Robert Bau

Closely Approaching the Silylium Ion (R3Si

The crystal structure of the tri-isopropyl silyl species, i-Pr3Si(Br6-CB11H6), where the brominated carborane Br6-CB11 H6– is perhaps the least nucleophilic anion presently known, has revealed the highest degree of silylium cation character (R3Si+) yet observed. The average C—Si—C angle is 117�, only 3� short of the planarity expected of a pure silylium ion(120�). This value compares to 114� recently reported for a toluene-solvated silyl cation, [Et3Si(toluene)]+ by Lambert and co-workers. The greater silylium ion character of i-Pr3Si(Br6-CB11H6) versus [i-Pr3Si(toluene)]+ is also reflected in the larger downfield shift of the silicon-29 nuclear magnetic resonance, 109.8 versus ≤94.0 parts per million. The unusual bonding of toluene to R3Si+ in Lamberts compound is reinterpreted as a significant covalent π interaction.

Structures of transition metal hydrides determined by neutron diffraction

Abstract Neutron diffraction structure determinations of transition metal hydrides have been reviewed. Molecular complexes as well as solid state compounds are surveyed; both structures determined by single-crystal and powder neutron diffraction are included. Tables of average MH distances have been compiled.

Crystal structure of rubredoxin from Pyrococcus furiosus at 0.95 Å resolution, and the structures of N-terminal methionine and formylmethionine variants of Pf Rd. Contributions of N-terminal interactions to thermostability

Abstract The high-resolution crystal structure of the small iron-sulfur protein rubredoxin (Rd) from the hyperthermophilic archeon Pyrococcus furiosus (Pf) is reported in this paper, together with those of its methionine ([_0M]Pf Rd) and formylmethionine (f[_0M]Pf Rd) variants. These studies were conducted to assess the consequences of the presence or absence of a salt bridge between the amino terminal nitrogen of Ala1 and the side chain of Glu14 to the structure and stability of this rubredoxin. The structure of wild-type Pf Rd was solved to a resolution of 0.95 Å and refined by full-matrix least-squares techniques to a crystallographic agreement factor of 12.8% [F>2σ(F) data, 25 617 reflections], while those of the [_0M]Pf and f[_0M]Pf Rd variants were solved at slightly lower resolutions (1.1 Å, R=11.5%, 17 213 reflections; 1.2 Å, R=13.7%, 12 478 reflections, respectively). The quality of the data was such that about half of the hydrogen atoms of the protein were clearly visible. All three structures were ultimately refined using the program SHELXL-93 with anisotropic atomic displacement parameters for all non-hydrogen protein atoms, and calculated hydrogen positions included but not refined. In this paper we also report thermostability data for all three forms of Pf Rd, and show that they follow the sequence wild-type >[_0M]Pf>formyl[_0M]Pf. Comparison of the three Pf Rd structures in the N-terminal region show that the structures of wild-type Pf Rd and f[_0M]Pf are rather similar, while that of [_0M]Pf Rd shows a number of additional hydrogen bonds involving the extra methionine group. While the salt bridge between the Ala1 amino group and the Glu14 carboxylate is not the primary determinant of the thermostability of Pf Rd, alterations to the amino terminus do have a moderate influence on the thermostability of this protein.

The crystal structures and absolute configurations of the anti-tumor complexes Pt(oxalato)(1R,2R-cyclohexanediamine) and Pt(malonato)(1R,2R-cyclohexanediamine)

Abstract The absolute configurations of the anti-tumor complexes [Pt(oxalato)(trans-l-dach)] and [Pt(malonato) (trans-l-dach)] (trans-l-dach = 1R,2R-cyclohexanediamine) have been determined by X-ray anomalous scattering techniques. These complexes are particularly interesting because they show higher anti-tumor activity than the corresponding Pt complexes with other 1,2-cyclohexanediamine(dach) ligands, namely those with trans-d-dach (1S,2S-dach) or cis-dach (1R,2S-dach). The oxalato and malonato ligands are found to bind to the Pt atom in a chelating fashion, through one oxygen atom from each of the two carboxylate groups. Crystallographic details: Pt(oxalato)(trans-l-dach): space group P2 1 (monoclinic); a = 11.230(11) A, b = 9.914(5) A, c = 4.716(3) A, β = 90.86(6)°; R = 4.0% for 1256 reflections. Pt(malonato)(trans-l-dach): space group P2 1 (monoclinic); a = 11.568(5) A, b = 10.007(5) A, c = 5.187(3) A, β = 99.16(4)°; R = 4.8% for 1675 reflections.

Neutron protein crystallography: beyond the folding structure of biological macromolecules

Neutron diffraction provides an experimental method of directly locating H atoms in proteins, a technique complementary to ultra-high-resolution X-ray diffraction. Three different types of neutron diffractometers for biological macromolecules have been constructed in Japan, France and the USA, 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 H-atom positions and hydration patterns in proteins have been obtained from these studies. Examples include the geometrical details of hydrogen bonds, the role of H atoms in enzymatic activity, CH3 configuration, H/D exchange in proteins and oligonucleotides, 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 and a database of hydrogen and hydration in proteins, are described.

The Crystal Structures of Metal Complexes of Nucleic Acids and Their Constituent

(1979). The Crystal Structures of Metal Complexes of Nucleic Acids and Their Constituent. CRC Critical Reviews in Biochemistry: Vol. 6, No. 3, pp. 245-336.

A new weakly coordinating anion: approaching the silylium (silicenium) ion

closo-6,7,8,9,10-Br5-CB9H5– is a new, chemically robust, soluble, weakly coordinating anion; it has been used to develop more cationic character in a trialkyl silicon moiety than has been conclusively demonstrated to date.

Concept of the H(δ+)...H(δ−) interaction. A low-temperature neutron diffraction study of cis-[IrH(OH)(PMe3)4]PF6

The structure of the rare hydridohydroxy complex cis-[IrH(OH)(PMe3)4]PF6 has been analyzed at 20 K by single-crystal neutron diffraction. The results confirm the geometry derived from an earlier X-ray analysis. Perhaps the most significant result concerns the bending of the O–H group towards the hydride ligand, with a smaller-than-usual Ir–O–H angle of 104.4(7)°, suggestive of an attractive interaction between the electron-deficient H atom of the hydroxy group and the electronegative hydride ligand.

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

Abstract.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 Å. 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.

Synthesis and x-ray crystallographic structural determination of the acetylene complex (η5−C5H5)2W2(CO)4(C2H2)

Abstract Reaction of photogenerated (η5−C5H5)2W2(CO)4 with acetylene at 25°C yields a complex of the formula (η5-C5H5)2W2(CO)4(C2H2). The crystal structure of the complex shows it to have a tetrahedrane-like W2C2 core. The C—C bond distance of the C2H2 unit is 1.33 A which is close to that of ethylene, considerably longer than the 1.20 A for acetylenes. The W—W distance is 2.987 A which is ∼0.25 A shorter than the W—W distance in (η5-C5H5)2W2(CO)6 but longer than that expected for (η5-C5H5)2W2(CO)4. By analogy to the parent (η5-C5H5)2M2(CO)6 species, the near-UV absorption in (η5-C5H5)2M2(CO)4(C2H2) is assigned to a σb → σ* transition. Owing to the shorter M—M bond in the C2H2 adducts, the σb → σ* absorption is at higher energy than in the (η5-C5H5)2M2(CO)6 complexes.