Oxygen Activation by Mononuclear Mn, Co, and Ni Centers in Biology and Synthetic Complexes

The active sites of metalloenzymes that catalyze O2-dependent reactions generally contain iron or copper ions. However, several enzymes are capable of activating O2 at manganese or nickel centers instead, and a handful of dioxygenases exhibit activity when substituted with cobalt. This minireview summarizes the catalytic properties of oxygenases and oxidases with mononuclear Mn, Co, or Ni active sites, including oxalate-degrading oxidases, catechol dioxygenases, and quercetin dioxygenase. In addition, recent developments in the O2 reactivity of synthetic Mn, Co, or Ni complexes are described, with an emphasis on the nature of reactive intermediates featuring superoxo-, peroxo-, or oxo-ligands. Collectively, the biochemical and synthetic studies discussed herein reveal the possibilities and limitations of O2 activation at these three “overlooked” metals

A Synthetic Model of the Nonheme Iron–Superoxo Intermediate of Cysteine Dioxygenase

A nonheme Fe(II) complex (1) that models substrate-bound cysteine dioxygenase (CDO) reacts with O2 at −80 °C to yield a purple intermediate (2). Analysis with spectroscopic and computational methods determined that 2 features a thiolate-ligated Fe(III) center bound to a superoxide radical, mimicking the putative structure of a key CDO intermediate

Henri Temianka Correspondence; (afischer)

This collection contains material pertaining to the life, career, and activities of Henri Temianka, violin virtuoso, conductor, music teacher, and author. Materials include correspondence, concert programs and flyers, music scores, photographs, and books.https://digitalcommons.chapman.edu/temianka_correspondence/3315/thumbnail.jp

Investigation of isocyanate-based dual-cure resins and their suitability for additive manufacturing

The development of materials for vat photopolymerization printing technologies has gained increasing attention within the last ten years. To date, the choice of materials for this printing technology is limited to photopolymers that tend to result in highly crosslinked, but usually brittle material properties. In this work, a dual-cure approach was developed, with the target to improve the overall material properties of 3D printed objects. For this purpose, the orthogonal and sequential polymerization of acrylate- and polyisocyanate based precursors was investigated. Photosensitive polyurethane acrylates were synthesized and mixed with low viscous polyisocyanates. The sequential polymerization was triggered by UV-light and heat, respectively. The reaction of the isocyanate groups in the heat, resulted in a polyurea network, confirmed by infrared spectroscopy. The morphology of resulting combined networks was systematically investigated by dynamic mechanical analysis and atomic force microscopy and showed that phase-separated morphologies with two distinct glass transition temperatures were formed, leading to a 10 fold increase in toughness in comparison to the neat polymeric networks. First printing trials showed the feasibility of the resin system for vat photopolymerization. The approach was extended to the sequential polymerization of isocyanate terminated prepolymers and low viscous acrylates. Varying the crosslinking densities of each polymeric network resulted in different morphologies as well as thermomechanical properties. Typical elastomers were obtained by a low crosslinking density of both polymeric networks. Cyclic loading-unloading measurements on 3D printed specimen showed the energy elastic behavior of the materials and an elongation at break of 101 % and a tensile strength of 3.4 N·mm-2 were achieved. A dual-cure resin based on acrylate-functionalized polyisocyanates was evaluated and resulted in thermally- and chemically stable 3D printed materials with the option to generate adoptable and catalytically active surfaces. The suitability of this resin to 3D print chemical reaction ware was demonstrated. In summary, the results obtained in this work show the tuneability and variability of combining acrylate- and polyisocyanate precursors, contributing to the increasing demand of new and improved materials for vat photopolymerization

Optimal exponential bounds for aggregation of estimators for the Kullback-Leibler loss

We study the problem of model selection type aggregation with respect to the Kullback-Leibler divergence for various probabilistic models. Rather than considering a convex combination of the initial estimators $f_1, \ldots, f_N$, our aggregation procedures rely on the convex combination of the logarithms of these functions. The first method is designed for probability density estimation as it gives an aggregate estimator that is also a proper density function, whereas the second method concerns spectral density estimation and has no such mass-conserving feature. We select the aggregation weights based on a penalized maximum likelihood criterion. We give sharp oracle inequalities that hold with high probability, with a remainder term that is decomposed into a bias and a variance part. We also show the optimality of the remainder terms by providing the corresponding lower bound results.Comment: 25 page

Spectroscopic and Computational Studies of Reversible O\u3csub\u3e2\u3c/sub\u3e Binding by a Cobalt Complex of Relevance to Cysteine Dioxygenase

The substitution of non-native metal ions into metalloenzyme active sites is a common strategy for gaining insights into enzymatic structure and function. For some nonheme iron dioxygenases, replacement of the Fe(II) center with a redox-active, divalent transition metal (e.g., Mn, Co, Ni, Cu) gives rise to an enzyme with equal or greater activity than the wild-type enzyme. In this manuscript, we apply this metal-substitution approach to synthetic models of the enzyme cysteine dioxygenase (CDO). CDO is a nonheme iron dioxygenase that initiates the catabolism of L-cysteine by converting this amino acid to the corresponding sulfinic acid. Two mononuclear Co(II) complexes (3 and 4) have been prepared with the general formula [Co2+(TpR2)(CysOEt)] (R = Ph (3) or Me (4); TpR2 = hydrotris(pyrazol-1-yl)borate substituted with R-groups at the 3- and 5-positions, and CysOEt is the anion of L-cysteine ethyl ester). These Co(II) complexes mimic the active-site structure of substrate-bound CDO and are analogous to functional iron-based CDO models previously reported in the literature. Characterization with X-ray crystallography and/or 1H NMR spectroscopy revealed that 3 and 4 possess five-coordinate structures featuring facially-coordinating TpR2 and S,N-bidentate CysOEt ligands. The electronic properties of these high-spin (S = 3/2) complexes were interrogated with UV-visible absorption and X-band electron paramagnetic resonance (EPR) spectroscopies. The air-stable nature of complex 3 replicates the inactivity of cobalt-substituted CDO. In contrast, complex 4 reversibly binds O2 at reduced temperatures to yield an orange chromophore (4-O2). Spectroscopic (EPR, resonance Raman) and computational (density functional theory, DFT) analyses indicate that 4-O2 is a S = 1/2 species featuring a low-spin Co(III) center bound to an end-on (η1) superoxo ligand. DFT calculations were used to evaluate the energetics of key steps in the reaction mechanism. Collectively, these results have elucidated the role of electronic factors (e.g., spin-state, d-electron count, metal–ligand covalency) in facilitating O2 activation and S-dioxygenation in CDO and related models