Significantly enhanced proteolytic activity of cyclen complexes by monoalkylation

Probing Unexpected Reactivity in Radiometal Chemistry: Indium-111-Mediated Hydrolysis of Hybrid Cyclen-Hydroxypyridinone Ligands

Chelators based on hydroxypyridinones have utility in incorporating radioactive metal ions into diagnostic and therapeutic agents used in nuclear medicine. Over the course of our hydroxypyridinone studies, we have prepared two novel chelators, consisting of a cyclen (1,4,7,10-tetraazacyclododecane) ring bearing two pendant hydroxypyridinone groups, appended via methylene acetamide motifs at either the 1,4-positions (L¹) or 1,7-positions (L²) of the cyclen ring. In radiolabeling reactions of L¹ or L² with the γ-emitting radioisotope, [¹¹¹In]In³⁺, we have observed radiometal-mediated hydrolysis of a single amide group of either L¹ or L². The reaction of either [¹¹¹In]In³⁺ or [^natIn]In³⁺ with either L¹ or L², in aqueous alkaline solutions at 80 °C, initially results in formation of [In(L¹)]⁺ or [In(L²)]⁺, respectively. Over time, each of these species undergoes In³⁺-mediated hydrolysis of a single amide group to yield species in which In³⁺ remains coordinated to the resultant chelator, which consists of a cyclen ring bearing a single hydroxypyridinone group and a single carboxylate group. The reactivity toward hydrolysis is higher for the L¹ complex compared to that for the L² complex. Density functional theory calculations corroborate these experimental findings and importantly indicate that the activation energy required for the hydrolysis of L¹ is significantly lower than that required for L². This is the first reported example of a chelator undergoing radiometal-mediated hydrolysis to form a radiometalated complex. It is possible that metal-mediated amide bond cleavage is a source of instability in other radiotracers, particularly those in which radiometal complexation occurs in aqueous, basic solutions at high temperatures. This study highlights the importance of appropriate characterization of radiolabeled products.

Molecular fusion of surfactant and Lewis-acid properties for attacking dirt by catalytic bond cleavage

The capability of ordinary surfactants in solubilizing hydrophobic compounds can come to a limit, if the extension of a contaminant is too large. An attractive goal is the development of surfactants which can actively reduce the size of dirt. Because strong Lewis acids are known to catalyze both bond formation and cleavage, an integration into the surfactant's molecular framework is tempting. End-group functionalized hepta-dentate ligands, which coordinate to metal ions preventing deactivation by hydrolysis over a broad range of pH values while maintaining strong Lewis-acidity, are herein presented. After proof of amphiphilicity and surfactant characteristics, catalytic properties are investigated for different reactions including the cleavage of proteins. The compounds perform better than benchmark catalysts concerning the attack of unreactive amide bonds. A study with two Sc³⁺ species as the active site, one non-amphiphilic, the other one being surface-active, underlines the positive effect of surfactant properties for boosting catalytic efficiency.

Peptide Hydrolysis by Metal (Oxa)cyclen Complexes: Revisiting the Mechanism and Assessing Ligand Effects

The mechanism responsible for peptide bond hydrolysis by Co(III) and Cu(II) complexes with (oxa)cyclen ligands has been revisited by means of computational tools. We propose that the mechanism starts by substrate coordination and an outer-sphere attack on the amide C atom of a solvent water molecule assisted by the metal hydroxo moiety as a general base, which occurs through six-membered ring transition states. This new mechanism represents a more likely scenario than the previously proposed mechanisms that involved an inner-sphere nucleophilic attack through more strained four-membered rings transition states. The corresponding computed overall free-energy barrier of 25.2 kcal mol^-1 for hydrolysis of the peptide bond in Phe-Ala by a cobalt(III) oxacyclen catalyst (1) is consistent with the experimental values obtained from rate constants. Also, we assessed the influence of the nature of the ligand throughout a systematic replacement of N by O atoms in the (oxa)cyclen ligand. Increasing the number of coordinating O atoms accelerates the reaction by increasing the Lewis acidity of the metal ion. On the other hand, the higher reactivity observed for the copper(II) oxacyclen catalyst with respect to the analogous Co(III) complex can be attributed to the larger Brönsted basicity of the copper(II) hydroxo ligand. Ultimately, the detailed understanding of the ligand and metal nature effects allowed us to identify the double role of the metal hydroxo complexes as Lewis acids and Brönsted bases and to rationalize the observed reactivity trends.