Insights into the Mechanism and Catalysis of Oxime Coupling Chemistry at Physiological pH

The dynamic covalent‐coupling reaction involving α‐effect nucleophiles has revolutionized bioconjugation approaches, due to its ease and high efficiency. Key to its success is the discovery of aniline as a nucleophilic catalyst, which made this reaction feasible under physiological conditions. Aniline however, is not so effective for keto substrates. Here, we investigate the mechanism of aniline activation in the oxime reaction with aldehyde and keto substrates. We also present carboxylates as activating agents that can promote the oxime reaction with both aldehyde and keto substrates at physiological pH. This rate enhancement circumvents the influence of α‐effect by forming H‐bonds with the rate‐limiting intermediate, which drives the reaction to completion. The combination of aniline and carboxylates had a synergistic effect, resulting in a ～14–31‐fold increase in reaction rate at pD 7.4 with keto substrates. The biocompatibility and efficiency of carboxylate as an activating agent is demonstrated by performing cell‐surface oxime labeling at physiological pH using acetate, which showed promising results that were comparable with aniline.     

Structure of a liquid crystal of 4-cyano-4′-n-octyloxybiphenyl

The structure of a liquid crystal of 4-cyano-4′-n-octyloxybiphenyl (C₂₁H₂₅NO) is determined by X-ray diffraction analysis. The compound crystallizes in the triclinic crystal system with unit cell parameters a = 7.322(1) Å, b = 12.693(3) Å, c = 20.393(2) Å, α = 92.45(1)°, β = 99.96(1)°, γ = 99.35(2)°, and space group $P\bar 1$ . The structure is solved by the direct method and refined to R = 0.057. Two independent molecules are located in the asymmetric unit. No short intermolecular contacts are observed in the crystal packing.     

2,3,12,13‐Tetrabromo‐5,10,15,20‐tetrakis(4‐butoxyphenyl)porphyrin 1,2‐dichloroethane solvate

Nonmesogenic 2,3,12,13‐tetrabromo‐5,10,15,20‐tetrakis(4‐butoxyphenyl)porphyrin crystallizes as the title 1,2‐dichloroethane solvate, C₆₀H₅₈Br₄N₄O₄·C₂H₄Cl₂. The porphyrin ring shows a nonplanar conformation, with an average mean plane displacement of the β‐pyrrole C atoms from the 24‐atom (C₂₀N₄) core of ±0.50 (3) Å. The 1,2‐dichloroethane solvent is incorporated between the porphyrin units and induces the formation of one‐dimensional chains via interhalogen Cl...Br and butyl–aryl C—H...π interactions. These chains are oriented along the unit‐cell a axis, with the macrocyclic ring planes lying almost parallel to the (010) plane. The chains are arranged in an offset fashion by aligning the butoxy chains approximately above or below the faces of the adjacent porphyrin core, resulting in decreased interporphyrin π–π interactions, and they are held together by weak intermolecular (C—Br...π, C—H...π and C—H...Br) interactions. The nonplanar geometry of the macrocyclic ring is probably due to the weak interporphyrin interactions induced by the solvent molecule and the peripheral butoxy groups. The nonplanarity of the mesogens could influence the mesogenic behaviour differently relative to planar porphyrin mesogens.     

Vibrational spectroscopic studies and ab initio calculations of 5-methyl-2-(p-methylaminophenyl)benzoxazole

FT-IR spectra of 5-methyl-2-(p-methylaminophenyl)benzoxazole was recorded and analysed. The vibrational frequencies of the compound have been computed using the Hartree-Fock/6-31G* basis and compared with the experimental values.     

Conformational preferences of the aglycon moiety in models and analogs of GlcNAc-Asn linkage: crystal structures and ab initio quantum chemical calculations of N-(beta-D-glycopyranosyl)haloacetamides

The biological addition of oligosaccharide structures to asparagine residues of N-glycoproteins influences the properties and bioactivities of these macromolecules. The linkage region constituents, 2-acetamino-2-deoxy-beta-D-glucopyranose monosaccharide (GlcNAc) and L-asparagine amino acid (Asn), are conserved in the N-glycoproteins of all eukaryotes. In order to gain information about the structure and dynamics of glycosylated proteins, two chloroacetamido sugars, Glc betaNAcNHCOCH2Cl and Man betaNHCOCH2Cl, have been synthesized, and their crystal structures have been solved. Structural comparison with a series of other models and analogs gives insight about the influence of the N-acetyl group at position C2 on the conformation of the glycan-peptide linkage at C1. Interestingly, this N-acetyl group also influences the packing and network of hydrogen bonds with involvement in weak hydrogen bonds C-H...X that are of biological importance. DFT ab initio calculations performed on a series of models and analogs also confirm that the GlcNAc derivatives present different preferred conformation about the N-CO-CH2-X (chi2) torsion angle of the glycan-peptide linkage, when compared to other monosaccharide derivatives. The energy profiles that have been obtained will be useful for parametrization of molecular mechanics force-field. The conjunction of crystallographic and computational chemistry studies provides arguments for the structural effect of the N-acetyl group at C2 in establishing an extended conformation that presents the oligosaccharide away from the protein surface.     

Vibrational spectroscopic studies and ab initio calculations of 2-cyanophenylisocyanid dichloride

FT-Raman and FT-IR spectra of 2-cyanophenylisocyanid dichloride were recorded and analyzed. The vibrational frequencies of the title compound have been computed using the Hartree-Fock/6-31G* basis and compared with the experimental values. The prepared compound was identified by NMR and mass spectra.     

Surface modification of cetyltrimethylammonium bromide-capped gold nanorods to make molecular probes

A chemical procedure to replace the cetyltrimethylammonium bromide (CTAB) cap on gold nanorods (GNRs) fabricated through seed-mediated growth with organothiol compounds [3-animo-5-mercapto-1,2,4-triazole (AMTAZ) and 11-mercaptoundecaonic acid (MUDA)] was developed to reduce the cytotoxity of GNRs and facilitate further biofunctionalization. Compared to phosphatidylcholine (PC) modification, our procedure yields stable GNRs that are biocompatible and suitable for whole-cell studies. The PC-, AMTAZ-, and MUDA-activated GNRs all showed low cytotoxicity. By choosing different organothiols, net positive or negative charges could be created on the nanorod surface, for different applications. Gold nanorod molecular probes (GNrMPs) were fabricated by subsequent attachment of antibodies to the activated GNRs and were used to visualize and detect cell surface biomarkers in normal and transformed human breast epithelial cells, demonstrating the potential of developing novel biosensors using gold nanorods. The sensitivity of GNrMPs made from organothiol-activated GNRs is considerably higher than that of CTAB/PC-activated GNRs, demonstrating that the protocol reported here is favored in developing molecular probes using GNRs.     

Self-assembled hybrid polymer-TiO2 nanotube array heterojunction solar cells

Films comprised of 4 microm long titanium dioxide nanotube arrays were fabricated by anodizing Ti foils in an ethylene glycol based electrolyte. A carboxylated polythiophene derivative was self-assembled onto the TiO2 nanotube arrays by immersing them in a solution of the polymer. The binding sites of the carboxylate moiety along the polymer chain provide multiple anchoring sites to the substrate, making for a stable rugged film. Backside illuminated liquid junction solar cells based on TiO2 nanotube films sensitized by the self-assembled polymeric layer showed a short-circuit current density of 5.5 mA cm-2, a 0.7 V open circuit potential, and a 0.55 fill factor yielding power conversion efficiencies of 2.1% under AM 1.5 sun. A backside illuminated single heterojunction solid state solar cell using the same self-assembled polymer was demonstrated and yielded a photocurrent density as high as 2.0 mA cm-2. When a double heterojunction was formed by infiltrating a blend of poly(3-hexylthiophene) (P3HT) and C60-methanofullerene into the self-assembled polymer coated nanotube arrays, a photocurrent as high as 6.5 mA cm-2 was obtained under AM 1.5 sun with a corresponding efficiency of 1%. The photocurrent action spectra showed a maximum incident photon-to-electron conversion efficiency (IPCE) of 53% for the liquid junction cells and 25% for the single heterojunction solid state solar cells.     

Piscidinol-C, a triterpenoid from Walsura piscidia leaves

The X-ray crystal structure of the triterpenoid piscidinol-C from the leaves of Walsura piscidia is determined. The compound, which is an apotirucallane derivative, crystallizes in the orthorhombic space group C222₁ with a = 18.985(4), b = 13.092(5), c = 24.690(7) Å, and D _calc = 1.209 g cm^–1 for Z = 8.     

Crystallographic Investigations of 3β‐acetoxy‐5α‐cholestan‐6‐one‐semicarbazone ‐ A Steroid

The crystal structure of 3b‐acetoxy‐5a‐cholestan‐6‐one‐semicarbazone (C₃₀H₅₁O₃N₃) has been determined by X‐ray diffraction methods. It crystallizes in the orthorhombic space group P2₁2₁2₁ with cell parameters a = 11.641(1), b = 16.552(1) c = 31.181(4) Å and Z = 8. The structure has been refined to an R‐value of 0.050 for 4407 observed reflections. Two molecules in the asymmetric unit have been observed. In both the crystallographically independent molecules, all the three six‐membered rings (A, B and C ) of steroid nucleus exist in chair conformation, while the five‐ membered ring D exists in 13β distorted‐envelope in molecule‐I and 13β, 14α half‐chair conformation in molecule‐II. Three intermolecular N‐H … O hydrogen bonds have been observed.