Shape-persistent octanuclear zinc salen clusters: synthesis, characterization, and catalysis

We describe a selective and template-controlled synthesis of a series of Zn(8) metal complexes based on a bis-nucleating salen ligand scaffold. Our results, a combination of X-ray analysis and solution studies, show that discrete, shape-persistent metal clusters can be prepared in high yield. Their activity in organic carbonate catalysis is a function of the metal-connecting fragment present in the exterior of the cluster complex. The high stability of the clusters has been confirmed by (1)H, (13)C (DEPTQ) and DOSY NMR, gel permeation chromatography, high-performance liquid chromatography, and mass spectrometry.     

Stabilization and Binding of [V4O12]4− and Unprecedented [V20O54(NO3)]n− to Lysozyme upon Loss of Ligands and Oxidation of the Potential Drug VIVO(acetylacetonato)2

High-resolution crystal structures of lysozyme in the presence of the potential drug V^IVO(acetylacetonato)₂ under two different experimental conditions have been solved. The crystallographic study reveals the loss of the ligands, the oxidation of V^IV to V^V and the subsequent formation of adducts of the protein with two different polyoxidovanadates: [V₄O₁₂]⁴⁻, which interacts with lysozyme non-covalently, and the unprecedented [V₂₀O₅₄(NO₃)]ⁿ⁻, which is covalenty bound to the side chain of an aspartate residue of symmetry related molecules.     

Fast stereoselective reactions in electrosprayed Co(II)/neurotransmitter nanodroplets

Collision induced dissociation (CID) of the m/z 479 ion, formed by ESI of Co(NO3)2-CH3OH solutions with either pure (1S,2S)-(+)-N-methylpseudoephedrine or its mixtures with (1S,2R)-(+)- or (1R,2S)-(-)-ephedrine, provides compelling evidence for fast, stereoselective reactions in Co(II)/neurotrasmitter(s) aggregates during solvent evaporation of the ESI droplets.     

Analytical and semipreparative high performance liquid chromatography separation of stereoisomers of novel 3,4-dihydropyrimidin-4(3H)-one derivatives on the immobilised amylose-based Chiralpak IA chiral stationary phase

Direct HPLC separation of stereoisomers of three novel 5-methyl-2-(alkylthio)-6-(2,6-difluorophenylalkyl)-3,4-dihydropyrimidin-4(3H)-ones endowed with antiviral and potential antiproliferative and morphological differentiation activity against melanoma cells was performed by using the new immobilised amylose-based Chiralpak IA chiral stationary phase. Stereoselective conditions were achieved using normal phase eluents containing "non-standard" solvents such as ethyl acetate, methyl tertbutyl ether, or dichloromethane. In order to study the chiroptical properties of single stereoisomers, mg-scale separations were performed on analytical and semipreparative size Chiralpak IA columns in combination with ethyl acetate-based eluents.     

Nitroxyl peptides as catalysts of enantioselective oxidations

The achiral, nitroxyl-containing alpha-amino acid TOAC (TOAC = 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid), in combination with the chiral alpha-amino acid C(alpha)-methyl valine [(alphaMe)Val], was used to prepare short peptides (from di- to hexa-) that induced the enantioselective oxidation of racemic 1-phenylethanol to acetophenone. The best catalyst was an N(alpha)-acylated dipeptide alkylamide with the -TOAC-(alphaMe)Val- sequence folded in a stable, intramolecularly hydrogen-bonded beta-turn conformation with large, lipophilic (hydrophobic) N- and C-terminal blocking groups. We rationalized our findings by proposing models for the diastereomeric intermediates between (R)-[and (S)]-1-phenylethanol and the catalyst Fmoc-TOAC-L-(alphaMe)Val-NHiPr, based on the X-ray diffraction structure of the latter.     

Insights into the free-energy dependence of intramolecular dissociative electron transfers

To study the relationship between rate and driving force of intramolecular dissociative electron transfers, a series of donor-spacer-acceptor (D-Sp-A) systems has been devised and synthesized. cis-1,4-Cyclohexanedyil and a perester functional group were kept constant as the spacer and acceptor, respectively. By changing the aryl substituents of the phthalimide moiety, which served as the donor, the driving force could be varied by 0.74 eV. X-ray diffraction crystallography and ab initio conformational calculations pointed to D-Sp-A molecules having the cis-(cyclohexane) equatorial(phthalimido)-axial(perester) conformation and the same D/A orientation. The intramolecular dissociative electron-transfer process was studied by electrochemical means in N,N-dimethylformamide, in comparison with thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular process consists of the electron transfer from the electrochemically generated phthalimide-moiety radical anion to the peroxide functional group. The electrochemical analysis provided clear evidence of a concerted dissociative electron-transfer mechanism, leading to the cleavage of the O-O bond. Support for this mechanism was obtained by ab initio MO calculations, which provided information about the LUMO of the acceptor and the SOMO of the donor. The intramolecular rate constants were determined and compared with the corresponding intermolecular values, the latter data being obtained by using the model molecules. As long as the effective location of the centroid of the donor SOMO does not vary significantly by changing the aryl substituent(s), the intramolecular dissociative electron transfer obeys the same main rules already highlighted for the corresponding intermolecular process. On the other hand, introduction of a nitro group drags the SOMO away from the acceptor, and consequently, the intramolecular rate drops by as much as 1.6 orders of magnitude from the expected value. Therefore, a larger solvent reorganization than for intermolecular electron transfers and the effective D/A distance and thus electronic coupling must be taken into account for quantitative predictions of intramolecular rates.     

Anomalous distance dependence of electron transfer across peptide bridges

The first investigation on the distance dependence of a dissociative electron transfer process across peptide bridges is reported. This study was carried out by using a series of donor-peptide-acceptor systems in which the donor is a phthalimido moiety, the peptide bridges are provided by alpha-aminoisobutyric acid (Aib) homooligomers, and the acceptor is a peroxide functional group. The intramolecular electron transfer from the electrogenerated phthalimido radical anion to the peroxide was studied in comparison with the thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular rate constants were determined in N,N-dimethylformamide by taking into account the corresponding intermolecular values. The experimental results point to an unusual non-exponential dependence of the intramolecular electron transfer rate on the number of bridge units. The same trend could be verified also by taking into account the actual donor-acceptor edge-to-edge distance. The peculiar distance dependence that was observed for the intramolecular electron transfer rate is attributed to the mediating effect of the intramolecular C=O...H-N hydrogen bonds.