bond lengths and bond angles associated with the S atom compared to those of O. These factors tend to weaken the electrostatic attraction for alkali cations. In contrast, covalent bonding plays an important part in the Ag + and Hg + complexes. The crystal structure of the hetero analogue o f ' l 2-crown* Author to whom correspondence should be addressed. 0108-2701/89/121871-04503.00 4', namely 2,5,8-trithia[9](2,6)pyridinophane (1), has been described by Weber, Jones & Sheldrick (1983). This compound was reported to form crystalline complexes with AgNO3, HgC12, HAuC14, PdC12, H2PtC16 and Co(SCN)2 (Weber & Vogtle, 1976). The crystal structure analysis of the title compound was undertaken to study the possible structural changes due to complexing of AgNO3. Experimental. Colourless needle shaped crystals, crystal size 0.1 × 0.1 × 0.4 mm, density measured by flotation, data collection on an automated Stoe fourcircle diffractometer using graphite-monochromated Mo Ka radiation (,~ = 0.71073 A), lattice parameters by least-squares analysis of 0 values of 24 high-angle reflections in the range 20 < 20 < 30°; to/20 scan technique, 20max = 60°; index range: 0 ___ h ___ 12, 0 ___ k ___ 14, 0 _< 1___ 23; three standard reflections monitored every 100 reflections showed no significant variations; 2412 reflections measured of which 2094 reflections with I___ 3o(/) considered observed; empirical absorption correction was applied using O scans (North, Phillips & Mathews, 1968), max. and min. transmission 1.11 and 0.83; Lorentz and polarization corrections applied. Structure was solved by heavy-atom method using SHELX76 (Sheldrick, 1976); Ag and S positions from the Patterson map; rest of the non-H atoms from the subsequent Fourier maps; H-atom positions from difference Fourier maps; anisotropic refinement for non-H atoms and isotropic refinement for H atoms using full-matrix least-squares refinement; the isotropic temperature factors of four of the hydrogens were kept fixed at 0.05 A 2 during refinement. Final R = 0.033 and wR =0.039 for 2094 observed reflections; weighting scheme used, w = 1.O0/[o~(Fo) + 0.012031Fo[2], (shift/ © 1989 International Union of Crystallography 1872 2,5,8-TRITHIA[9](2,6)PYRIDINOPHANE-SILVER NITRATE (1.1) e.s.d.)max in the final cycle of refinement for non-H atoms was 0.039, reflections/parameters refined ratio was 8-87; the minimum and maximum heights in the final difference Fourier map were -0.82 and +0.81 elk -3. Atomic scattering factors for non-H atoms from Cromer & Mann (1968), for H atoms from Stewart, Davidson & Simpson (1965); anomalous-dispersion correction from Cromer & Liberman (1970). Fractional coordinates and equivalent isotropic thermal parameters of the non-H atoms are given in Table 1; bond lengths and angles in Table 2.* The atom labels correspond to Fig. 1.

The structure of the serine endopeptidase proteinase K (279 amino acid residues; 28,790 daltons) has been refined by restrained least-squares methods to a conventional R value of 16.7% employing synchrotron film data of 30,812 reflections greater than 3 sigma in the 5.0 to 1.5 A resolution range. During refinement, the molecular structure was restrained to known stereochemistry, with root-mean-square (r.m.s.) deviation of 0.015 A from ideal bond lengths. The average atomic temperature factor, B, is 11.1 A2 for all atoms. The final model comprises 2020 protein atoms and 174 solvent molecules (which were given unit occupancies). Four corrections to the amino acid sequence were made, which were confirmed later by sequence analysis of the proteinase K gene: a deletion of one glycine in position 80; a change of sequence in position 207-208 and insertions of the dipeptide 210-211 and of residue 270. The r.m.s. deviation in the alpha-C atomic positions between the final refined model and the initial model built on the basis of a 3.3 A mini-map is 1.72 A for 227 out of 266 residues, which were originally traced in the mini-map without sequence information. The positions of the remaining 39 residues deviate by more than 8 A from the original ones and are located in regions where extensive revision of the structural model was necessary.

Na2CO 3. 7H20, FW 232.09, crystallizes in the orthorhombic space group Pbca with a = 14.492 (7), b = 19.490 (5), c = 7.017 (3)/~, V = 1981.9/~3, Z = 8 and D x -- 1.559 Mg m -3. The crystal structure has been determined with 1230 observed X-ray diffractometer data by full-matrix least-squares calculations to R -- 6.7%. The packing arrangement in the crystal unit cell is characterized by two types of Na-water octahedra. These are connected edgewise to produce endless, linear chains [Na(H20)4],~ +. The chains are cross-linked by [NaE(H20)lo ]2+ units. This network extends largely in the bc plane and contains interstices which are filled by carbonate anions. The O atoms of the latter are hydrogen bonded to three or four water molecules.