Synthesis and structural characterization of novel tetranuclear organotitanoxane derivatives

Synthesis of the novel titanoxane compounds, [(TiCl)(TiOH){(Ti)[μ-(η(5)-C(5)Me(4)SiMe(2)O-κO)](2)(μ-O)}(2)(μ-O)] (4) and [{Ti[μ-(η(5)-C(5)Me(4)SiMe(2)O-κO)](μ-O)}(4)] (5) by controlled reaction of the dinuclear titanium oxo complex [{Ti{μ-(η(5)-C(5)Me(4)SiMe(2)O-κO)}Cl](2)(μ-O)] (1) with 2 equiv of LiOH is reported. Complex 4 is innovative and remarkable. It is one of the rare known examples of tetranuclear stable terminal hydroxo titanium complexes, with an open-chained structure, which coincides with the transient metal monohydroxo proposed in the stepwise pathway employed to justify the formation of the hexanuclear complex [{Ti[μ-(η(5)-C(5)Me(4)SiMe(2)O-κO)](μ-O)}(6)] (3) from 1. (1)H DOSY experiments were used to characterize complex 4. In addition, the structures of compound 5 and of precursor 1 were determined by single-crystal X-ray diffraction studies.     

Studies on the active species in olefin polymerisation generated from phenoxo-amido titanium “chiral-at-metal” compounds

Treatment of RHN-CH₂-(3,5-tBu₂C₆H₂-2-OH) (R = C₆H₅ 1a, p-MeC₆H₄ 1b, Cy 1c; Cy = cyclohexyl) with 1 equiv of TiCp1Cl₃ (Cp1 = η⁵-C₅Me₅) in the presence of 2.5 equiv of NEt₃ in pentane or hexane at room temperature gives the monocyclopentadienyl phenoxo-amido monochloro complexes [TiCp1{RN-CH₂-(3,5-tBu₂C₆H₂-2-O)}Cl] (R = C₆H₅ 2, p-MeC₆H₄ 4, Cy 5). In a more polar solvent the phenoxo-amino complex [TiCp1{(C₆H₅)(H)N–CH₂-(3,5-tBu₂C₆H₂-2-O)}Cl₂] (3) is obtained from the reaction with 1a. The reaction of TiCp1Cl₃ with tBu(H)N–CH₂-(3,5-tBu₂C₆H₂-2-OH) (1d) affords the complex [TiCp1{tBu(H)N–CH₂-(3,5-tBu₂C₆H₂-2-O)}Cl₂] (6) in which no coordination of the amino group to the metal centre is observed as a consequence of the high steric requirements of the amino substituent in the phenol-amine. All the reported compounds were characterised by the usual analytical and spectroscopic methods and the molecular structures of 2 and 5 were determined by X-ray diffraction analysis from suitable single crystals.Studies of catalytic activity for ethylene or propylene polymerisation using boron or aluminium reagents as cocatalysts were performed under different conditions. In general the trends observed for the phenoxo-amido precatalysts with the aluminium reagent as cocatalyst in the α-olefin polymerisation reactions might suggest a catalyst decomposition process through ligand abstraction by sMAO. The activity found for ethylene or propylene polymerisation when B(C₆F₅)₃ or [CPh₃][B(C₆F₅)₄] are used as cocatalysts is related to the strength of the cation-anion interactions.     

3D comparative structural study of 6-hydroxy-4-methyl-5,7-dinitrocoumarin using experimental and theoretical approaches

The compound 6-hydroxy-4-methyl-5,7-dinitrocoumarin (2) was synthesized with high yield by the nitration of compound 1. Its molecular structure was determined using X-ray diffractometry and compared with the structure obtained from semiempirical, HF, and B3LYP methods. B3LYP calculations offer the best conformity with X-ray diffractometry for bond lengths and bond angles, whereas AM1 results are significantly close to them and provide the best superimposition with the entire 3D crystal structure.     

Effect of solvation on the structure conformation of human serum albumin in aqueous–alcohol mixed solvents

In the present paper, we analyze the effect of ethanol in the structure conformation of human serum albumin (HSA) by far- and near circular dichroism, UV–Vis spectroscopy and density and ultrasound velocity measurements. Both circular dichroism and absorbance data show changes in both tertiary and secondary structures at ethanol concentrations below 30% (v/v), which can be indicative of the presence of an equilibrium intermediate state. At higher ethanol concentrations in the mixed solvent (>30% (v/v)), a change from a rich--helix to a β-sheet and unordered secondary structure is observed. Moreover, it seems that certain protein aggregation starts to occur. From ethanol concentrations higher than 55% (v/v), a certain redissolution of these aggregates takes place, which seems to be accompanied by an increase in -helix content. Trends found by volume and compressibility determinations seem to be in accordance. Both volume and compressibility increase with alcohol concentration up to 50% (v/v) but in two well-defined steps indicating a different nature of these two changes, the first of them with a variation close to that shown by a molten globule intermediate state. Moreover, at ethanol concentrations higher than 55% (v/v), a decrease in both quantities occurs confirming the breakdown of protein aggregates.     

Determining a 'safe' high-mass limit in matrix-assisted laser desorption/ionisation time-of-flight mass spectra of coal derived materials with reference to instrument noise

Three methods for determining a 'safe' estimate for high-mass limits of MALDI spectra of coal derived liquids were explored, using a sample of coal-tar pitch and its pyridine-insoluble fraction. Co-addition of increasing numbers of single-shot spectra (10, 30, 50 and 100 pulses) showed visually observable reductions in noise levels, consistent with robust and statistically meaningful signals. Three separate types of post-acquisition calculation were used to identify high-mass limits of the spectra. (i) A literature method indicated high-mass limits similar to those observed visually-as a shift from baseline at the highest masses, nearly 350 000 u for the coal tar pitch and about 390 000 u for its pyridine insoluble fraction. (ii) Comparing instrument signal with pre-selected multiples of the standard deviation, upper mass estimates of between 40-60 000 u for the coal-tar pitch and about 95 000 u for its pyridine-insoluble fraction were found. (iii) Calculation of the slope was used to identify 'lift-off' of the spectrum from baseline. The angle between the smoothed spectrum and the baseline was matched to a pre-selected value (e.g. 0.5 degrees and 1 degrees ). However, the arbitrary specification of the key parameter did not establish this last method on a firm basis. The choice of a criterion for estimating high-mass limits of MALDI spectra remains a semi-quantitative procedure; a reasonably conservative high-mass limit may be estimated by comparison of signal with five times the standard deviation. However, evaluation of size exclusion chromatograms of the present samples using polystyrene standards suggests that molecular mass distributions of pitch samples arrived at by MALDI mass spectrometry are, at least partly, determined by the limitations of available instruments. Copyright 1999 John Wiley & Sons, Ltd.     

Analysis of the interactions between human serum albumin/amphiphilic penicillin in different aqueous media: an isothermal titration calorimetry and dynamic light scattering study

The complexation process of the amphiphilic penicillins sodium cloxacillin and sodium dicloxacillin with the protein human serum albumin (HSA) in aqueous buffered solutions of pH 4.5 and 7.4 at 25 °C was investigated through isothermal titration calorimetry (ITC) and dynamic light scattering. ITC experiments were carried out in the very dilute regime and showed that although hydrophobic interactions are the leading forces for complexation, electrostatic interactions also play an important role. The possibility of the formation of hydrogen bonds is also deduced from experimental data. The thermodynamic quantities of the binding mechanism, i.e, the enthalpy, , entropy, , Gibbs energy, , binding constant, and the number of binding sites, n_i, were obtained. The binding was saturable and is characterised by Langmuir adsorption isotherms. From ITC data and following a theoretical model, the number of bound and free penicillin molecules was calculated. From Scatchard plots, and n_i were obtained and compared with those from ITC data. The interaction potential between the HSA–penicillin complexes and their stability were determined at pH 7.4 from the dependence of the diffusion coefficients on protein concentration by application of the DLVO colloidal stability theory. The results indicate decreasing stability of the colloidal dispersion of the drug–protein complexes with increase in the concentration of added drug.     

Modification of the thermal unfolding pathways of myoglobin upon drug interaction in different aqueous media

In this work, we have analyzed the influence of two structurally related phenothiazine drugs, promazine and triflupromazine hydrochlorides, when bound to myoglobin, a model protein, and how the drug concentration and solution conditions may affect the denaturation process of this protein. In this manner, we derive the thermodynamic quantities of the unfolding process by using a spectroscopic technique such as UV-vis spectroscopy at different drugs concentrations and at pH 2.5, 5.5, and 9.0. To do this, a thermodynamic model was used which included experimental data corresponding to the pre- and post-transition into the observable transition. It has been found that both drugs play a destabilizing role for the protein, at least at low concentrations. In addition, at acidic pH and higher drug concentrations, a stabilizing effect can be observed, which may be related to the formation of some type of protein refolding, subsequent aggregation, or both. The reason for this behavior has been suggested to be the different protein conformations at acidic pH, the increase of solvent-exposed hydrophobic and hydrophilic residues after denaturation and/or binding, and the different strength of drug-protein interactions when changing the solution conditions. For this reason, thermodynamic quantities such as Gibbs energies, DeltaG, and entropies of unfolding, DeltaS(m), increase as the solution pH increases provided that additional solvent-exposed hydrophobic residues are present, which were previously buried at room temperature. Moreover, the larger binding affinity at pH 9.0 due to enhanced electrostatic interactions between protein and drug molecules (drug and protein differ in their net electrical charge) additionally collaborates to this residue exposition to solvent as a consequence of the alteration of protein conformation as due to drug binding. Comparison of thermodynamic data between promazine and triflupromazine hydrochlorides also shows that drug-protein affinity and hydrophobicity also affect the thermodynamic denaturation parameters.