A Facile Method for the Separation of Methionine Sulfoxide Diastereomers,Structural Assignment,and DFT Analysis

Methionine (Met) oxidation is an important biological redox node, with hundreds if not thousands of protein targets. The process yields methionine oxide (MetO). It renders the sulfur chiral, producing two distinct, diastereomerically related products. Despite the biological significance of Met oxidation, a reliable protocol to separate the resultant MetO diastereomers is currently lacking. This hampers our ability to make peptides and proteins that contain stereochemically defined MetO to then study their structural and functional properties. We have developed a facile method that uses supercritical CO₂ chromatography and allows obtaining both diastereomers in purities exceeding 99 %. ¹H NMR spectra were correlated with X-ray structural information. The stereochemical interconversion barrier at sulfur was calculated as 45.2 kcal mol⁻¹, highlighting the remarkable stereochemical stability of MetO sulfur chirality. Our protocol should open the road to synthesis and study of a wide variety of stereochemically defined MetO-containing proteins and peptides.     

Iridium‐Catalyzed Allylic Substitutions with Cyclometalated Phosphoramidite Complexes Bearing a Dibenzocyclooctatetraene Ligand: Preparation of (π‐Allyl)Ir Complexes and Computational and NMR Spectroscopic Studies

(π‐Allyl)Ir complexes derived from dibenzocyclooctatetraene and phosphoramidites by cyclometalation are effective catalysts for allylic substitution reactions of linear monosubstituted allylic carbonates. These catalysts provide exceptionally high degrees of regioselectivity and allow the reactions to be run under aerobic conditions. A series of (π‐allyl)Ir complexes were prepared and characterized by X‐ray crystal structure analyses. An allylic amination with aniline displayed different resting states depending on the presence of a strong base. DFT calculations were carried out on the mechanistic aspects of these reactions. The results suggest that for the (π‐allyl)Ir complexes, the formation and reactions with nucleophiles proceed with comparable rates.     

A crystal-structural study of Pauling–Corey rippled sheets

Following the seminal theoretical work on the pleated β-sheet published by Pauling and Corey in 1951, the rippled β-sheet was hypothesized by the same authors in 1953. In the pleated β-sheet the interacting β-strands have the same chirality, whereas in the rippled β-sheet the interacting β-strands are mirror-images. Unlike with the pleated β-sheet that is now common textbook knowledge, the rippled β-sheet has been much slower to evolve. Much of the experimental work on rippled sheets came from groups that study aggregating racemic peptide systems over the course of the past decade. This includes MAX1/DMAX hydrogels (Schneider), L/D-KFE8 aggregating systems (Nilsson), and racemic Amyloid β mixtures (Raskatov). Whether a racemic peptide mixture is “ripple-genic” (i.e., whether it forms a rippled sheet) or “pleat-genic” (i.e., whether it forms a pleated sheet) is likely governed by a complex interplay of thermodynamic and kinetic effects. Structural insights into rippled sheets remain limited to only a very few studies that combined sparse experimental structural constraints with molecular modeling. Crystal structures of rippled sheets are needed so we can rationally design rippled sheet architectures. Here we report a high-resolution crystal structure, in which (l,l,l)-triphenylalanine and (d,d,d)-triphenylalanine form dimeric antiparallel rippled sheets, which pack into herringbone layer structures. The arrangements of the tripeptides and their mirror-images in the individual dimers were in excellent agreement with the theoretical predictions by Pauling and Corey. A subsequent mining of the PDB identified three orphaned rippled sheets among racemic protein crystal structures.

Following the seminal theoretical work on the pleated β-sheet published by Pauling and Corey in 1951, the rippled β-sheet was hypothesized by the same authors in 1953.     

Asymmetric catalysis with 7-ring chelate diphosphines: DIOP,BINAP and conformational mobility

The conformations of two classes of 7-ring diphosphine metal chelate have been analysed, based on DIOP- or BINAP-type structures. A combination of X-ray analysis, DFT calculations, data analysis based on the CDS structure database and solution NMR studies has been employed. The conformational flexibility of DIOP-type structures has been endorsed, and the scope of BINAP complex flexibility defined. BINAP complexes possess an intrinsic conformational mobility. Analysis of the metal-adjacent torsion angles C–P–M–P′ and C′–P′–M–P provides a useful probe for the ligand–metal environment, and may be more generally useful.     

Pharmacokinetics of Py-Im polyamides depend on architecture: cyclic versus linear

The pharmacokinetic properties of three pyrrole-imidazole (Py-Im) polyamides of similar size and Py-Im content but different shape were studied in the mouse. Remarkably, hairpin and cyclic oligomers programmed for the same DNA sequence 5'-WGGWWW-3' displayed distinct pharmacokinetic properties. Furthermore, the hairpin 1 and cycle 2 exhibited vastly different animal toxicities. These data provide a foundation for design of DNA binding Py-Im polyamides to be tested in vivo.     

Well defined thermostable cellulose nanocrystals via two-step ionic liquid swelling-hydrolysis extraction

Cellulose nanocrystals were prepared from cotton fibers by a two-stage method involving ionic liquid swelling treatment followed by hydrolysis under mild acid conditions. Controlled swelling of cellulosic fibers was achieved in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) at 80 °C, while avoiding extensive dissolution of crystalline regions. Since the accessibility of the substrate was considerably enhanced, the hydrolysis occurred even under mild conditions, using up to 60 times less sulfuric acid than the traditional extraction methods based on concentrated sulfuric acid. The effects of process parameters on nanoparticle morphology, composition and stability were investigated. The individual rod-like nanocrystals, observed under field emission gun scanning electron microscopy, exhibited an average diameter of around 20 nm and a length ranging from 150 to 350 nm. According to X-ray photoelectron spectroscopy and thermogravimetric analysis, the surface of the so-extracted nanoparticles proved to be deprived of contaminating sulfate groups leading to significantly higher thermal stability with respect to cellulose nanocrystals extracted by traditional method in concentrated sulfuric acid.     

The rippled β-sheet layer configuration—a novel supramolecular architecture based on predictions by Pauling and Corey

The rippled β-sheet is a peptidic structural motif related to but distinct from the pleated β-sheet. Both motifs were predicted in the 1950s by Pauling and Corey. The pleated β-sheet was since observed in countless proteins and peptides and is considered common textbook knowledge. Conversely, the rippled β-sheet only gained a meaningful experimental foundation in the past decade, and the first crystal structural study of rippled β-sheets was published as recently as this year. Noteworthy, the crystallized assembly stopped at the rippled β-dimer stage. It did not form the extended, periodic rippled β-sheet layer topography hypothesized by Pauling and Corey, thus calling the validity of their prediction into question. NMR work conducted since moreover shows that certain model peptides rather form pleated and not rippled β-sheets in solution. To determine whether the periodic rippled β-sheet layer configuration is viable, the field urgently needs crystal structures. Here we report on crystal structures of two racemic and one quasi-racemic aggregating peptide systems, all of which yield periodic rippled antiparallel β-sheet layers that are in excellent agreement with the predictions by Pauling and Corey. Our study establishes the rippled β-sheet layer configuration as a motif with general features and opens the road to structure-based design of unique supramolecular architectures.

The rippled sheet was proposed by Pauling and Corey in 1953, yet structural foundation remains extremely limited. Here we report on three X-ray crystal structures of rippled β-sheets, thus providing the long-sought structural basis for the field.     

The transfer of natural (226Ra,?232Th, 40K) and artificial (137Cs, 90Sr) radionuclides from soil to the wood of a Scots pine (Pinus sylvestris L.)

On the Stabatishke site near to Ignalina Nuclear Power Plant a near surface repository for low and intermediate-level short-lived radioactive waste is being constructed to store the waste which was produced during the decommissioning of the Ignalina Nuclear Power Plant. A possible spread of radionuclides from the near surface repository and the radiation level of expression are one at the most important aspects while evaluating the safety of the repository. The article examines the specific activity of artificial (¹³⁷Cs, ⁹⁰Sr) and natural (²²⁶Ra, ²³²Th, ⁴⁰K) radionuclides in the soil of Stabatishke site of Ignalina NPP in pinewood; the spread of these radionuclides is determined in the system “soil-tree”. The change of long term soil pollution with artificial radionuclides was estimated. This is a background soil pollution which had been there before the exploitation of the near surface waste repository. The transfer factors of natural (²²⁶Ra, ²³²Th, ⁴⁰K) and artificial (¹³⁷Cs, ⁹⁰Sr) radionuclides from the 0-to-20-cm soil layer to different annual pinewood rings were measured. After estimation of the position of pine roots in accordance to the soil vertical and the annual change of the plant biomass, the transfer factor of ¹³⁷Cs movement to different annual pinewood rings from the separate 0 to 20-cm soil layers was determined.