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1. A mixed-valent Fe(II)Fe(III) species converts cysteine to an oxazolone/thioamide pair in methanobactin biosynthesis

   https://doi.org/10.1073/pnas.2123566119  

   Park, Yun Ji; Jodts, Richard J.; Slater, Jeffrey W.; Reyes, Reyvin M.; Winton, Valerie J.; Montaser, Rana A.; Thomas, Paul M.; Dowdle, William B.; Ruiz, Anahi; Kelleher, Neil L.; et al (March 2022, Proceedings of the National Academy of Sciences)

   The iron-containing heterodimeric MbnBC enzyme complex plays a central role in the biosynthesis of methanobactins (Mbns), ribosomally synthesized, posttranslationally modified natural products that bind copper with high affinity. MbnBC catalyzes a four-electron oxidation of a cysteine residue in its precursor-peptide substrate, MbnA, to an oxazolone ring and an adjacent thioamide group. Initial studies of MbnBC indicated the presence of both diiron and triiron species, complicating identification of the catalytically active species. Here, we present evidence through activity assays combined with electron paramagnetic resonance (EPR) and Mössbauer spectroscopic analysis that the active species is a mixed-valent, antiferromagnetically coupled Fe(II)Fe(III) center. Consistent with this assignment, heterologous expression of the MbnBC complex in culture medium containing less iron yielded purified protein with less bound iron but greater activity in vitro. The maximally activated MbnBC prepared in this manner could modify both cysteine residues in MbnA, in contrast to prior findings that only the first cysteine could be processed. Site-directed mutagenesis and multiple crystal structures clearly identify the two essential Fe ions in the active cluster as well as the location of the previously detected third Fe site. Moreover, structural modeling indicates a role for MbnC in recognition of the MbnA leader peptide. These results add a biosynthetic oxidative rearrangement reaction to the repertoire of nonheme diiron enzymes and provide a foundation for elucidating the MbnBC mechanism. 

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2. Synthetic Metallodithiolato Ligands as Pendant Bases in [Fe ^I Fe ^I ], [Fe ^I [Fe(NO)] ^II ], and [(μ-H)Fe ^II Fe ^II ] Complexes

   https://doi.org/10.1021/acs.inorgchem.9b03409  

   Kariyawasam Pathirana, Kavindu Dilshan; Ghosh, Pokhraj; Hsieh, Chung-H.; Elrod, Lindy Chase; Bhuvanesh, Nattamai; Darensbourg, Donald J.; Darensbourg, Marcetta Y. (March 2020, Inorganic Chemistry)

   null (Ed.)
3. Selenium oxyanion exchange with Mg(II)-Fe(III) and Fe(II)-Fe(III) layered double hydroxides

   https://doi.org/10.1016/j.clay.2020.105959  

   Schellenger, Alexandra E.P.; Choi, Sunho; Onnis-Hayden, Annalisa; Larese-Casanova, Philip (January 2021, Applied Clay Science)

   null (Ed.)
4. Fe-Ti vs. Fe-Fe charge transfers: A comprehensive review and its applications in minerals and glasses

   https://doi.org/10.2138/am-2025-9835  

   Vigier, Maxence; Evans, Helen; Rossman, George R; Jobic, Stéphane; Fritsch, Emmanuel (August 2025, American Mineralogist)

   Tomascak, P; Nestola, F (Ed.)

   Abstract Iron-titanium (Fe-Ti) charge transfer is mentioned in numerous articles as the source of the coloration of many natural minerals and some man-made materials, but no global review of this phenomenon has been provided so far. Iron and titanium are ubiquitous in nature and are often found in the same material as Fe2+ and Fe3+, and Ti4+ (more rarely Ti3+). When Fe and Ti ions are in close geometric proximity in an oxide or (alumino)silicate structure, charge transfer can occur between the two ions, even though their concentration might be below 100 ppm. This results in a variety of absorption features that contributes to the color of minerals. Adebate remains on the exact nature of Fe/Ti electronic transition, i.e. Fe2+ + Ti4+ → Fe3+ + Ti3+ or the reverse, but solving this issue is not within the scope of the present work. Ascertaining a metal-metal charge transfer is often not straightforward. This review compiles existing knowledge on Fe-Ti charge transfer in both crystalline and amorphous materials and identifies several key characteristics in more than 40 different materials. A charge transfer is associated with broad, intense, optical absorption bands that decrease in intensity at elevated temperatures. It is also strongly pleochroic in non-isotropic materials. Until now, Fe-Ti charge transfer transitions have been primarily described in the 2.25 to 3.1 eV range, corresponding to yellow to orange to brown colors, with notable exceptions such as blue sapphire or kyanite, and green andalusite. This review suggests that Fe-Ti charge transfer can occur across the entire visible spectrum, and the position of the absorption band correlates with the Fe-Ti nteratomic distance. This correlation highlights the presence of multiple crystallographic sites for both Fe and Ti in many oxides, leading to multiple Fe-Ti bands within these materials (e.g. sapphire, ilmenite, pseudobrookite). Finally, the use of metal-metal distances is suggested to differentiate this heteronuclear Fe-Ti charge transfer from the common homonuclear charge transfer Fe2+-Fe3+. 

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5. Enhanced Fe-Centered Redox Flexibility in Fe–Ti Heterobimetallic Complexes

   https://doi.org/10.1021/acs.inorgchem.9b00442  

   Moore, James T.; Chatterjee, Sudipta; Tarrago, Maxime; Clouston, Laura J.; Sproules, Stephen; Bill, Eckhard; Bernales, Varinia; Gagliardi, Laura; Ye, Shengfa; Lancaster, Kyle M.; et al (April 2019, Inorganic Chemistry)