The Manganese Ion of the Heterodinuclear Mn/Fe Cofactor in <i>Chlamydia trachomatis</i> Ribonucleotide Reductase R2c Is Located at Metal Position 1

The essential catalytic radical of Class-I ribonucleotide reductase is generated and delivered by protein R2, carrying a dinuclear metal cofactor. A new R2 subclass, R2c, prototyped by the <i>Chlamydia trachomatis</i> protein was recently discovered. This protein carries an oxygen-activating heterodinuclear Mn­(II)/Fe­(II) metal cofactor and generates a radical-equivalent Mn­(IV)/Fe­(III) oxidation state of the metal site, as opposed to the tyrosyl radical generated by other R2 subclasses. The metal arrangement of the heterodinuclear cofactor remains unknown. Is the metal positioning specific, and if so, where is which ion located? Here we use X-ray crystallography with anomalous scattering to show that the metal arrangement of this cofactor is specific with the manganese ion occupying metal position 1. This is the position proximal to the tyrosyl radical site in other R2 proteins and consistent with the assumption that the high-valent Mn­(IV) species functions as a direct substitute for the tyrosyl radical

Effects of Terminal Dimethylation and Metal Coordination of Proline-2-formylpyridine Thiosemicarbazone Hybrids on Lipophilicity, Antiproliferative Activity, and hR2 RNR Inhibition

The nickel­(II), copper­(II), and zinc­(II) complexes of the proline-thiosemicarbazone hybrids 3-methyl-(<i>S</i>)-pyrrolidine-2-carboxylate-2-formylpyridine thiosemicarbazone (l-Pro-FTSC or (<i>S</i>)-H₂L¹) and 3-methyl-(<i>R</i>)-pyrrolidine-2-carboxylate-2-formylpyridine thiosemicarbazone (d-Pro-FTSC or (<i>R</i>)-H₂L¹), as well as 3-methyl-(<i>S</i>)-pyrrolidine-2-carboxylate-2-formylpyridine 4,4-dimethyl-thiosemicarbazone (dm-l-Pro-FTSC or (<i>S</i>)-H₂L²), namely, [Ni­(l-Pro-FTSC–2H)]₂ (<b>1</b>), [Ni­(d-Pro-FTSC–2H)]₂ (<b>2</b>), [Ni­(dm-l-Pro-FTSC–2H)]₂ (<b>3</b>), [Cu­(dm-l-Pro-FTSC–2H)] (<b>6</b>), [Zn­(l-Pro-FTSC–2H)] (<b>7</b>), and [Zn­(d-Pro-FTSC–2H)] (<b>8</b>), in addition to two previously reported, [Cu­(l-Pro-FTSC–2H)] (<b>4</b>), [Cu­(d-Pro-FTSC–2H)] (<b>5</b>), were synthesized and characterized by elemental analysis, one- and two-dimensional ¹H and ¹³C NMR spectroscopy, circular dichroism, UV–vis, and electrospray ionization mass spectrometry. Compounds <b>1</b>–<b>3</b>, <b>6</b>, and <b>7</b> were also studied by single-crystal X-ray diffraction. Magnetic properties and solid-state high-field electron paramagnetic resonance spectra of <b>2</b> over the range of 50–420 GHz were investigated. The complex formation processes of l-Pro-FTSC with nickel­(II) and zinc­(II) were studied in aqueous solution due to the excellent water solubility of the complexes via pH potentiometry, UV–vis, and ¹H NMR spectroscopy. The results of the antiproliferative activity <i>in vitro</i> showed that dimethylation improves the cytotoxicity and hR2 RNR inhibition. Therefore, introduction of more lipophilic groups into thiosemicarbazone-proline backbone becomes an option for the synthesis of more efficient cytotoxic agents of this family of compounds

Electronic Structural Flexibility of Heterobimetallic Mn/Fe Cofactors: R2lox and R2c Proteins

The electronic structure of the Mn/Fe cofactor identified in a new class of oxidases (R2lox) described by Andersson and Högbom [<i>Proc. Natl. Acad. Sci. U.S.A.</i> <b>2009</b>, 106, 5633] is reported. The R2lox protein is homologous to the small subunit of class Ic ribonucleotide reductase (R2c) but has a completely different in vivo function. Using multifrequency EPR and related pulse techniques, it is shown that the cofactor of R2lox represents an antiferromagnetically coupled Mn^III/Fe^III dimer linked by a μ-hydroxo/bis-μ-carboxylato bridging network. The Mn^III ion is coordinated by a single water ligand. The R2lox cofactor is photoactive, converting into a second form (R2lox_Photo) upon visible illumination at cryogenic temperatures (77 K) that completely decays upon warming. This second, unstable form of the cofactor more closely resembles the Mn^III/Fe^III cofactor seen in R2c. It is shown that the two forms of the R2lox cofactor differ primarily in terms of the local site geometry and electronic state of the Mn^III ion, as best evidenced by a reorientation of its unique ⁵⁵Mn hyperfine axis. Analysis of the metal hyperfine tensors in combination with density functional theory (DFT) calculations suggests that this change is triggered by deprotonation of the μ-hydroxo bridge. These results have important consequences for the mixed-metal R2c cofactor and the divergent chemistry R2lox and R2c perform