Enzymatische Acyl- und Phosphoryltransferreaktionen unter Beteiligung von zwei Metallionen

Lanthanide hydroxide cubane clusters anchoring ferrocenes: model compounds for fixation of organometallic fragments on a lanthanide oxide surface

The reaction of the lanthanide trichloride hexahydrates [LnCl(3).6H(2)O] (Ln = Yb, Lu) with two equivalents of benzoylferrocenoylmethane resulted in the tetranuclear lanthanide hydroxo clusters [Ln(4)(mu(3)-OH)(4)(FcacacPh)(8)] (Ln = Yb (1), Lu (2); FcacacPh = benzoylferrocenoylmethanide). Compounds 1 and 2 are made up of a distorted tetranuclear lanthanide Ln(4)O(4) cubane core consisting of four mu(3)-oxygen atoms while the eight FcacacPh ligands build up the peripheral part of the cluster. These compounds contain the maximum number of ferrocene units anchored to any molecular metal-heteroatom framework reported so far and for which the X-ray structures are known.

Effect of Zn⋅⋅⋅Zn Separation on the Hydrolytic Activity of Model Dizinc Phosphodiesterases

From the study of highly preorganized model systems, experimental support has been obtained for a possible functional role of the Zn-(H)O...HO(H)-Zn motif in oligozinc hydrolases. The mechanistic relevance of such an array, which may be described as a hydrated form of a pseudo-terminal Zn-bound hydroxide, has recently been supported by DFT calculations on various metallohydrolase active sites. In the present targeted approach, the Zn...Zn distance in two related dizinc complexes has been controlled through the use of multifunctional pyrazolate-based ligand scaffolds, giving either a tightly bridged Zn-O(H)-Zn or a more loosely bridged Zn-(H)O...HO(H)-Zn species in the solid state. Zn-bound water has been found to exhibit comparable acidity irrespective of whether the resulting hydroxide is supported by strong hydrogen-bonding in the O(2)H(3) moiety or is in a bridging position between two zinc ions, indicating that water does not necessarily have to adopt a bridging position in order for its pK(a) to be sufficiently lowered so as to provide a Zn-bound hydroxide at physiological pH. Comparative reactivity studies on the cleavage of bis(4-nitrophenyl)phosphate (BNPP) mediated by the two dizinc complexes have revealed that the system with the larger Zn...Zn separation is hydrolytically more potent, both in the hydrolysis and the transesterification of BNPP. The extent of active site inhibition by the reaction products has also been found to be governed by the Zn...Zn distance, since phosphate diester coordination in a bridging mode within the clamp of two zinc ions is only favored for Zn...Zn distances well above 4 A. Different binding affinities are rationalized in terms of the structural characteristics of the product-inhibited complexes for the two different ligand scaffolds, with dimethyl phosphate found as a bridging ligand within the bimetallic pocket.

Polyoxometalate-Supported Y- and YbIII-Hydroxo/Oxo Clusters from Carbonate-Assisted Hydrolysis

Carbonate-assisted hydrolysis of Y or Yb(III) ions in the presence of the trivacant Wells-Dawson polyoxoanion, alpha-[P(2)W(15)O(56)](12-), produced two polyoxometalate-supported Y- or Yb(III)-hydroxo/oxo clusters, which have been characterized by single-crystal X-ray structure determination. The structure of the Y complex consists of a distorted Y(4)(OH)(4) cubane cluster encapsulated by two lacunary alpha-[P(2)W(15)O(56)](12-) units, while the Yb cluster features a hexametallic core centered around a mu(6)-oxo atom with each Yb(3)(III) triangular face capped by an oxo or a hydroxo group. Magnetization measurements of the ytterbium(III) derivative suggested that intermolecular dipolar exchange is present at low temperatures (below 15 K). Despite its absence in the structures themselves, control experiments show that carbonate not only functions in the hydrolysis, it also influences the structure of the complexes by complexation to yttrium and the f-block elements.

Correlation of Structure and Function in Oligonuclear Zinc(II) Model Phosphatases

A series of pyrazolate-based dizinc(II) complexes has been synthesized and investigated as functional models for phosphoesterases, focusing on correlations between hydrolytic activity and molecular parameters of the bimetallic core. The Zn...Zn distance, the (bridging or nonbridging) position of the Zn-bound hydroxide nucleophile, and individual metal ion coordination numbers are controlled by the topology of the compartmental ligand scaffold. Species distributions of the various dizinc complexes in solution have been determined potentiometrically, and structures in the solid state have been elucidated by X-ray crystallography. The hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) promoted by the dinuclear phosphoesterase model complexes has been investigated in DMSO/buffered water (1:1) at 50 degrees C as a function of complex concentration, substrate concentration, and pH. Coordination of the phosphodiester has been followed by ESI mass spectrometry, and bidentate binding could be verified crystallographically in two cases. Drastic differences in hydrolytic activity are observed and can be attributed to molecular properties. A significant decrease of the pK(a) of zinc-bound water is observed if the resulting hydroxide is involved in a strongly hydrogen-bonded intramolecular O(2)H(3) bridge, which can be even more pronounced than for a bridging hydroxide. Irrespective of the pK(a) of the Zn-bound water, a hydroxide in a bridging position evidently is a relatively poor nucleophile, while a nonbridging hydroxide position is more favorable for hydrolytic activity. Additionally, the metal array has to provide a sufficient number of coordination sites for activating both the substrate and the nucleophile, where phosphate diesters such as BNPP preferentially bind in a bidentate fashion, requiring a third site for water binding. Product inhibition of the active site by the liberated (p-nitrophenyl)phosphate is observed, and the product-inhibited complex could be characterized crystallographically. In that complex, the phosphate monoester is found to cap a rectangular array of four zinc ions composed of two bimetallic entities.

Metal-mediated reactions modeled after nature

The Collaborative Research Center (CRC) 436 'Metal-Mediated Reactions Modeled after Nature' was founded for the express purpose of analyzing the catalytic principles of metallo-enzymes in order to construct efficient catalysts on a chemical basis. The structure of the active center and neighboring chemical environment in enzymes serves as a focal point for developing reactivity models for the chemical redesign of catalysts. Instead of simply copying enzyme construction, we strive to achieve new chemical intuition based on the results of long-lasting natural evolution. We hope for success, since nature uses a limited set of building blocks, whereas we can apply the full repertoire of chemistry. Key substrates in this approach are small molecules, such as CO2, O2 NO3- and N2. Nature complexes these substrates, activates them and performs chemical transformations--all within the active center of a metalloenzyme. In this article, we report on some aspects and first results of the Collaborative Research Center (CRC) 436, such as nitrate reductase, sphingolipid desaturase, carbonic anhydrase, leucine aminopeptidase and dopamine beta-monooxygenase.

Tuning the activity of catechol oxidase model complexes by geometric changes of the dicopper core

Dicopper(II) complexes of a series of different pyrazolate-based dinucleating ligands [L1](-)-[L4](-) have been synthesized and characterized structurally and spectroscopically. A major difference between the four complexes is the individual metal-metal separation that is enforced by the chelating side arms of the pyrazolate ligand scaffold: it varies from 3.45 A in 2 x (BF4)4 to 4.53 A in 4 x (ClO4)2. All complexes have been evaluated as model systems for the catechol oxidase enzyme by using 3,5-di-tert-butylcatechole (DTBC) as the test substrate. They were shown to exhibit very different catecholase activities ranging from very efficient to poor catalysts (k(obs) between 2430+/-202 and 22.8+/-1.2 h(-1)), with an order of decreasing activity 2 x (ClO4)4 > 1 x (ClO4)2 > 3 x (ClO4)2 >> 4 x (ClO4)2. A correlation of the catecholase activities with the variation in Cu...Cu distances, as well as other effects resulting from the distinct redox potentials, neighboring groups, and the individual coordination spheres are discussed. Saturation behavior for the rate dependence on substrate concentration was observed in only two cases, that is, for the most active 2 x (ClO4)4 and for the least active 4 x (ClO4)2, whereas a catalytic rate that is almost independent of substrate concentration (within the range studied) was observed for 1 x (ClO4)2 and 3 x (ClO4)2. H2O2 was detected as the product of O2 reduction in the catecholase reaction of the three most active systems. The structures of the adducts of "L3Cu2" and "L4Cu2" with a substrate analogue (tetrachlorocatecholate, TCC) suggest a bidentate substrate coordination to only one of the copper ions for those catalysts that feature short ligand side arms and correspondingly exhibit larger metal-metal separations; this possibly contributes to the lower activity of these systems. TCC binding is supported by several H-bonding interactions to water molecules at the adjacent copper or to ligand-side-arm N-donors; this emphasizes the importance of functional groups in proximity to the bimetallic active site.

Dinuclear titanium(IV) complexes from amino acid bridged dicatechol ligands: formation, structure, and conformational analysis

Amino acid bridged dicatechol ligands 3a-e-H4 form dinuclear double-stranded coordination compounds [(3a-e)2Ti2(OCH3)2]2- with titanium(IV) ions. Due to the directionality of the ligands, the chirality of the strand, and the chiral complex units, up to seven isomers, I-VII, can be obtained for the double-stranded complexes of ligands 3a-e-H4. The composition of the mixture of isomeric compounds in solution is strongly dependent on the conditions of complex formation. Under thermodynamic control, only a few isomers are obtained, one of which is the major component of the mixture. X-ray structure analyses were performed for K2[(3b)2Ti2(OH)2] and K2[(3d)2Ti2(OH)2] (type I), and for the meso complex Na2[(3e)(3e')Ti2(OCH3)2]. A conformational analysis that uses Ramachandrans method revealed that the conformation of the amino acids in the ligand strands can be compared with those found for amino acids in helical peptide structures. The most favored isomer of [(3)2Ti2(OCH3)2]2- appears to be of type I, with the catecholamide unit located at the N terminus of the ligand strand that binds to a lambda-configurated titanium(IV) complex unit and the dihydroxybenzyl group at the C terminus that coordinates to a delta-configurated titanium(IV) complex unit. The lambda configuration at the N terminus induces the conformation of a right-handed helix in the amino acid residue, while the delta configuration induces the less favored left-handed helix.

Hemilability of Hybrid Ligands and the Coordination Chemistry of Oxazoline-Based Systems

Ligand design is becoming an increasingly important part of the synthetic activity in chemistry. This is of course because of the subtle control that ligands exert on the metal center to which they are coordinated. Ligands which contain significantly different chemical functionalities, such as hard and soft donors, are often called hybrid ligands and find increasing use in molecular chemistry. Although the interplay between electronic and steric properties has long been recognized as essential in determining the chemical or physical properties of a complex, predictions remain very difficult, not only because of the considerable diversity encountered within the Periodic Table-different metal centers will behave differently towards the same ligand and different ligands can completely modify the chemistry of a given metal-but also because of the small energy differences involved. New systems may-even through serendipity-allow the emergence of useful concepts that can gain general acceptance and help design molecular structures orientated towards a given property. The concept of ligand hemilability, which finds numerous illustrations with hybrid ligands, has gained increased acceptance and been found to be very useful in explaining the properties of metal complexes and in designing new systems for molecular activation, homogeneous catalysis, functional materials, or small-molecule sensing. In the field of homogeneous enantioselective catalysis, in which steric and/or electronic control of a metal-mediated process must occur in such a way that one stereoisomer is preferentially formed, ligands containing one or more chiral oxazoline units have been found to be very valuable for a wide range of metal-catalyzed reactions. The incorporation of oxazoline moieties in multifunctional ligands of increasing complexity makes such ligands good candidates to display hemilabile properties, which until recently, had not been documented in oxazoline chemistry. Herein, we briefly recall the definition and scope of hemilabile ligands, present the main classes of ligands containing one or more oxazoline moieties, with an emphasis on hybrid ligands, and finally explain why the combination of these two facets of ligand design appears particularly promising.

Pyrazolylborate-Zinc Complexes of RNA Precursors and Analogues Thereof

The Tp ligand tris(3-cumenyl-5-methylpyrazolyl)borate was found to stabilize zinc complexes of nucleobases, of their natural precursors, and of nucleoside and nucleotide derivatives. Dihydroorotic acid and orotic acid are bound as monodentate carboxylate ligands. Uracil is coordinated via its deprotonated N1; 6-methylthiouracil acts as a bidentate ligand via N1 and S. Analogously, xanthine is a monodentate ligand bound by its deprotonated N7, while 6-mercaptopurine seems to bind in a bidentate fashion via N7 and S. Spectroscopic evidence indicates coordination of uridine and 2',3'-O-isopropylideneuridine via their deprotonated N3, as well as of xanthosine via N7. The hydrolytic cleavage of 2',3'-O-isopropylideneuridine 5'-(bis(p-nitrophenyl) phosphate) by TpZn-OH is preceded by an attachment of one TpZn unit to the deprotonated uracil base, presumably via N3.