Synthesis and X-ray characterization of the phosphido-carbonyl cluster anions.

The [Co(9)P(CO)(21)](2)(-) anion has been isolated from the products of the reaction between Na[Co(CO)(4)] and PCl(5) in tetrahydrofuran at reflux. The structure of the cluster anion [Co(9)P(CO)(21)](2)(-) in its tetraphenylphosphonium salt has been elucidated by X-ray analysis. The crystals are monoclinic, space group P2(1)/n, a = 12.528(3), b = 14.711(5), c = 19.312(6) A, beta = 93.68(2) degrees, Z = 2. Final R = 0.065 for 2300 unique reflections having I > 3sigma(I). The anion, which is disordered about an inversion center, consists of a monocapped square antiprismatic cluster containing an interstitial phosphide and surrounded by 13 terminal and 8 edge-bridging carbonyl ligands. Average values are: Co-Co 2.685 A, and Co-P 2.256 A. The [Co(10)P(CO)(22)](3)(-) anion has been obtained by condensation of the [Co(9)P(CO)(21)](2)(-) anion with [Co(CO)(4)](-) in tetrahydrofuran at reflux. While the [Co(9)P(CO)(21)](2)(-) anion is stable under CO, the [Co(10)P(CO)(22)](3)(-) anion is decomposed to [Co(9)P(CO)(21)](2)(-) and [Co(CO)(4)](-). The benzyltrimethylammonium salt of the [Co(10)P(CO)(22)](3)(-) anion has been studied by X-ray analysis. It gives triclinic crystals, space group P_1, a = 11.452(3), b = 23.510(6), c = 25.606(4) A, alpha = 112.46(1), beta = 95.79(1), gamma = 73.548(2) degrees, Z = 4. Final R = 0.041 for 8600 unique reflections having I > 3sigma(I). There are two independent trianions in the asymmetric unit, both showing similar geometries, consisting of bicapped square antiprismatic clusters with a central P atom, each bearing 10 terminal and 12 edge-bridging carbonyl ligands, 8 of which, bound to the capping metals, are markedly asymmetric. Average values are: Co-Co 2.678 A, and Co-P 2.262 A. Electrochemistry shows that [Co(9)P(CO)(21)](2)(-) and [Co(10)P(CO)(22)](3)(-) in acetonitrile solution undergo either a one-electron oxidation or a two-electron reduction. This latter process appears as a single step in the case of the dianion and as two separated one-electron steps in the case of the trianion. All the processes are accompanied by slow chemical complications, thus testifying that no stable redox congeners exist for these phosphide clusters.

Mono-, di-, and trimetallic complexes of the nonalternating polycondensed pi-perimeter decacyclene, C36H18: synthesis, structure, and spectroelectrochemistry of

Reaction of the half-sandwich complexes [(eta5-Me4RC5)M(eta2:O-acac)] (M = Co, Ni; R = Me or Et) with di- and trianions of the polycondensed pi-hydrocarbon decacyclene results in formation of the first Co and Ni triple-decker complexes of this hydrocarbon. For the title compound NMR spectra as well as a crystal structure analysis reveal an antarafacial coordination of two (eta5-Me4EtC5)Co fragments at the central six-membered ring and one of the neighboring five-membered rings of decacyclene. The bridging pi-perimeter decacyclene displays a bowl-shaped topology. In the case of Ni, coordination of two (eta5-Me5C5)Ni fragments at the central six-membered ring of decacyclene is observed, based on the results of 1H and 13C NMR studies. This coordination mode is without precedent for nickel organometallic compounds reported so far. The cobalt complex shows a rich spectroelectrochemistry. Results of cyclic voltammetry and coupled ESR experiments reveal a strong interaction of both metal centers in the mixed-valent monocation of [(eta5-Me4EtC5)Co2(mu-eta5:eta4-C36H18)]. This categorizes the title compound into Robin Day class III.

Synthesis, structure, and spectroelectrochemical investigation of novel ternary Co/S(Se)/Sn clusters derived from binary cobalt stannanediyl complexes.

The synthesis and structure of heterobimetallic Co/Sn complexes [(eta5-CpR)Co-Sn[CH(SiMe3)2]2] (CpR = C5Me5 2; C5EtMe4 3) are described. Insertion reactions of sulfur and selenium into the unbridged heteronuclear Co-Sn bonds of 1, 2, and 3 (R= H5 1, Me5 2, EtMe4 3) have been studied. Depending on the stoichiometry of the chalcogen element used, novel ternary Sn-chalcogen-Co clusters (8, 9, 15, and 16) can be synthesized, and their molecular structures, which represent rare examples of crystallographically characterized cases of ternary transition metal/chalcogen/tin complexes, have been determined. Electrochemistry shows that complexes 8 and 9 are able to support reversibly either the removal or addition of one electron. Insertion of a further (Cp)Co-E (E = chalcogen) fragment significantly affects the electron distribution and causes complexes 9 and 16 to undergo two consecutive one-electron oxidations. The EPR spectra of the respective monocations have been recorded. In all cases, the unpaired electron strongly interacts with the cobalt nucleus(i), thus testifying that the main contribution to the relevant HOMO orbitals comes from the cobalt atom(s).

Tendamistat surface accessibility to the TEMPOL paramagnetic probe.

TEMPOL, the soluble spin-label 4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl, has been used to determine the surface characteristics of tendamistat, a small protein with a well-characterised structure both in solution and in the crystal. A good correlation has been found between predicted regions of exposed protein surface and the intensity attenuations induced by the probe on 2D NMR TOCSY cross peaks of tendamistat in the paramagnetic water solution. All the high paramagnetic effects have been interpreted in terms of more efficient competition of TEMPOL with water molecules at some surface positions. The active site of tendamistat coincides with the largest surface patch accessible to the probe. A strong hydration of protein N and C termini can also be suggested by this structural approach, as these locations exhibit reduced paramagnetic perturbations. Provided that the solution structure is known, the use of this paramagnetic probe seems to be well suited to delineate the dynamic behaviour of the protein surface and, more generally, to gain relevant information about the molecular presentation processes.

Histidyl-glycyl containing peptides. Characterization complexation properties of H(L-His-Gly)2-R with hydrogen and alkali metal ions in the gas phase.

The two histidyl-glycyl containing peptides, H(L-His-Gly)2-OH and its methyl ester (H(L-His-Gly)2-OCH3, have been structurally characterized by liquid secondary-ion mass spectrometry. Both high-internal-energy ion fragmentations produced in the source and metastable decompositions occurring in the first field-free region have been studied. The mass spectra show the presence of y-, a- and b-type ions, a1 being the most abundant fragment ion. The metastable decompositions are dominated by the loss of a water molecule and by y-type ions. The interactions of the two peptides with alkali metal ions (Li, Na, K) have been evaluated both by normal mass spectrometry and by tandem mass spectrometry to obtain selected daughter-ion spectra. The occurrence of mono-, bi- and trimetallated species has been detected in the gas phase. While, in the case of the protonated species, y-type ions are predominant, in the presence of an alkali metal ion (Cat), they show lower abundances (Cat = Li) or are absent (Cat = Na, K) both in the mass spectra produced in the source and in metastable decompositions. In most of these cases, a very intense low-internal-energy ion, which is represented by [a3 + Cat - H]+ and which can be produced by interaction of the metal with a deprotonated amide nitrogen, is recorded. This mechanism should be favored by the ¿anchoring¿ effect exerted by the imidazole ring of the histidine which promotes interaction with metals. Other metastable decompositions yield abundant [b3 + Cat - H]+ ions or involve the loss of the side-chain of the histidine. The formation of [b3 + Cat +OH]+ ions, observed only in the case of the free acid peptide, is due to the interaction of the metal ion with the C-terminus carboxyl group. Bi- and trimetallated species have also been detected in the gas phase and characterized.

Interaction of purine nucleotides with inert paramagnetic Cr(III) probes evaluated by NMR relaxation effects. Molecular mechanics calculations on Cr(III) and Co(III) polyphosphate complexes.

The 1H NMR relaxation effects produced by paramagnetic Cr(III) complexes on nucleoside 5'-mono- and -triphosphates in D2O solution at pH' = 3 were measured. The paramagnetic probes were [Cr(III)(H2O)6]3+, [Cr(III)(H2O)3(HATP)], [Cr(III)(H2O)3(HCTP)] and [Cr(III)(H2O)3(UTP)-, while the matrix nucleotides (0.1 M) were H2AMP, HIMP-, and H2ATP2-. For the aromatic base protons, the ratios of the transverse to longitudinal paramagnetic relaxation rates (R2p/R1p) for the [Cr(III)(H2O)6]3+/H2ATP2-, [Cr(III)(H2O)3(HATP)]/H2ATP2-, [Cr(III)(H2O)3(HCTP)]/H2ATP2 and [Cr(III)(H2O)3(UTP)]-/H2ATP2 systems were below 2.33 so the dipolar term predominates. For a given nucleotide, R1p for the purine H(8) signal was larger than for the H(2) signal with the [Cr(III)(H2O)6]3+ probe, while R1p for the H(2) signal was larger with all the other Cr(III) probes. Molecular mechanics computations on the [Cr(III)(H2O)4(HPP)(alpha,beta)], [Cr(III)(NH3)4(HPP)(alpha,beta)], [Co(III)(NH3)3(H2PPP)(alpha,beta,gamma)] and [Co(III)(NH3)4(HPP)(alpha,beta)] complexes gave calculated energy-minimized geometries in good agreement with those reported in crystal structures. The molecular mechanics force constants found were then used to calculate the geometry of the inner sphere [Cr(III)(H2O)6]3+ and [Cr(III)(H2O)3(HATP)(alpha,beta,gamma)] complexes as well as the structures of the outer sphere [Cr(III)(H2O)6]3(+)-(H2AMP) and [Cr(III)(H2O)6]-(HIMP)- species. The gas-phase structure of the [Cr(III)(H2O)3(HATP)(alpha,beta,gamma)] complex shows the existence of a hydrogen bond interaction between a water ligand and the adenine N(7)(O...N = 2.82 A). The structure is also stabilized by intramolecular hydrogen bonds involving the -O(2')H group and the adenine N(3) (O...N = 2.80 A) as well as phosphate oxygen atoms and a water molecule (O...O = 2.47 A). The metal center has an almost regular octahedral coordination geometry. The structures of the two outer-sphere species reveal that the phosphate group interacts strongly with the hexa-aquochromium probe. In both complexes, the nucleotides have a similar "anti" conformation around the N(9)-C(1') glycosidic bond. However, a very important difference characterizes the two structures. For the (HIMP)- complex, strong hydrogen bond interactions exist between one and two water ligands and the inosine N(7) and O(6) atoms, respectively (O...O = 2.63 A; O...N = 2.72, 2.70 A). For the H2AMP complex, the [Cr(III)(H2O)6]3+ cation does not interact with N(7) since it is far from the purine system. Hydrogen bonds occur between water ligands and phosphate oxygens. The Cr-H(8) and Cr-H(2) distances revealed by the energy-minimized geometries for the two outer sphere species were used to calculate the R1p values for the H(8) and H(2) signals for comparison with the observed R1p values: 0.92(c), 1.04(ob) (H(8)) and 0.06(c), 0.35(ob) (H(2)) for H2AMP; and 3.76(c), 4.53(ob) (H(8)) and 0.16(c), 0.77(ob) s-1 (H(2)) for HIMP-.(ABSTRACT TRUNCATED AT 400 WORDS)

Spin label studies of erythrocytes during storage of blood.

The kinetic behavior of the spin label MAL-6 in the interaction with differently aged human erythrocyte membranes was evaluated by monitoring the rate of disappearance of the room temperature ESR signal due to the MAL-6 spin label added to blood after storage at 4 degrees C or after incubation of red cells at 37 degrees C in a protein-free medium. After 35 days of blood storage or 60 h of erythrocytes incubation at 37 degrees C the decrease of the intensity of the MAL-6 ESR spectra in respect to control samples is markedly enhanced and the correspondent kinetic constants significantly increase. Signal decay of MAL-6 is a further proof that during storage of blood under blood bank conditions or during an artificial ageing of erythrocytes at 37 degrees C, profound modifications occur in the human erythrocyte membrane.

Brain death in moderately injured rat brains by in vivo P-31 NMR spectroscopy.

In the course of a systematic in vivo P-31 NMR study of moderately insulted rat brains, some brains died after such injury. Changes in high-energy phosphate metabolism of these brains after injury were rationalised by the aid of a model based on the depletion of an insoluble Pi pool followed by depletion of PCr and ATP pools.

Structural and dynamical characterization of piroxicam by 1H- and 13C-NMR relaxation studies.

Carbon spin-lattice relaxation rates of an anti-inflammatory drug, piroxicam, have been measured. These results have been used in determining the reorientational rates of the proton carbon vectors. An analysis of internal motions within the pyridinyl moiety of piroxicam was carried out. Selective proton-carbon nuclear Overhauser effect (NOE) measurements were made in order to determine the solution structure of piroxicam. The effect of indirect NOE arising from exchangeable protons has been analyzed and considered.