Biochemistry of Nucleic Acids Functionalized with Sulfur, Selenium, and Tellurium: Roles of the Single-Atom Substitution

Photoredox-Catalyzed Synthesis of C-Benzoselenazolyl/Benzothiazolyl Glycosides from 2-Isocyanoaryl Selenoethers/Thioethers and Glycosyl Bromides

Molecules containing heteroatoms, such as Se and S, play an indispensable role in the discovery and design of pharmaceuticals, whereas Se has been less studied. Here, we described a photoredox strategy to synthesize C-benzoselenazolyl (Bs) glycosides from 2-isocyanoaryl selenoethers and glycosyl bromides. This reaction was carried out under mild conditions with high efficiency. C-Benzothiazolyl (Bt) glycosides could also be synthesized from 2-isocyanoaryl thioethers using this strategy. This method can access novel seleno/thiosugars, which will benefit Se/S-containing drug discovery.

Optical and electronic properties enhancement via chalcogenides: promising materials for DSSC applications

CONTEXT

Comparatively, metal-free sensitizers featuring the chalcogen family receive less attention despite known electronic properties for metal-chalcogenide materials. This work reports an array of optoelectronic properties using quantum chemical methods. Observed red-shifted bands within the UV/Vis to NIR regions with absorption maxima > 500 nm were consistent with increasing chalcogenide size. There is a monotonic down-shift in the LUMO and ESOP energy consistent with O 2p, S 3p, Se 4p, to Te 5p atomic orbital energies. Excited-state lifetime and charge injection free energies follow the decreasing order of chalcogenide electronegativity. Adsorption energies of dyes on TiO₂ anatase (101) range between - 0.08 and - 0.77 eV. Based on evaluated properties, selenium- and tellurium-based materials show potential use in DSSCs and futuristic device applications. Therefore, this work motivates continued investigation of the chalcogenide sensitizers and their application.

METHODS

The geometry optimization was performed at B3LYP/6-31 + G(d,p) and B3LYP/LANL2DZ level of theory for lighter and heavier atoms, respectively, using Gaussian 09. The equilibrium geometries were confirmed by the absence of imaginary frequencies. Electronic spectra were obtained at CAM-B3LYP/6-31G + (d,p)/LANL2DZ level of theory. Adsorption energies for dyes on a 4 × 5 supercell TiO₂ anatase (101) were obtained using VASP. The dye-TiO₂ optimizations were employed using GGA and PBE with the PAW pseudo-potentials. The energy cutoff was set at 400 eV and convergence threshold for self-consistent iteration was set to 10^-4, and van der Waals were accounted using DFT-D3 model and on-site Coulomb repulsion potential set at 8.5 eV for Ti.

Mechanistic Photophysics of Tellurium-Substituted Uracils: Insights from Multistate Complete-Active-Space Second-Order Perturbation Calculations

The photophysical mechanisms of tellurium-substituted uracils were studied at the multistate complete-active-space second-order perturbation level with a particular focus on how the position and number of tellurium substitutions affect their nonadiabatic relaxation processes. Electronic structure analysis reveals that the lowest several excited states are closely concerned with the n and π orbitals at the Te₇-C₂ [Te₈-C₄] moiety of 2-tellurouracil (2TeU) [4TeU and 24TeU]. Both planar and twisted minima were optimized for 2TeU, whereas only planar ones were obtained for 4TeU and 24TeU, except for a twisted T₁ minimum of 4TeU. Based on intersection structures and linearly interpolated internal coordinate paths, we proposed several feasible excited-state deactivation paths. It is found that the relaxation channels for 2TeU are more complicated than those of 4TeU and 24TeU. The electronic population transfer to the T₁ state for 2TeU is easier than that for 4TeU and 24TeU in consideration of the barrier heights from the S₂ Franck-Condon point to the S₂/S₁ or S₂/T₂ intersections. In addition, the recovery of the ground state from the T₁ state for 2TeU will be more efficient than that for the other two systems as well.

Synthesis of 4-Selenothreofuranose Derivatives via Pummerer-Type Reactions of trans-3,4-Dioxygenated Tetrahydroselenophenes Mediated by a Selenonium Intermediate

Selenosugars are interesting targets of organic synthesis as they would possess potential biological activities. However, 4-selenotherofuranose derivatives, which have trans configuration for the two dihydroxy substituents at the 2,3-positions and a glycoside bond at the anomeric position, are not available in the current selenosugar library. In this study, racemic 4-selenothreofuranose derivatives were synthesized from trans-3,4-dioxygenated tetrahydroselenophenes in 77-99% yields with the α/β selectivity about 7:3 via oxidation and subsequent seleno-Pummerer rearrangement. The acetoxy group introduced at the anomeric position was then substituted with various nucleophiles, including activated 6-chloropurine, which afforded 4'-selenothreonucleoside derivatives, in the presence of BF₃·OEt₂ or TMSOTf. The stereochemistry of the selenosugar products was established by ¹H NMR spectroscopy as well as X-ray analysis. The similar α/β selectivity observed in the latter glycosylation reaction to that in the former seleno-Pummerer rearrangement suggested the mediation of a common selenonium intermediate (-Se⁺=C<). It was also suggested that an unexpected interaction between the ester protecting group at the 3-position of the selenofuranose ring and the anomeric carbon atom decreases the α/β selectivity.

Validation of l-Tellurienylalanine as a Phenylalanine Isostere

Mass cytometry (MC) and imaging mass cytometry (IMC^TM ) have emerged as important tools for the study of biological heterogeneity. We recently demonstrated the use of l-2-tellurienylalanine (TePhe), a mimic of phenylalanine (Phe), as an MC- and IMC-compatible protein synthesis reporter. In this work, the biochemical similarity of TePhe and its cognate analogue, Phe, are examined in the context of the RNase S complex. Isothermal titration calorimetry studies show that incorporation of TePhe preserves the interaction of S-peptide with S-protein, and the dissociation constants for the interaction of the Phe and TePhe peptides are within a factor of two. The resulting RNase S complex is catalytically active without significant alterations in the enzyme's kinetic parameters. Furthermore, circular dichroism spectroscopy does not reveal any changes to the secondary structure of TePhe-substituted RNase S. These findings provide strong evidence that TePhe functions as a Phe isostere in the context of a folded protein. It is anticipated that incorporation of TePhe into peptides or peptidomimetic scaffolds will enable facile generation of MC and IMC^TM probes.