Olga Kennard

The development of versions 3 and 4 of the cambridge structural database system

ed, together with any associated supplementary (deposited) data. The CSD itself acts as a computerized depository for large-volume numerical results for some 30 journals. A total of 584 primary sources are now referenced in the CSD, of which 74 are regularly scanned in-house to provide ca. 80% of current input. Remaining references are located via a scan of secondary sources, particularly Chemical Abstracts. Each entry in the CSD relates to a specific crystal structure determination of a specific chemical compound. Each entry is identified by a CSD reference code (REFCODE). This consists of eight characters: the first six are alphabetic and identify the chemical compound (initially assigned as a mnemonic of the compound name, now generated automatically for new compounds), the last two characters are digits which trace the publication history and define (a) whether the paper is a republication by the same authors (perhaps reporting an improved coordinate set) or (b) whether the paper is a redetermination by a different set of authors. The information recorded for each entry may conveniently be categorized according to its dimensionality, as described below and illustrated in Figure 1. 1 D information consists of bibliographic and chemical text strings, together with certain individual numeric items: comBATCH OR VERSION 4 GRAPHICS VERSION 4 GRAPHICS

DNA conformation is determined by economics in the hydration of phosphate groups

Mixed sequence DNA can exist in two right-handed and one left-handed double helical conformations—A, B and Z1–3. Under conditions of high water activity the B conformation prevails. If the water activity is reduced on addition of salt or organic solvents, transformation occurs to A-DNA or, in DNAs with alternating purine-pyrimidine sequences, to the left-handed Z-DNA. In crystal structure analyses of oligonucleotides, the free oxygen atoms of adjacent phosphate groups along the polynucleotide chain in B-DNA are found at least 6.6 Å apart and individually hydrated4 whereas they are as close as 5.3 Å in A-DNA and 4.4 Å in Z-DNA, and bridged by water molecules5–7. We suggest that this more economical hydration in A- and Z-DNA compared with B-DNA is the underlying cause of B → A and B → Z transitions.

The molecular structures of nucleosides and nucleotides: Part 1. The influence of protonation on the geometries of nucleic acid constituents

Abstract A survey of nucleoside and nucleotide crystal structures using structural data retrieved from the Cambridge Crystallographic Database is presented. The molecular geometries of base residues and terminal phosphate groups in various protonation states have been determined and rationalised by simple valence bond theory. Empirical formulae for inferring the protonation states of base residues and phosphate groups from their molecular dimensions have been derived. The formulae were tested on 43 relatively imprecise crystal structures. The protonation states of 47 out of 51 base residues, and 15 out of 19 terminal phosphate groups, were correctly predicted in the tests.

Chiral phosphorothioate analogues of B-DNA: The crystal structure of Rp-d¦Gp(S)CpGp(S)CpGp(S)C¦☆

The compound Rp-d[Gp(S)CpGp(S)CpGp(S)C], an analogue of the deoxyoligomer d(G-C)3, crystallizes in space group P2(1)2(1)2(1) with a = 34.90 A, b = 39.15 A and c = 20.64 A. The structure, which is not isomorphous with any previously determined deoxyoligonucleotide, was refined to an R factor of 14.5% at a resolution of 2.17 A, with 72 solvent molecules located. The two strands of the asymmetric unit form a right-handed double helix, which is a new example of a B-DNA conformation and brings to light an important and overlooked component of flexibility of the double helix. This flexibility is manifest in the alternation of the backbone conformation between two states, defined by the adjacent torsion angles epsilon and zeta, trans . gauche-(BI) and gauche-. trans (BII). BI is characteristic of classical of B-DNA and has an average C(1) to C(1) separation of 4.5 A. The corresponding separation for BII is 5.3 A. Each state is associated with a distinct phosphate orientation where the plane of the PO2 (or POS) group is alternately near horizontal or vertical with respect to the helix axis. The BI and BII conformations are out of phase on the two strands. As a consequence, on one strand purine-pyrimidine stacking is better than pyrimidine-purine, while the converse holds for the other strand. At each base-pair step, good and bad stacking alternate across the helix axis. The pattern of alternation is regular in the context of a fundamental dinucleotide repeat. Re-examination of the B-DNA dodecamer d(C-G-C-G-A-A-T-T-C-G-C-G) shows that the C-G-C-G regions contain the BI and BII conformations, and the associated dual phosphate orientation and asymmetric base stacking. Different mechanisms are used in the two structures to avoid clashes between guanine residues on opposite strands, a combination of lateral slide, tilt and helical twist in the present structure, and base roll, tilt and longitudinal slide (Calladine rules) in the dodecamer. The flexibility of the phosphate orientations demonstrated in this structure is important, since it offers a structural basis for protein-nucleic acid recognition.

Structural Studies of DNA Fragments: The G·T Wobble Base Pair in A, B and Z DNA; The G·A Base Pair in B-DNA

The crystal structures of five double helical DNA fragments containing non-Watson-Crick complementary base pairs are reviewed. They comprise four fragments containing G.T base pairs: two deoxyoctamers d(GGGGCTCC) and d(GGGGTCCC) which crystallise as A type helices; a deoxydodecamer d(CGCGAATTTGCG) which crystallises in the B-DNA conformation; and the deoxyhexamer d(TGCGCG), which crystallises as a Z-DNA helix. In all four duplexes the G and T bases form wobble base pairs, with bases in the major tautomer forms and hydrogen bonds linking N1 of G with O2 of T and O6 of G with N3 of T. The X-ray analyses establish that the G.T wobble base pair can be accommodated in the A, B or Z double helix with minimal distortion of the global conformation. There are, however, changes in base stacking in the neighbourhood of the mismatched bases. The fifth structure, d(CGCGAATTAGCG), contains the purine purine mismatch G.A where G is in the anti and A in the syn conformation. The results represent the first direct structure determinations of base pair mismatches in DNA fragments and are discussed in relation to the fidelity of replication and mismatch recognition.

Parallel and antiparallel (G.GC)2 triple helix fragments in a crystal structure

Nucleic acid triplexes are formed by sequence-specific interactions between single-stranded polynucleotides and the double helix. These triplexes are implicated in genetic recombination in vivo and have application to areas that include genome analysis and antigene therapy. Despite the importance of the triple helix, only limited high-resolution structural information is available. The x-ray crystal structure of the oligonucleotide d(GGCCAATTGG) is described; it was designed to contain the d(G˙GC)2 fragment and thus provide the basic repeat unit of a DNA triple helix. Parameters derived from this crystal structure have made it possible to construct models of both parallel and antiparallel triple helices.

Refined crystal structure of an octanucleotide duplex with G · T mismatched base-pairs☆

Single crystal X-ray diffraction techniques have been used to determine the structure of the DNA octamer d(G-G-G-G-C-T-C-C) at a resolution of 2.25 A. The asymmetric unit consists of two strands coiled about each other to produce an A-type DNA helix. The double helix contains six G . C Watson-Crick base-pairs and two G . T mismatched base-pairs. The mismatches adopt a wobble type structure in which both bases retain their major tautomer forms. The double helix is able to accommodate this G . T pairing with little distortion of the overall helical conformation. Crystals of this octamer melt at a substantially lower temperature than do those of a related octamer also containing two G . T base-pairs. We attribute this destabilization to disruption of the hydration network around the mismatch site combined with changes in intermolecular packing. Full details are given of conformational parameters, base stacking, intermolecular contacts and hydration involving 52 solvent molecules.

Structural Features and Hydration of d(C-G-C-G-A-A-T-T-A-G-C-G); a Double Helix Containing Two G.A Mispairs

Single crystal X-ray diffraction techniques have been used to characterise the molecular structure of the title compound to 2.5A resolution. The structure consists of ten standard Watson-Crick base pairs and two G.A mismatched base pairs. The purine-purine mismatches have guanine in the usual anti orientation with respect to the sugar and adenine in syn orientation. There are two hydrogen bonds formed between the mismatch bases, N-1 and O-6 of guanine with N-7 and N-6 of adenine respectively. The bulky purine-purine mismatches are accommodated with minor perturbation of the sugar-phosphate backbone. There is a slight improvement in base pair overlap at the mismatch sites. Details of the backbone conformation, base stacking interactions and hydration are presented and compared with those of the parent compound d(C-G-C-G-A-A-T-T-C-G-C-G).

Accuracy of Crystal Structure Error Estimates

A statistical analysis of 100 crystal structures retrieved from the Cambridge Structural Database is reported. Each structure has been determined independently by two different research groups. Comparison of the independent results leads to the following conclusions: (a) The e.s.d.s of non-hydrogen-atom positional parameters are almost invariably too small. Typically, they are underestimated by a factor of 1.4-1-45. (b) The extent to which e.s.d.s are underestimated varies significantly from structure to structure and from atom to atom within a structure. (c) Errors in the positional parameters of atoms belonging to the same chemical residue tend to be positively correlated. (d) The e.s.d.s of heavy-atom positions are less reliable than those of light-atom positions. (e) Experimental errors in atomic positional parameters are normally, or approximately normally, distributed. (f) The e.s.d.s of cell parameters are grossly underestimated, by an average factor of about 5 for cell lengths and 2.5 for cell angles. There is marginal evidence that the accuracy of atomic-coordinate e.s.d.s also depends on diffractometer geometry, refinement procedure, whether or not the structure has a centre of symmetry, and the degree of precision attained in the structure determination.

Isolation of centaurepensin, a guaianolide sesquiterpene lactone ester containing two chlorine atoms; determination of structure and absolute configuration by X-ray crystallography

Centaurepensin, an unusual guaianolide sesquiterpene lactone ester containing two chlorine atoms was isolated from Centaurea repens and its structure and absolute configuration were determined by X-ray diffraction methods.