Search for: All records

Abstract The structures and associated functions of biological molecules are driven by noncovalent interactions, which have classically been dominated by the hydrogen bond (H‐bond). Introduction of the σ‐hole concept to describe the anisotropic distribution of electrostatic potential of covalently bonded elements from across the periodic table has opened a broad range of nonclassical noncovalent ( nc NC) interactions for applications in chemistry and biochemistry. Here, we review how halogen bonds, chalcogen bonds and tetrel bonds, as they are found naturally or introduced synthetically, affect the structures, assemblies, and potential functions of peptides and proteins. This review intentionally focuses on examples that introduce or support principles of stability, assembly and catalysis that can potentially guide the design of new functional proteins. These three types of nc NC interactions have energies that are comparable to the H‐bond and, therefore, are now significant concepts in molecular recognition and design. However, the recently described H‐bond enhanced X‐bond shows how synergism among nc NC interactions can be exploited as potential means to broaden the range of their applications to affect protein structures and functions. 

Abstract CeOs₄Sb₁₂, a member of the skutterudite family, has an unusual semimetallic low-temperature $\mathcal{L}$-phase that inhabits a wedge-like area of the fieldH—temperatureTphase diagram. We have conducted measurements of electrical transport and megahertz conductivity on CeOs₄Sb₁₂single crystals under pressures of up to 3 GPa and in high magnetic fields of up to 41 T to investigate the influence of pressure on the differentH–Tphase boundaries. While the high-temperature valence transition between the metallic $\mathcal{H}$-phase and the $\mathcal{L}$-phase is shifted to higherTby pressures of the order of 1 GPa, we observed only a marginal suppression of the $\mathcal{S}$-phase that is found below 1 K for pressures of up to 1.91 GPa. High-field quantum oscillations have been observed for pressures up to 3.0 GPa and the Fermi surface of the high-field side of the $\mathcal{H}$-phase is found to show a surprising decrease in size with increasing pressure, implying a change in electronic structure rather than a mere contraction of lattice parameters. We evaluate the field-dependence of the effective masses for different pressures and also reflect on the sample dependence of some of the properties of CeOs₄Sb₁₂which appears to be limited to the low-field region. 

Abstract The imaging of individual Ba²⁺ions in high pressure xenon gas is one possible way to attain background-free sensitivity to neutrinoless double beta decay and hence establish the Majorana nature of the neutrino. In this paper we demonstrate selective single Ba²⁺ion imaging inside a high-pressure xenon gas environment. Ba²⁺ions chelated with molecular chemosensors are resolved at the gas-solid interface using a diffraction-limited imaging system with scan area of 1 × 1 cm²located inside 10 bar of xenon gas. This form of microscopy represents key ingredient in the development of barium tagging for neutrinoless double beta decay searches in¹³⁶Xe. This also provides a new tool for studying the photophysics of fluorescent molecules and chemosensors at the solid-gas interface to enable bottom-up design of catalysts and sensors. 

Abstract Modified DNA bases functionally distinguish the taxonomic forms of life—5-methylcytosine separates prokaryotes from eukaryotes and 5-hydroxymethylcytosine (5hmC) invertebrates from vertebrates. We demonstrate here that mouse endonuclease G (mEndoG) shows specificity for both 5hmC and Holliday junctions. The enzyme has higher affinity (>50-fold) for junctions over duplex DNAs. A 5hmC-modification shifts the position of the cut site and increases the rate of DNA cleavage in modified versus unmodified junctions. The crystal structure of mEndoG shows that a cysteine (Cys69) is positioned to recognize 5hmC through a thiol-hydroxyl hydrogen bond. Although this Cys is conserved from worms to mammals, a two amino acid deletion in the vertebrate relative to the invertebrate sequence unwinds an α-helix, placing the thiol of Cys69 into the mEndoG active site. Mutations of Cys69 with alanine or serine show 5hmC-specificity that mirrors the hydrogen bonding potential of the side chain (C–H < S–H < O–H). A second orthogonal DNA binding site identified in the mEndoG structure accommodates a second arm of a junction. Thus, the specificity of mEndoG for 5hmC and junctions derives from structural adaptations that distinguish the vertebrate from the invertebrate enzyme, thereby thereby supporting a role for 5hmC in recombination processes.