Martin Bröring

Axial ligand substitution enables modulation of photophysics in tin( iv ) corroles, governing singlet oxygen sensitization and photon upconversion activity.

The encasement of a Cu(ii)-ion by the ring-contracted corrin-ligand of vitamin B₁₂ leads to a tightly bound and nearly planar, 4-coordinate d₉-Cu(ii)-centre, whose unpaired electron occupies a largely metal-located, antibonding d_x²–y² orbital.

Sb(V) dihalide corrole complexes, in particular difluoro-5,15-di(4-cyanophenyl)-10-(2,4,5-trimethoxyphenyl)corrolatoantimony(V) (complex 1), show distinct emission properties and efficient intersystem crossing rates. Furthermore, complex 1 is characterised by its extended triplet excited state lifetime and an impressive singlet oxygen quantum yield exceeding 95%. This emphasises its potential for effective photooxidation reactions, positioning it as a leader in Sb(V) complex applications.

Antimony corrole cations have been prepared by one‐electron oxidation of antimony(III) congeners with silver(I) and copper(II) salts. Isolation and crystallization was successful for the first time, and the X‐ray crystallographic investigation unraveled structural similarities with antimony(III)corroles. EPR experiments showed strong hyperfine interactions of the unpaired electron with the ¹²¹Sb (I=5/2) and ¹²³Sb nuclei (I=7/2). A DFT analysis supports the description of the oxidized form as a Sb^III corrole radical with less than 2 % Sb^IV character. In the presence of water or a fluoride source like PF₆⁻, the compounds undergo a redox disproportionation to yield known antimony(III)corroles and either difluorido‐antimony(V)corroles, or bis‐μ‐oxido‐di[antimony(V)corroles] via novel cationic hydroxo‐antimony(V) derivatives.

Antimony corrole cations have been prepared by one‐electron oxidation of antimony(III) congeners with silver(I) and copper(II) salts. Isolation and crystallization was successful for the first time, and the X‐ray crystallographic investigation unraveled structural similarities with antimony(III)corroles. EPR experiments showed strong hyperfine interactions of the unpaired electron with the ¹²¹Sb (I=5/2) and ¹²³Sb nuclei (I=7/2). A DFT analysis supports the description of the oxidized form as a Sb^III corrole radical with less than 2 % Sb^IV character. In the presence of water or a fluoride source like PF₆⁻, the compounds undergo a redox disproportionation to yield known antimony(III)corroles and either difluorido‐antimony(V)corroles, or bis‐μ‐oxido‐di[antimony(V)corroles] via novel cationic hydroxo‐antimony(V) derivatives.

The methyltransferase FliB posttranslationally modifies surface-exposed ɛ-N-lysine residues of flagellin, the protomer of the flagellar filament in Salmonella enterica (S. enterica). Flagellin methylation, reported originally in 1959, was recently shown to enhance host cell adhesion and invasion by increasing the flagellar hydrophobicity. The role of FliB in this process, however, remained enigmatic. In this study, we investigated the properties and mechanisms of FliB from S. enterica in vivo and in vitro. We show that FliB is an S-adenosylmethionine (SAM) dependent methyltransferase, forming a membrane associated oligomer that modifies flagellin in the bacterial cytosol. Using X-band electron paramagnetic resonance (EPR) spectroscopy, zero-field ⁵⁷Fe Mössbauer spectroscopy, methylation assays and chromatography coupled mass spectrometry (MS) analysis, we further found that FliB contains an oxygen sensitive [4Fe-4S] cluster that is essential for the methyl transfer reaction and might mediate a radical mechanism. Our data indicate that the [4Fe-4S] cluster is coordinated by a cysteine rich motif in FliB that is highly conserved among multiple genera of the Enterobacteriaceae family. The bacterial flagella are tail-like appendages that play important roles in motility and host cell infection. In Salmonella, the surface of the flagellar filaments is heavily modified by methylation on their lysine residues, contributing to efficient gut colonization and successful invasion of the host. However, little is known about the properties of the methylase FliB, that catalyzes flagellar methylation. In this study, we isolated and characterized FliB of S. enterica for the first time. We show that FliB forms oligomers that localize in close proximity to the bacterial cell membrane. The FliB methyltransferase coordinates a [4Fe-4S] cluster at its active site by several cysteine residues that were highly conserved in many entero-pathogens. We further found that FliB modifies flagellar subunits using an iron-sulfur cluster dependent mechanism and requires a reducing environment. Our work sets a stage for understanding the roles of oxygen sensitive iron-sulfur cluster in flagellar modification and entero-bacterial pathogenesis.

Invited for the cover of this issue are Jens Krumsieck and Martin Bröring from the TU Braunschweig, Germany. The cover image allows a glance into a futuristic lab with an innovative imaging device analyzing structural details of a metal corrole. Read the full text of the article at 10.1002/chem.202101243.

The one‐pot synthesis of zinc isoporphyrins from pyrrole, an aryl aldehyde and zinc acetate was investigated with regard to the formation of by‐products. The optical spectra of the product mixtures differ strongly from each other depending on the aldehyde used and the water content in the solvent mixture of the condensation step. The compounds belonging to the two most prominent bands at approx. 500 nm and at approx. 640 nm could be isolated and structurally characterised. These are the homoleptic complexes (dipy)₂Zn with a dipyrrolic ligand, and unexpected zinc‐tripyrrin species (trpy)ZnCl, respectively. The tripyrrin species further react under the conditions of chromatography to green intermediates and yellow end products, whose mass spectroscopic investigations indicate dimers and loss of one ZnCl₂ equivalent. The final products could be crystallographically identified as zinc‐porphodimethene complexes and analysed with respect to the stereochemical situation and optical behaviour. In one case, the green intermediate form could be enriched. The investigations indicate a helical pentapyrrolic zinc complex with a non‐coordinated pyrrolyl side arm and give clues to the stereochemical course of the stepwise dimerisation.