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R Bau

Publications and source records attributed to R Bau.

16 recordsLinked to original sources
PubMed2006

Recent results on hydrogen and hydration in biology studied by neutron macromolecular crystallography.

Neutron diffraction provides an experimental method of directly locating hydrogen atoms in proteins, a technique complimentary to ultra-high-resolution [1, 2] X-ray diffraction. Three different types of neutron diffractometers for biological macromolecules have been constructed in Japan, France and the United States, and they have been used to determine the crystal structures of proteins up to resolution limits of 1.5-2.5 A. Results relating to hydrogen positions and hydration patterns in proteins have been obtained from these studies. Examples include the geometrical details of hydrogen bonds, H/D exchange in proteins and oligonucleotides, the role of hydrogen atoms in enzymatic activity and thermostability, and the dynamical behavior of hydration structures, all of which have been extracted from these structural results and reviewed. Other techniques, such as the growth of large single crystals, the preparation of fully deuterated proteins, the use of cryogenic techniques, and a data base of hydrogen and hydration in proteins, will be described.

Biology↗
PubMed2001

Crystal structure of ClF(4)(+)SbF(6)(-), normal coordinate analyses of ClF(4)(+), BrF(4)(+), IF(4)(+), SF(4), SeF(4), and TeF(4), and simple method for calculating the effects of fluorine bridging on the structure and vibrational spectra of ions in a strongly interacting ionic solid.

The crystal structure of the 1:1 adduct ClF(5).SbF(5) was determined and contains discrete ClF(4)(+) and SbF(6)(-) ions. The ClF(4)(+) cation has a pseudotrigonal bipyramidal structure with two longer and more ionic axial bonds and two shorter and more covalent equatorial bonds. The third equatorial position is occupied by a sterically active free valence electron pair of chlorine. The coordination about the chlorine atom is completed by two longer fluorine contacts in the equatorial plane, resulting in the formation of infinite zigzag chains of alternating ClF(4)(+) and cis-fluorine bridged SbF(6)(-) ions. Electronic structure calculations were carried out for the isoelectronic series ClF(4)(+), BrF(4)(+), IF(4)(+) and SF(4), SeF(4), TeF(4) at the B3LYP, MP2, and CCSD(T) levels of theory and used to revise the previous vibrational assignments and force fields. The discrepancies between the vibrational spectra observed for ClF(4)(+) in ClF(4)(+)SbF(6)(-) and those calculated for free ClF(4)(+) are largely due to the fluorine bridging that compresses the equatorial F-Cl-F bond angle and increases the barrier toward equatorial-axial fluorine exchange by the Berry mechanism. A computationally simple model, involving ClF(4)(+) and two fluorine-bridged HF molecules at a fixed distance as additional equatorial ligands, was used to simulate the bridging in the infinite chain structure and greatly improved the fit between observed and calculated spectra.

Journal Article↗
PubMed2001

Synthesis and characterization of phosphorescent cyclometalated iridium complexes.

The preparation, photophysics, and solid state structures of octahedral organometallic Ir complexes with several different cyclometalated ligands are reported. IrCl3.nH2O cleanly cyclometalates a number of different compounds (i.e., 2-phenylpyridine, 2-(p-tolyl)pyridine, benzoquinoline, 2-phenylbenzothiazole, 2-(1-naphthyl)benzothiazole, and 2-phenylquinoline), forming the corresponding chloride-bridged dimers, CwedgeN2Ir(mu-Cl)2IrCwedgeN2 (CwedgeNis a cyclometalated ligand) in good yield. These chloride-bridged dimers react with acetyl acetone (acacH) and other bidentate, monoanionic ligands such as picolinic acid (picH) and N-methylsalicylimine (salH), to give monomeric CwedgeN2Ir(LX) complexes (LX = acac, pic, sal). The emission spectra of these complexes are largely governed by the nature of the cyclometalating ligand, leading to lambda(max) values from 510 to 606 nm for the complexes reported here. The strong spin-orbit coupling of iridium mixes the formally forbidden 3MLCT and 3pi-pi* transitions with the allowed 1MLCT, leading to a strong phosphorescence with good quantum efficiencies (0.1-0.4) and room temperature lifetimes in the microsecond regime. The emission spectra of the CwedgeN2Ir(LX) complexes are surprisingly similar to the fac-IrCwedgeN3 complex of the same ligand, even though the structures of the two complexes are markedly different. The crystal structures of two of the CwedgeN2Ir(acac) complexes (i.e., CwedgeN = ppy and tpy) have been determined. Both complexes show cis-C,C', trans-N,N' disposition of the two cyclometalated ligands, similar to the structures reported for other complexes with a "CwedgeN2Ir" fragment. NMR data (1H and 13C) support a similar structure for all of the CwedgeN2Ir(LX) complexes. Close intermolecular contacts in both (ppy)2Ir(acac) and (tpy)2Ir(acac) lead to significantly red shifted emission spectra for crystalline samples of the ppy and tpy complexes relative to their solution spectra.

Journal Article↗
PubMed2000

Hydroxo hydrido complexes of iron and cobalt (Sn-Fe-Sn, Sn-Co-Sn): probing agostic Sn...H-M interactions in solution and in the solid state

Bis(toluene)iron 9 reacts with Lappert's stannylene [Sn[CH(SiMe3)2]2] (4) to form the paramagnetic bis-stannylene complex [[(eta6-toluene)Fe-Sn-[CH(SiMe3)2]2]2] (10). Compound 10 reacts with H2O to form the hydroxo hydrido complex [(eta6-C7H8)(mu-OH)(H)-Fe-[Sn[CH(SiMe3)2]2]2] (12) in high yield; its solid-state structure has been elucidated by X-ray and neutron diffraction analysis. In agreement with the 1H NMR results, 12 contains a hydridic ligand whose exact coordination geometry could be determined by neutron diffraction. The 1H and 119Sn NMR analysis of 12 suggested a multicenter Sn/Sn/H/Fe bonding interaction in solution, based on significantly large values of J(Sn,H,Fe) = 640+/-30 Hz and J(119Sn,119Sn) = 4340+/-100 Hz. In solution, complex 12 exists as two diastereomers in a ratio of about 2:1. Neutron diffraction analysis has characterized 12 as a classical metal hydride complex with very little Sn...H interaction and a typical Fe-H single bond (1.575(8) A). This conclusion is based on the fact that the values of the Sn...H contact distances (2.482(9) and 2.499(9) A) are not consistent with strong Fe-H...Sn interactions. This finding is discussed in relation to other compounds containing M-H...Sn units with and without strong three-center interactions. The neutron diffraction analysis of 12 represents the first determination of a Sn-H atomic distance employing this analytical technique. The cobalt analogues [(eta5-Cp)(mu-OH)(H)Co-[Sn[CH(SiMe3)2]2]2] (15) and [(eta5-Cp)(OD)(D)Co-[Sn[CH-(SiMe3)2]2]2] [D2]15, which are isolobal with 12, were prepared by the reaction of [(eta5-Cp)Co-Sn[CH(SiMe3)2]2] (14) with H2O and D2O, respectively. The magnitude of J(Sn,H) (539 Hz) in 15 is in the same range as that found for 12. The molecular structure of 15 has been determined by X-ray diffraction which reveals it to be isostructural with 12. The coordination geometries of the Co(Fe)-Sn1-O-Sn2 arrangements in 12 and 15 are fully planar within experimental error. Compounds 10 and 15 are rare examples of fully characterized complexes obtained as primary products from water activation reactions.

Journal Article↗
PubMed1997

Five-Coordinate Hydrogen: Neutron Diffraction Analysis of the Hydrido Cluster Complex

Pentacoordinate hydrogen atoms were identified by single-crystal neutron diffraction analysis of [N(CH3)4]3[H2Rh13(CO)24]. The hydrogen atoms are located in square pyramidal cavities of the Rh13 cluster, in positions almost coplanar with the Rh4 faces on the surface of the cluster. They are slightly displaced inward, toward the central rhodium atom of the cluster, with average H-Rh(central) and H-Rh(surface) distances of 1.84(2) and 1.97(2) angstroms, respectively. This result shows that hydrogen, which normally forms only one bond, can be attached to five other atoms simultaneously in a large metal cluster.

Journal Article↗
PubMed1995

Absolute configuration of (+)-[fluoro(hydroxyphenylphosphinyl)methyl]-phosphonic acid, a specific inhibitor of Na(+)-gradient-dependent Na(+)-phosphate cotransport across renal brush border membrane, by X-ray crystallographic analysis of its (-)-quinine salt.

Racemic [fluoro(hydroxyphenylphosphinyl)methyl]phosphonic acid (1) and its individual enantiomers [(+), 98% ee; (-), 67% ee] were previously shown to inhibit Na(+)-gradient-dependent Na(+)-phosphate cotransport across renal brush border membrane, without measurable stereospecificity. Resolution of 1 was effected by fractional recrystallization of its (-)-quinine salts. The more levorotatory, diquinine product 2, corresponding to (+)-1, has now been analyzed by X-ray crystallography and found to be composed of the S enantiomer of 1. This result confirms the absence of stereochemical preference in inhibition of the cotransporter by the enantiomers of 1 and provides the first absolute configuration assignment of an asymmetrical alpha-halomethylene pyrophosphate analogue.

Carrier Proteins↗
PubMed1994

Determination of the absolute configuration of (+)-neopentyl-1-d alcohol by neutron and x-ray diffraction analysis.

The absolute configuration of (+)-neopentyl-1-d alcohol, prepared by the reduction of 2,2-dimethylpropanal-1-d by actively fermenting yeast, has been determined to be S by neutron diffraction. The neutron study was carried out on the phthalate half ester of neopentyl-1-d alcohol, crystallized as its strychnine salt. The absolute configuration of the (-)-strychninium cation was first determined by an x-ray anomalous dispersion study of its iodide salt. The chiral skeleton of strychnine then served as a reference from which the absolute configuration of the -O-CHD-C(CH3)3 group of neopentyl phthalate was determined. Difference Fourier maps calculated from the neutron data showed unambiguously that the -O-CHD-C(CH3)3 groups of both independent molecules in the unit cell had the S configuration. This work proves conclusively that the yeast system reduces aldehydes by delivering hydrogen to the re face of the carbonyl group. Crystallographic details: (-)-strychninium (+)-neopentyl-1-d phthalate, space group P2(1) (monoclinic), a = 18.564(6) A, b = 7.713(2) A, c = 23.361(8) A, beta = 94.18(4) degrees, V = 3336.0(5) A3, Z = 2 (T = 100 K). Final agreement factors are R(F) = 0.073 for 2768 reflections collected at room temperature (x-ray analysis) and R(F) = 0.144 for 960 reflections collected at 100 K (neutron analysis).

Crystallography, X-Ray↗
PubMed1993

Nonsymmetrical bipiperidyls as inhibitors of vesicular acetylcholine storage.

Introduction of a nitrogen atom into the cyclohexane ring of 2-(4-phenylpiperidinyl)cyclohexanol (vesamicol, AH5183) yielded two positional isomers, 5-azavesamicol (5, prezamicol) and 4-azavesamicol (6, trozamicol). As inhibitors of vesicular acetylcholine transport, 5 and 6 were found to be 147 and 85 times less potent than vesamicol. N-Benzoylation of 5 (to yield 9a) increased the potency 3-fold. In contrast, 10a, a compound derived from N-benzoylation of 6, was 50 times more potent than the latter and almost equipotent with vesamicol, thereby suggesting a preference for the 4-azavesamicol series. Although (-)-vesamicol is more potent than its dextrorotary isomer, (+)-10a was found to be 3 times more potent than (-)-10a, suggesting a reversal of the sign of rotation in the azavesamicol series. Reduction of 9a and 10a (to yield the corresponding N-benzyl derivatives 11a and 12a) increased potency 20- and 2-fold, respectively, indicating a preference for a basic nitrogen. The reaction of 5 or 6 with substituted benzyl halides yielded several potent inhibitors of vesicular acetylcholine transport, including N-(p-fluorobenzyl)trozamicol, 12d, which is twice as potent as vesamicol. Thus the introduction of a nitrogen atom into the cyclohexane ring of vesamicol provides opportunities for developing a new class of anticholinergic agents.

Acetylcholine↗
PubMed1988

Reaction products of a new anti-cancer agent, Pt(IV)(cyclohexyldiamine)Cl4, with guanosine and 9-methylguanine: first crystal structures of Pt(cyclohexyldiamine) complexes with a nucleobase and a nucleoside.

Pt(IV)(dach)Cl4 (dach = cyclohexyldiamine) was reacted with guanosine and 9-methylguanine and their reaction products were analyzed by single-crystal x-ray diffraction. In both cases the resulting complexes, [Pt(dach)(guanosine)2]2+ and [Pt(dach)(9-methylguanine)2]2+ respectively, corresponded to an unanticipated reduction of the octahedral Pt(IV) starting material to a square planar Pt(II) species. The nature of the reducing agent is presently unknown.

Antineoplastic Agents↗
PubMed1988

Octahedral complexes of anti-cancer Pt(IV)(cyclohexyldiamine) agents with 9-methylguanine.

The compounds [Pt(IV)(dach)(9-methylguanine)2X2]2+ (X = Cl, OH) have been prepared and structurally characterized. Their existence shows that it is possible to accommodate two purine bases (in a cis configuration) and four other ligands around a Pt(IV) atom. The geometries of these complexes have very different orientations of the guanine rings as compared to their corresponding Pt(II) counterparts.

Antineoplastic Agents↗
PubMed1988

Determination of the absolute configuration of (-)-(2R)-succinic-2-d acid by neutron diffraction study: unambiguous proof of the absolute stereochemistry of the NAD+/NADH interconversion.

The absolute configuration of the CHD group (D = deuterium) in (-)-(2R)-succinic-2-d acid, as prepared from (-)-(2S,3R)-malic-3-d acid, has been shown unambiguously to be R by the technique of single-crystal neutron diffraction. The optically active cation (+)-phenylethylammonium was used as the chiral reference. The structure of [C6H5CH3CHNH3]+[HOOCCH2CHDCOO]- has been studied with x-ray diffraction at room temperature and neutron diffraction at 100 K. Crystal data from the neutron diffraction analysis of the phenylethylammonium salt of the title compound at 100 K: space group P21; a = 8.407(2) A, b = 8.300(4) A, c = 8.614(2) A, beta = 91.20(3) degrees; unit cell volume = 600.9(3) A3, zeta = 2 (numbers in parentheses are the estimated standard deviations). Final agreement factors are R(F2) = 0.0355 and R(wF2) = 0.0457 for 1690 independent neutron reflections and 297 parameters varied. The result confirms the stereochemistry of the malate/succinate transformation, as well as the NAD+/NADH interconversion, and demonstrates the usefulness of the single-crystal neutron diffraction method for determining the absolute configuration of molecules having a chiral monodeuteriomethylene group.

Chemical Phenomena↗
PubMed1983

Absolute configuration of a chiral CHD group via neutron diffraction: confirmation of the absolute stereochemistry of the enzymatic formation of malic acid.

Neutron diffraction has been used to monitor the absolute stereochemistry of an enzymatic reaction. (-)(2S)malic-3-d acid was prepared by the action of fumarase on fumaric acid in D2O. After a large number of cations were screened, it was found that (+)(R) alpha-phenylethylamine forms the large crystals necessary for a neutron diffraction analysis. The subsequent structure determination showed that (+)(R) alpha-phenylethylammonium (-)(2S)malate-3-d has an absolute configuration of R at the CHD site (i.e., the C3 carbon of malate). This result confirms the absolute stereochemistry of fumarate-to-malate transformation as catalyzed by the enzyme fumarase.

Malates↗