Enzymatische Acyl- und Phosphoryltransferreaktionen unter Beteiligung von zwei Metallionen

Resolution of Expanded Porphyrinoids: A Path to Persistent Chirality and Appealing Chiroptical Properties

Chirality is a fundamental characteristic of nature. Expanded porphyrinoids and their analogues offer an attractive platform for delving into the intricacies of chirality. Expanded porphyrinoids comprise pyrrolic macrocycles and related heterocyclic systems. As a class, expanded porphyrinoids are widely recognized for their flexible structural features, nontrivial coordination capabilities, and intriguing optical and electronic properties. With limited exceptions, their inherent conformational flexibility coupled with a low racemization barrier allows for the facile interchange between enantiomers. As a result, achieving the effective chiral resolution of individual enantiomers and the subsequent exploration of their chiroptical properties represents a significant challenge. This review summarizes strategies used to realize the chiral resolution of expanded porphyrinoids and the understanding of intrinsic chiroptical properties that has emerged from these separation efforts. It is our hope that this review will serve not only to codify our current understanding of chiral expanded porphyrinoids, but also inspire advances in the generalized area of chiral functional materials.

Controlled Hydrolysis of Phosphate Esters: A Route to Calixarene-Supported Rare-Earth Clusters

Phosphate ester bonds are widely present in nature (e. g. DNA/RNA) and can be extremely stable against hydrolysis without the help of catalysts. Previously, we showed how the combination of phosphoryl and calix[4]arene moieties in the same organic framework (L_PO ) allows isolation of single lanthanide (Ln) metal ions as [Ln^III (L_PO )₂ ](O₃ SCF₃ )₃ . Here we report how by controlling the reaction conditions a new hydrolyzed phosphoryl-calix[4]arene ligand (H₃ L_HPO ) is formed as a result of Ln^III -mediated P-OEt bond cleavage in three out of the eight possible sites in L_PO . The chelating nature of H₃ L_HPO traps the Ln^III species in the form of [Ln^III (L_HPO )((EtO)₂ P(O)OH)]₂ dimers (Ln=La, Dy, Tb, Gd), where the Dy derivative shows slow magnetization relaxation. The strategy presented herein could be extended to access a broader library of hydrolyzed platforms (H_x L_HPO ; x=1-8) that may represent mimics of nuclease enzymes.

Di-2,7-pyrenidecaphyrin(1.1.0.0.0.1.1.0.0.0) and Its Bis-Organopalladium Complexes: Synthesis and Chiroptical Properties

A non-aromatic expanded carbaporphyrinoid, incorporating two built-in 2,7-pyrenylene moieties was synthesized. The intrinsically labile structure was demonstrated by proton-triggered conformational changes between the figure-of-eight and quasi-Möbius conformers. Upon treatment with Pd(OAc)₂ , the reaction produces two bis-Pd^II complexes with distinct coordination modes. Metal coordination serves to fix the macrocyclic frameworks with the net result that both bis-Pd^II complexes could be resolved by high performance liquid chromatography (HPLC) on a chiral stationary phase. The isolated enantiomers showed persistent chiroptical properties as evidenced by the intense response in the circular dichroism (CD) spectra and the record high absorption dissymmetry factors (g_abs of up to 0.038) seen in the near-infrared spectral region. Moreover, the mutual interconversion of these two Pd^II complexes was found to be stereospecific and to favor the more stable isomers under weakly acidic conditions.

Metal Cation-Driven Dynamic Covalent Formation of Imine and Hydrazone Ligands Displaying Synergistic Co-catalysis and Auxiliary Amine Effects

Optimizing C=N bond formation and C/N component exchange has major significance in Dynamic Covalent Chemistry (DCC). Imine and hydrazone generation from their aldehyde, amine and hydrazine components showed large accelerations in presence of AgOTf or Zn(OTf)₂ , up to 10⁴ for the Zn(II)-(p-anisidine)imine complex. Zn(OTf)₂ and auxiliary p-anisidine together accelerated 630 times the formation of the Zn(II)-hydrazone complex, revealing a strong synergistic effect, traced to very fast initial formation of the reactive Zn(II)-imine complex presenting a C=N bond metallo-activated towards reaction with the hydrazine component. Reactions involving more entities showed kinetically faster and thermodynamically simpler outputs due to dynamic competition within a mixture of higher complexity. Catalytic amounts of metal salts and auxiliary amine gave similar marked rate accelerations and turnover, indicating true catalysis. The synergistic effect achieved by combining metallo- and organo-catalysis points to a powerful co-catalysis strategy of bond-formation in DCC through interconnected chemical transformations.

Chiral Nanozymes-Gold Nanoparticle-Based Transphosphorylation Catalysts Capable of Enantiomeric Discrimination

Enantioselectivity in RNA cleavage by a synthetic metalloenzyme has been demonstrated for the first time. Thiols containing chiral Zn(II) -binding head groups have been self-assembled on the surface of gold nanoparticles. This results in the spontaneous formation of chiral bimetallic catalytic sites that display different activities (kcat ) towards the enantiomers of an RNA model substrate. Substrate selectivity is observed when the nanozyme is applied to the cleavage of the dinucleotides UpU, GpG, ApA, and CpC, and remarkable differences in reactivity are observed for the cleavage of the enantiomerically pure dinucleotide UpU.

Study of the Phosphoryl-Transfer Mechanism of Shikimate Kinase by NMR Spectroscopy

The phosphoryl-transfer mechanism of shikimate kinase from Mycobacterium tuberculosis and Helicobacter pylori, which is an attractive target for antibiotic drug discovery, has been studied by 1D (1)H and (31)P NMR spectroscopy. Metaphosphoric acid proved to be a good mimetic of the metaphosphate intermediate and facilitated the ready and rapid evaluation by NMR spectroscopic analysis of a dissociative mechanism. The required closed form of the active site for catalysis was achieved by the use of ADP (product) or two synthetic ADP analogues (AMPNP, AMPCP). Molecular dynamics simulation studies reported here also revealed that the essential arginine (Arg116/Arg117 in H. pylori and M. tuberculosis, respectively), which activates the γ-phosphate group of ATP for catalysis and triggers the release of the product for turnover, would also be involved in the stabilisation of the metaphosphate intermediate during catalysis. We believe that the studies reported here will be helpful for future structure-based design of inhibitors of this attractive target. The approach is also expected be useful for studies on the possible dissociative mechanism of other kinase enzymes.

Cleavage of phosphodiesters and of DNA by a bis(guanidinium)naphthol acting as a metal-free anion receptor

Phosphoric acid diesters form anions at neutral pH. As a result of charge repulsion they are notoriously resistant to hydrolysis. Nucleophilic attack, however, can be promoted by different types of electrophilic catalysts that bind to the anions and reduce their negative charge density. Although in most cases phosphodiester-cleaving enzymes and synthetic catalysts rely on Lewis acidic metal ions, some exploit the guanidinium residues of arginine as metal-free electrophiles. Here we report that a combination of two guanidines and a hydroxy group yields highly reactive receptor molecules that can attack a broad range of phosphodiester substrates by nucleophilic displacement at phosphorus in a single-turnover mode. Some stable O-phosphates were isolated and characterized further by NMR spectroscopy. The bis(guanidinium)naphthols also cleave plasmid DNA, presumably by a transphosphorylation mechanism.

Interfacial enzyme kinetics of a membrane bound kinase analyzed by real-time MAS-NMR

The simultaneous observation of interdependent reactions within different phases as catalyzed by membrane-bound enzymes is still a challenging task. One such enzyme, the Escherichia coli integral membrane protein diacylglycerol kinase (DGK), is a key player in lipid regulation. It catalyzes the generation of phosphatidic acid within the membrane through the transfer of the γ-phosphate from soluble MgATP to membrane-bound diacylglycerol. We demonstrate that time-resolved (31)P magic angle spinning NMR offers a unique opportunity to simultaneously and directly detect both ATP hydrolysis and diacylglycerol phosphorylation. This experiment demonstrates that solid-state NMR provides a general approach for the kinetic analysis of coupled reactions at the membrane interface regardless of their compartmentalization. The enzymatic activity of DGK was probed with different lipid substrates as well as ATP analogs. Our data yield conclusions about intersubunit cooperativity, reaction stoichiometries and phosphoryl transfer mechanism and are discussed in the context of known structural data.

Self-assembly approach toward chiral bimetallic catalysts: bis-urea-functionalized (salen)cobalt complexes for the hydrolytic kinetic resolution of epoxides

A series of novel bis-urea-functionalized (salen)Co complexes has been developed. The complexes were designed to form self-assembled structures in solution through intermolecular urea-urea hydrogen-bonding interactions. These bis-urea (salen)Co catalysts resulted in rate acceleration (up to 13 times) in the hydrolytic kinetic resolution (HKR) of rac-epichlorohydrin in THF by facilitating cooperative activation, compared to the monomeric catalyst. In addition, one of the bis-urea (salen)Co(III) catalyst efficiently resolves various terminal epoxides even under solvent-free conditions by requiring much shorter reaction time at low catalyst loading (0.03-0.05 mol %). A series of kinetic/mechanistic studies demonstrated that the self-association of two (salen)Co units through urea-urea hydrogen bonds was responsible for the observed rate acceleration. The self-assembly study with the bis-urea (salen)Co by FTIR spectroscopy and with the corresponding (salen)Ni complex by (1)H NMR spectroscopy showed that intermolecular hydrogen-bonding interactions exist between the bis-urea scaffolds in THF. This result demonstrates that self-assembly approach by using non-covalent interactions can be an alternative and useful strategy toward the efficient HKR catalysis.

Enhanced complexity and catalytic efficiency in the hydrolysis of phosphate diesters by rationally designed helix-loop-helix motifs

HJ1, a 42-residue peptide that folds into a helix-loop-helix motif and dimerizes to form a four-helix bundle, successfully catalyzes the cleavage of "early stage" DNA model substrates in an aqueous solution at pH 7.0, with a rate enhancement in the hydrolysis of heptyl 4-nitrophenyl phosphate of over three orders of magnitude over that of the imidazole-catalyzed reaction, k(2)(HJ1)/k(2)(Im) = 3135. The second-order rate constant, k(2)(HJ1) was determined to be 1.58x10(-4) M(-1) s(-1). The catalyst successfully assembles residues that in a single elementary reaction step are capable of general-acid and general-base catalysis as well as transition state stabilization and proximity effects. The reactivity achieved with the HJ1 polypeptide, rationally designed to catalyze the hydrolysis of phosphodiesters, is based on two histidine residues flanked by four arginines and two adjacent tyrosine residues, all located on the surface of a helix-loop-helix motif. The introduction of Tyr residues close to the catalytic site improves efficiency, in the cleavage of activated aryl alkyl phosphates as well as less activated dialkyl phosphates. HJ1 is also effective in the cleavage of an RNA-mimic substrate, uridine-3'-2,2,2-trichloroethyl phosphate (leaving group pK(a) = 12.3) with a second-order rate constant of 8.23x10(-4) M(-1) s(-1) in aqueous solution at pH 7.0, some 500 times faster than the reaction catalyzed by imidazole, k(2)(HJ1)/k(2)(Im) = 496.

Structural, Kinetic, and Theoretical Studies on Models of the Zinc-Containing Phosphodiesterase Active Center: Medium-Dependent Reaction Mechanisms

Dinuclear zinc(II) complexes [Zn(2)(bpmp)(mu-OH)](ClO(4))(2) (1) and [Zn(2)(bpmp)(H(2)O)(2)](ClO(4))(3) (2) (H-BPMP=2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol) have been synthesized, structurally characterized, and pH-driven changes in metal coordination observed. The transesterification reaction of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) in the presence of the two complexes was studied both in a water/DMSO (70:30) mixture and in DMSO. Complex 2 was not reactive whereas for 1 considerable rate enhancement of the spontaneous hydrolysis reaction was observed. A detailed mechanistic investigation by kinetic studies, spectroscopic measurements ((1)H, (31)P NMR spectroscopy), and ESI-MS analysis in conjunction with ab initio calculations was performed on 1. Based on these results, two medium-dependent mechanisms are presented and an unusual bridging phosphate intermediate is proposed for the process in DMSO.

Antibiotic resistance: mono- and dinuclear zinc complexes as metallo-beta-lactamase mimics

Biomimetic systems containing one or two zinc(II) ions supported by phenolate ligands were developed as functional mimics of metallo-beta-lactamase. These complexes were shown to catalytically hydrolyze beta-lactam substrates, such as oxacillin and penicillin G. The dinuclear zinc complex 1, which has a coordinated water molecule, exhibits high beta-lactamase activity, whereas the dinuclear zinc complex 2, which has no water molecules, but labile chloride ligands, shows a much lower activity. The high beta-lactamase activity of complex 1 can be ascribed to the presence of a zinc-bound water molecule that is activated by being hydrogen bonded to acetate substituents. The kinetics of the hydrolysis of oxacillin by complex 1 and the effect of pH on the reaction rates are reported in detail. In addition, the kinetic parameters obtained for the synthetic analogues are compared with those of the natural metallo-beta-lactamase from Bacillus cereus (BcII). To understand the role of the second metal ion in hydrolysis, the syntheses and catalytic activities of two mononuclear complexes (3 and 4) that include coordinated water molecules are described. Interestingly, the mononuclear zinc complexes 3 and 4 also exhibit high activity, supporting the assumption that the second zinc ion is not crucial for the beta-lactamase activity.

Mimicking Enzymes: Cooperation between Organic Functional Groups and Metal Ions in the Cleavage of Phosphate Diesters

A series of ligands derived from the bis-2-pyridinylmethylamine structure, which bear either additional hydroxyl or aromatic amino groups, were prepared and their Zn(II) complexes were studied as catalysts for the cleavage of bis-p-nitrophenyl phosphate (BNP) and 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP) diesters. A comparative kinetic study indicated that the insertion of organic groups, capable of acting as nucleophiles or as hydrogen-bond donors, substantially increases the hydrolytic activity of the metal complex. Dissection of the effects of the individual groups revealed that the increase in reactivity can reach up to three orders of magnitude. The improved efficiency of the systems studied, combined with the benefits resulting from the low pK(a) value of the active nucleophile, result in an acceleration of the BNP cleavage at pH 7 of six orders of magnitude. The pH-dependent reactivity profiles follow a bell-shaped curve and the maximum reactivity is observed at pH 9. The mechanism of the reactions and the structure of the complexes were investigated in detail by means of kinetic analysis, NMR spectroscopy experiments, and theoretical calculations. The reactivity of the complexes that cleave HPNP closely resembles the reactivity observed for BNP, but the accelerations achieved are lower as a result of different reaction mechanisms.

Tripodal, Cooperative, and Allosteric Transphosphorylation Metallocatalysts

Three artificial amino acids derived from l-serine by replacing the hydroxyl moiety with 1,4,7-triazacyclononane, 1,5,9-triazacyclododecane, and 1,4,7,10-tetraazacyclododecane, respectively, have been connected to the three arms of the tetraamine tris(2-aminoethyl)amine, Tren, to obtain tripodal ligands. They are able to bind up to four metal ions (like CuII and ZnII), three with the polyazamacrocycles and one with the Tren platform. Some of the ZnII complexes of these tripodal ligands proved to be good catalysts for the cleavage of the RNA model substrate 2-hydroxypropyl-p-nitrophenylphosphate (HPNP). Studies of the catalytic activity in the presence of increasing amounts of ZnII show that the complexes represent minimalist examples of metallocatalysts with cooperativity between the metal centers and allosteric control by a metal ion. The Tren binding site constitutes the allosteric regulation unit, while the three ZnII-azacrown complexes provide the cooperative, catalytic site. The allosteric role of the ZnII ion located in the Tren binding site was unambiguously demonstrated by studying the catalytic activity of a derivative unable to complex ZnII in that site. In this case, the cooperativity between the three ZnII ions bound to the peripheral azacrowns was totally suppressed. The kinetic analysis has shown that cooperativity is due to neither the occurrence of general-acid/general-base catalysis nor a decreased binding of the substrate because of the deprotonation of a water molecule bound to the complex but, rather, stabilization of the complexed substrate in its transformation into the transition state.