The Zn(II)-1,4,7-Trimethyl-1,4,7-Triazacyclononane Complex: A Monometallic Catalyst Active in Two Protonation States

Organometallic modification confers oligonucleotides new functionalities

To improve their properties or to introduce entirely new functionalities, the intriguing scaffolds of nucleic acids have been decorated with various modifications, most recently also organometallic ones. While challenging to introduce, organometallic modifications offer the potential of expanding the field of application of metal-dependent functionalities to metal-deficient conditions, notably those of biological media. So far, organometallic moieties have been utilized as probes, labels and catalysts. This Feature Article summarizes recent efforts and predicts likely future developments in each of these lines of research.

Organomercury oligonucleotide conjugates as artificial ribonucleases

Two oligonucleotide conjugates sharing the same sequence but incorporating a different 5'-terminal organometallic moiety were synthesized, by either direct mercuration in solution or oximation with an organomercury aldehyde on solid support. The potential of these conjugates to serve as new type of artificial ribonucleases was tested with a complementary 2´-O-methyl-RNA target sequence featuring a single cleavable RNA phosphodiester linkage. Both organomercury oligonucleotides greatly outperformed their metal-free counterparts as well as the previously reported small molecule organomercury RNA cleaving agent in catalytic activity, providing an important proof-of-concept. Compared to state-of-the-art metal-dependent artificial ribonucleases, however, the observed activity was modest.

Design, Synthesis, and Biological Evaluation of Boron-Containing Macrocyclic Polyamines and Their Zinc(II) Complexes for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a binary therapeutic method for cancer treatment based on the use of a combination of a cancer-specific drug containing boron-10 (¹⁰B) and thermal neutron irradiation. For successful BNCT, ¹⁰B-containing molecules need to accumulate specifically in cancer cells, because destructive effect of the generated heavy particles is limited basically to boron-containing cells. Herein, we report on the design and synthesis of boron compounds that are functionalized with 9-, 12-, and 15-membered macrocyclic polyamines and their Zn²⁺ complexes. Their cytotoxicity, intracellular uptake activity into cancer cells and normal cells, and BNCT effect are also reported. The experimental data suggest that mono- and/or diprotonated forms of metal-free [12]aneN₄- and [15]aneN₅-type ligands are uptaken into cancer cells, and their complexes with intracellular metals such as Zn²⁺ would induce cell death upon thermal neutron irradiation, possibly via interactions with DNA.

Cleavage of an RNA Model Compound by an Arylmercury Complex

A water-soluble arylmercury complex has been synthesized, and its ability to catalyze the cleavage of the phosphodiester linkage of the RNA model compound adenylyl-3',5'-(2',3'-O-methyleneadenosine) has been assessed over a pH range of 3-8.5 and a catalyst concentration range of 0-7 mM. In the presence of 1 mM catalyst, the observed pH-rate profile featured a new pH-independent region between pH 6 and 7, the catalyzed reaction being as much as eight times faster than the background reaction. At pH 7, the acceleration increased linearly from three- to 17-fold upon increasing the catalyst concentration from 1 to 7 mM. The linear dependence indicates a relatively low affinity of the catalyst for the substrate and, hence, the potential for considerable improvement on tethering to an appropriate targeting group, such as an oligonucleotide.

Host-Guest Allosteric Control of an Artificial Phosphatase

The activity of many enzymes is regulated by associative processes. To model this mechanism, we report here that the conformation of an unstructured bimetallic Zn(II) complex can be controlled by its inclusion in the cavity of a γ-cyclodextrin. This results in the formation of a catalytic bimetallic site for the hydrolytic cleavage of the RNA model substrate HPNP, whose reactivity is 30-fold larger with respect to the unstructured complex. Competitive inhibition with 1-adamantanecarboxylate displaces the metal complex from the cyclodextrin decreasing the reactivity.