J. Pletcher

Refined structure of rat Clara cell 17 kDa protein at 3.0 Å resolution

The rat Clara cell 17 kDa protein (previously referred to as the rat Clara cell 10 kDa protein) has been reported to inhibit phospholipase A2 and papain, and to also bind progesterone. It has been isolated from rat lung lavage fluid and crystallized in the space group P6(5)22. The structure has been determined to 3.0 A resolution using the molecular replacement method. Uteroglobin, whose amino acid sequence is 55.7% identical, was used as the search model. The structure was then refined using restrained least-squares and simulated annealing methods. The R-factor is 22.5%. The protein is a covalently bound dimer. Two disulfide bonds join the monomers together in an antiparallel manner such that the dimer encloses a large internal hydrophobic cavity. The hydrophobic cavity is large enough to serve as the progesterone binding site, but access to the cavity is limited. Each monomer is composed of four alpha-helices. The main-chain structure of the Clara cell protein closely resembles that of uteroglobin, but the nature of many of the exposed side-chains differ. This is true, particularly in a hypervariable region between residues 23 and 36, and in the H1H4 pocket.

Structure and Absolute Configuration of (R)-Baclofen Monohydrochloride

C~oHI3C1NO+.C1 -, M, = 250.13, ortho- rhombic, P2~2t2 ~, a = 6.373 (1), b = 7.318 (2), c = 25.699 (5) A, 2(Cu) = 1.54180 A, V= 1198.5 A 3, Z = 4, D c- 1.386 g cm -3, ~ = 47.35 cm-k The phase problem was solved by the direct method (MULTAN 78); R(F) -- 0.029 for 1169 reflections. The molecules are linked into infinite chains along the b axis by hydrogen bonding. There is no significant ring stacking. Introduction. Baclofen (y-amino-fl-(p-chlorophenyl)- butyric acid) is a derivative of the inhibitory neuro- transmitter y-aminobutyric acid (GABA) but, unlike GABA, it can cross the blood/brain barrier (Birkmayer, 1972). It has been shown that baclofen reduces excitatory transmitter effects, especially sub- stance P (Pier & Zimmerman, 1973; Polc & Haefely, 1976; Potashner, 1979; Saito, Konishi & Otsuka, 1975). Baclofen has become the drug of choice in the treatment of spasticity of spinal origin due to its antispastic efficacy at doses which do not produce Cl sedation, its low frequency of serious side effects and its 0(1) lack of organ toxicity (Sachais & Logue, 1977). Recent o(2) studies have shown baclofen to be a promising new N drug in the treatment of the paroxysmal pain of c(1) trigeminal neuralgia (Fromm, Terrence, Chattha & c(2) Glass, 1980). c(4) White, tabular crystals were grown from water by c(5) slow evaporation. The space group was uniquely C(6) determined from Weissenberg photographs as P212~21. C(7) The unit-cell parameters were obtained from a least- c(8) squares fitting of the setting angles for 12 reflections c(10) measured on a Picker FACS-1 diffractometer. The H(O) intensity data were collected using graphite-mono- H 1(2) chromated Cu Ka radiation with the 0:20 scan H2(2) technique. Within the 20 range of 5.0 ° to 125.0 °, 68 Hl(4) out of 1 169 reflections were considered unobserved by H2(4) the criterion I < 3o(1). The E map generated by H(6) MULTAN 78 (Main, 1978) revealed positions of two H(7) CI atoms. All C and N atoms were located from a H(10) subsequent Fourier synthesis. The structure was refined by the full-matrix least-squares refinement procedures and all H atoms (with the exception of those of the NH 3 group) were located in a difference Fourier map. A close look at the difference Fourier map around N showed vague positions of three H atoms. The coordinates of these H atoms were fixed from consideration of the hydrogen bonding to CI-. In the final stages of the refinement all atoms except H were refined with anisotropic temperature factors. The final R factor is 0.029 while the weighted R factor,

Thiamine Pyrophosphate Hydrochloride: Stereochemical Aspects from an X-Ray Diffraction Study

The crystal structure of thiamine pyrophosphate has been determined by a three-dimensional x-ray analysis. The conformation of the molecule in the crystalline state is determined by the formal charge distribution within the molecule which exists as a zwitterion with the negative pyrophosphate chain folded back over the positive, ring portion of the molecule. The oxygen atoms in the pyrophosphate group are in the staggered conformation when viewed along the phosphorus-phosphorus axis. Even though the pyrophosphate is present in this compound as the monoionized monoester, the configuration is the same as that present in the inorganic pyrophosphate ion. From a comparison of three different crystal structures containing the thiamine moiety and from studies with atomic models, it seems plausible that the basic molecular conformation observed in this crystal is maintained in the catalytically active molecule. Knowledge of the detailed crystal structure provides new insight into the biochemical mechanism of reactions catalyzed by thiamine pyrophosphate.

EFFECTS OF STRUCTURAL VARIATIONS IN THIAMIN, ITS DERIVATIVES AND ANALOGUES

The structural features of thiamin undoubtedly influence its catalytic properties. Although the exact nature of the correlation is not always evident, various structural features are suggestive of mechanistic importance. One such feature, which has received considerable discussion already,’** is the conformation of the bridged-ring system. From a consideration of steric factors there is only a limited restriction to rotation about both bonds to the methylene-bridge carbon. Even in the C(2) derivative compounds, where rotation is more restricted, a reasonable amount of structural variability seems possible. Contrary to these expectations, the tabulation of torsion angles (TABLE 1) indicates that only three basic conformations are observed. Representative molecular structures illustrating these three conformations are shown in FIGURE 1. That intramolecular forces predominate in determining the conformation of the rings is readily apparent from a comparison with the torsion angles of the C(5) substituent for which the conformation can be drastically altered in order to optimize molecular packing interactions. This does not say that neighboring molecules can not influence the conformation of the thiamin ring system in the catalytic reaction pathway, but it does indicate that there are significant intramolecular forces to be considered. Intramolecular forces also contribute to the conformations that are observed for the specific C(2) substituents. The electrostatic S ( 1)---0(2a,) interaction is the one most frequently observed. The structure of 2-(a-hydroxyethyl)thiamin3 (FIGURE 2a) illustrates a substituent conformation utilizing this interaction. 2-(a-hydroxybenzyl) thiamin4 (HBT) and 2-(a-hydroxybenzyl) oxythiamin’ (FIGURE 2b) also exhibit the intramolecular S---0, but in addition they possess intramolecular ring stacking which also influences the conformation. The position of the pyrimidine with respect to the C(2) substituent is affected by the ring stacking. The conformational difference is most clearly seen by comparing the position of H(2a) in FIGURES 2a and 2b. If the carboxyl group of the C(2) lactyl derivative forms a similar association with the pyrimidine ring, then the conformation of the initial intermediate with pyruvate would be similar to that of HBT. Although the structure of the lactyl derivative has not been determined, the methyl acetyl phosphonate analogue (MAP) of this derivative6 has

Crystallization and preliminary X-ray study of staphylococcal enterotoxin B

Single crystals of staphylococcal enterotoxin B have been grown by vapour diffusion against polyethylene glycol 4000. The space group is P212121 with unit cell dimensions a = 45·4 A, b = 69·0 A and c = 79·0 A. The crystals diffract at least to 2·5 A resolution.

CHARGE DENSITY ANALYSIS OF THIAMIN

Several years ago we decided to study the distributions of the valence electrons and the atomic charges in thiamin. We planned to do this by analyzing a rather extensive collection of X-ray diffraction data we had accumulated over the years from single-crystal studies. Our objective was to understand thiamin chemistry on a more fundamental level. The link between charge density and chemical understanding follows naturally from the tenet that the wave functions describing a molecule determine its properties. Unfortunately wave functions are available only through theoretical ab initio quantum calculations which are expensive, mathematically complex, limited by computer size to relatively small molecules, and often use many overlap approximations. Fortunately the electron density (charge density) that is related to the absolute value of the squared wave function is measurable by X-ray diffraction. More specifically, the probability of finding an electron in a finite volume is given by:

Crystal and molecular structure of the enol form of 1,1′-diphenyl-2,2′-dithiobis(butane-1,3-dione)

The crystal structure of the title compound has been determined by X-ray diffraction methods. Crystals are orthorhombic, space group Pcab, with Z= 8 in a unit cell of dimensions: a= 26.708(1), b= 18.917(7), c= 7.591(1)A. The structure was solved by the heavy-atom method and refined by full-matrix least squares to a final R of 0.072 for 2 218 observed reflections. The butane-1,3-dione groups are planar and the phenyl groups lie at 47.8 and 50.3° to these planes. The distances between the oxygen atoms within the butanedione groups are 2.374 and 2.436 A, indicating strong, intramolecular hydrogen bonds. The compound is in fact better named 3,3′-dihydroxy-3,3′-diphenyl-2,2′-dithiobisbut-2-en-1-one. S(1)–S(2) is 2.078 ± 0.005 A and the dihedral angle, C(5)–S(2)–S(1)–C(17), is 66.4°.

Crystallization and preliminary X-ray study of rat Clara cell 10,000 Mr protein

Single crystals of Clara cell 10,000 Mr protein have been grown by vapour diffusion in the presence of ammonium sulphate. The space group is P4(1)32 or P4(3)32 with unit cell dimension a = 156.9 A. Crystals diffract to about 3.8 A resolution.

Crystal and molecular structure of bis(8-amino-2-methylquinoline)nitratonickel(II) nitrate

An X-ray diffraction study of the title compound has been carried out and the structure solved by direct methods. Crystals are monoclinic, space group P2l/c, Z= 4, a= 15.905(3), b= 8.040(5), c= 10.736(3)A, β= 117.78(4)°. The structure was refined by block-diagonal least-squares to R and R′ both 4.2%. The nitrato-group in the cation acts as a bidentate ligand. The presence of the methyl group on the organic ligand produces various distortions of bond lengths and angles, and in the geometrical arrangement about the metal.