Transition metal complexes with 2,4-substituted tetradentate, 2,3,4- and 2,4,7-substituted pentadentate, and 2,3,4,7-substituted hexadentate bispidine ligands (bispidine = 3,7-diazabicyclo[3.3.1]nonane) with two tertiary amine and two, three, or four pyridine donors are relatively stable (10 < log K(CuL) < 18). Interestingly, the two isomeric pentadentate ligands have very different stabilities with a variety of metal ions and, depending on the metal ion, one of the isomers leads to more stable complexes than the hexadentate and the other to less stable complexes than the tetradentate ligand. Another interesting observation is that the complex stabilities of all bispidine ligands reported here do not follow the Iriving-Williams series since the stability constants of the cobalt(II) complexes are up to 4 log units larger than those of the corresponding nickel(II) complexes. All these observations are analyzed on the basis of subtle distortions of the coordination geometries, and these have been related previously to Jahn-Teller-derived distortions for the copper(II) complexes. However, similar but less pronounced structural properties are observed with other metal centers, as shown, e.g., with the experimental structures of the two zinc(II) complexes with the isomeric pentadentate ligands reported here. The structural properties and the related stabilities are also discussed on the basis of force field calculations.
Introduction Historical Outline Structure Chemical Structure Superstructure/Secondary Structure Occurrence Physiological Function Analysis and Detection Chemical Characterization Physical Characterization Biosynthesis Degradation Biotechnological Production General Description of the Process Process Improvement Modeling the Fermentation Process Post‐fermentation Treatment Properties Viscosity Flow Behavior Weak Network Formation Gelation Interaction of Xanthan with Other Macromolecules Applications Food Applications Non‐food Applications Relevant Patents Current Problems and Limitations Outlook and Perspectives Acknowledgements
A mechanism for the oxidation of 3,5-di-tert-butylcatechol (dtbc) with dioxygen to the corresponding quinone (dtbq), catalyzed by bispidine-dicopper complexes (bispidines are various mono- and dinucleating derivatives of 3,7-diazabicyclo[3.3.1]nonane with bis-tertiary-amine-bispyridyl or bis-tertiary-amine-trispyridyl donor sets), is proposed on the basis of (1) the stoichiometry of the reaction as well as the stabilities and structures [X-ray, density functional theory (B3LYP, TZV)] of the bispidine-dicopper(II)-3,4,5,6-tetrachlorcatechol intermediates, (2) formation kinetics and structures (molecular mechanics, MOMEC) of the end-on peroxo-dicopper(II) complexes and (3) kinetics of the stoichiometric (anaerobic) and catalytic (aerobic) copper-complex-assisted oxidation of dtbc. This involves (1) the oxidation of the dicopper(I) complexes with dioxygen to the corresponding end-on peroxo-dicopper(II) complexes, (2) coordination of dtbc as a bridging ligand upon liberation of H(2)O(2) and (3) intramolecular electron transfer to produce dtbq, which is liberated, and the dicopper(I) catalyst. Although the bispidine complexes have reactivities comparable to those of recently published catalysts with macrocyclic ligands, which seem to reproduce the enzyme-catalyzed process in various reaction sequences, a strikingly different oxidation mechanism is derived from the bispidine-dicopper-catalyzed reaction.
The branched pentasaccharide chain of ganglioside GM1 is a prominent cell surface ligand, for example, for cholera toxin or tumor growth-regulatory homodimeric galectins. This activity profile via protein recognition prompted us to examine the binding properties of peptides with this specificity. Our study provides insights into the mechanism of molecular interaction of this thus far unexplored size limit of the protein part. We used three pentadecapeptides in a combined approach of mass spectrometry, NMR spectroscopy and molecular modelling to analyze the ligand binding in solution. Availability of charged and hydrophobic functionalities affected the intramolecular flexibility of the peptides differently. Backfolding led to restrictions in two cases; the flexibility was not reduced significantly by association of the ligand in its energetically privileged conformations. Major contributions to the interaction energy arise from the sialic acid moiety contacting Arg/Lys residues and the N-terminal charge. Considerable involvement of stacking between the monovalent ligand and aromatic rings could not be detected. This carbohydrate binding strategy is similar to how an adenoviral fiber knob targets sialylated glycans. Rational manipulation for an affinity enhancement can now be directed to reduce the flexibility, exploit the potential for stacking and acquire the cross-linking capacity of the natural lectins by peptide attachment to a suitable scaffold.
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