Precursor Backbone Research Articles

Preparation and properties of polybenzoxazole–silica nanocomposites via sol–gel process

A novel organic/inorganic hybrid material has been prepared through the sol–gel process. A high temperature polymer, polybenzoxazole (PBO), was chosen as the organic phase due to its inherent low dielectric constant and low water absorption. The inorganic phase was generated via sol–gel reaction from a silica precursor, phenyltriethoxysilane (PTEOS). Due to the hydroxyl groups in the PBO precursor backbone and the water release during the cyclization of the precursor, the sol–gel reaction proceeded without the addition of water and any catalyst. After curing at 350 °C, we obtained the PBO/silica nanocomposites. From TEM and SEM photographs, the silica particles dispersed in the PBO matrix were nano-sized. With an addition of 100 wt% of PTEOS, the T g of PBO was increased 35 °C. The dielectric constant of the hybrid materials increased with the increasing amount of PTEOS.

Synthesis of Large-Pore Methylene-Bridged Periodic Mesoporous Organosilicas and Its Implications

In this article, we report the synthesis of methylene-bridged periodic mesoporous organosilicas (PMOs) of the SBA-15 type. The materials were characterized by SAXS, BET, NMR, FESEM, and TEM. It was found that the synthesis of methylene-bridged SBA-15 PMOs requires more rigorous conditions than that of SBA-15 PMOs bearing organic bridges other than methylene. A mild acidic environment, which slows down the hydrolysis and condensation rates of the precursor, with the assistance of a salt, which enhances precursor-template interaction, should be used to synthesize high-quality large-pore methylene-bridged PMOs. We attributed this to the fast hydrolysis and condensation rates and the rigid backbone of precursor 1,2-bis(triethoxysilyl)methylene. By examining and comparing the synthesis of three large-pore PMOs with different bridges, we concluded that the inductive, bridging, and conformation effects of the organic bridging group play an important role in the synthesis of large-pore PMO materials.

Synthesis, characterization and crystal structure of a Er III mono-substituted [α-SiW 11O 39] 8− polyoxotungstate containing one-dimensional zigzag chainlike structure

A Keggin-type monovacant silicotungstate derivative K 2H 2{Er(H 2O) 6[Er(H 2O) 3SiW 11O 39]} 2·22H 2O ( 1) has been synthesized by reaction of the polyoxometalate α-K 8SiW 11O 39·13H 2O ligands with erbium III cations in the aqueous solution under moderate condition, and characterized by elemental analyses, IR, UV-visible spectroscopy and X-ray single-crystal diffraction. Crystal data: K 2H 2{Er(H 2O) 6[Er(H 2O) 3SiW 11O 39]} 2·22H 2O, triclinic, P−1, a=12.463(3), b=14.849(3), c=16.772(3) Å, α=95.37(3), β=94.65(3), γ=106.76°, Z=1, R 1=0.0638, wR 2=0.1140. X-ray single-crystal diffraction reveals that the polyanion of crystalline 1 is made up of two crystallographically identical polyoxometalate-based {Er(H 2O) 6[Er(H 2O) 3SiW 11O 39]} 2− building block units, which are linked through erbium centers by means of the terminal oxygen atoms from the backbone of precursor [α-SiW 11O 39] 8−, generating a novel 1D chain-like structure. Variable-temperature magnetic susceptibility study indicates the presence of the relatively strong spin-orbital coupling in compound 1 in the wide temperature region of 70–300 K.

Toward the total synthesis of methyl isosartortuoate: construction of the backbone of the diene unit

The backbone of diene precursor in the proposed biogenesis of methyl isosartortuoate through a Diels–Alder reaction has been constructed via dehydration to establish the conjugated system; and double cyclization by using Horner–Wittig–Emmons reaction and the chiral epoxy ring opening.

Toward the total synthesis of methyl isosartortuoate: construction of the backbone of the diene unit

The backbone of diene precursor in the proposed biogenesis of methyl isosartortuoate through a Diels–Alder reaction has been constructed via dehydration to establish the conjugated system; and double cyclization by using Horner–Wittig–Emmons reaction and the chiral epoxy ring opening.

Glass transition behaviours in aromatic poly(amic dialkyl ester) precursors with various chain rigidities

Five different poly(amic dialkyl ester) precursors with various chain rigidities were synthesized and their powder and film samples were prepared in residual solvent free: poly(4,4′-oxydiphenylene biphenyltetracarboxamic diethyl ester); poly( p-phenylene biphenyltetracarboxamic diethyl ester); poly( p-phenylene biphenyltetracarboxamic dimethyl ester); poly(4,4′-oxydiphenylene pyromellitamic diethyl ester); and poly( p-phenylene pyromellitamic diethyl ester). Glass transition temperatures ( T gs) of these precursors, which have never been reported so far, were successfully determined in the present study by the new oscillating differential scanning calorimetry (o.d.s.c.). In addition, their imidization behaviours and morphological structures were investigated by o.d.s.c. and X-ray diffraction, respectively. The measured T gs are in the range 179.2–235.4°C, depending upon the precursor backbone structure as well as the history of sample preparation: these T gs are very close to the imidization temperatures ( T is), causing difficulties in the T g measurements. The measured T is are in the range 198.1–235.4°C, depending upon the precursor backbone structure as well as the history of sample preparation. The T gs are dependent on the chain rigidity and the morphological structure, whereas the T is are influenced by the chemical nature in addition to the chain rigidity and the morphological structure.

Synthesis of well-defined poly(alkyl methacrylate)-graft-polylactone by sequential living polymerization

A novel combination of living polymerization reactions has been proposed for the controlled synthesis of poly(alkyl methacrylate)-graft-polylactones. This strategy relies upon the sequential living polymerization of alkyl methacrylates and aliphatic lactones, with an intermediate chemical transformation for shifting from the first mechanism to the second one. In the first step, an alkyl methacrylate (methyl and butyl) is copolymerized with 2-trimethylsiloxyethyl methacrylate (TMSEMA). This living anionic polymerization is initiated with diphenylhexyllithium( DPHLi) in the presence of a μ-ligand, lithium chloride, in THF at –78°C. The trimethylsiloxy groups are then hydrolyzed with release of hydroxyl groups which are reacted with triethylaluminum in order to form a multifunctional macroinitiator of the Al alkoxide type. The second step consists of the ring opening polymerization (ROP) of aliphatic lactones (ϵ-caprolactone, δ-valerolactone and 1,4,8-trioxaspiro[4.6]-9-undecanone (TOSUO)) with the formation of novel graft copolymers. As a result of the livingness of both the anionic and the ROP polymerization steps, the molecular weight of both the main backbone and the grafts is predictable, the apparent polydispersity is narrow (Mw/Mn from 1.05 to 1.30) and the grafting density can be controlled being dependent on the distribution of the hydroxyl groups within the precursor backbone.

Synthesis of well-defined poly(alkyl methacrylate)-graft-polylactone by sequential living polymerization

A novel combination of living polymerization reactions has been proposed for the controlled synthesis of poly(alkyl methacrylate)-graft-polylactones. This strategy relies upon the sequential living polymerization of alkyl methacrylates and aliphatic lactones, with an intermediate chemical transformation for shifting from the first mechanism to the second one. In the first step, an alkyl methacrylate (methyl and butyl) is copolymerized with 2-trimethylsiloxyethyl methacrylate (TMSEMA). This living anionic polymerization is initiated with diphenylhexyllithium( DPHLi) in the presence of a μ-ligand, lithium chloride, in THF at –78°C. The trimethylsiloxy groups are then hydrolyzed with release of hydroxyl groups which are reacted with triethylaluminum in order to form a multifunctional macroinitiator of the Al alkoxide type. The second step consists of the ring opening polymerization (ROP) of aliphatic lactones (ϵ-caprolactone, δ-valerolactone and 1,4,8-trioxaspiro[4.6]-9-undecanone (TOSUO)) with the formation of novel graft copolymers. As a result of the livingness of both the anionic and the ROP polymerization steps, the molecular weight of both the main backbone and the grafts is predictable, the apparent polydispersity is narrow (Mw/Mn from 1.05 to 1.30) and the grafting density can be controlled being dependent on the distribution of the hydroxyl groups within the precursor backbone.

Synthesis and Properties of an EGF-like Domain (residues 361-406) in the Extreme N-terminal Region of the Mouse EGF Precursor

Various proteins contain EGF-like domains that are not ligands for the EGF receptor. In the present study a cognate polypeptide for residues 361-406 of the mouse EGF precursor was synthesized by the solid-phase method. The product was renatured under oxidative conditions since it probably has an EGF-like array of three cystine disulfide bonds in its native state. HPLC analysis of the renaturation reaction revealed formation of a peak material with no apparent free-SH groups. Accordingly, the HPLC retention time of this product was readily increased by treatment(reduction of disulfides) with dithiothreitol. The renatured 46-mer (PEGF-1) did not displace 125 I-EGF bound to rat liver membranes and 125I-PEGF-l did not exhibit specific binding to membrane preparations from the mouse liver, mammary gland, or kidney, with or without Ca2+ in the binding medium. Although PEGF-1 contains a putative Ca2+ binding motif, specific binding of this cation by the polypeptide could not be demonstrated by electromobility shiff or incubation with 45Ca2+. Immunoassay of PEGF-1 and EGF in fractions obtained following gel filtration of mouse urine revealed multiple peaks of PEGF-1 immunoreactivity with the major peaks eluting at an Mr > 30 kDa. In contrast, virtually all the EGF immunoreactivity eluted at a volume similar to that of 125I-EGF. These data suggest that selective cleavage of the PEGF-1 domain from the precursor does not occur with the proclivity known for that of EGF. Instead, the PEGF-1 probably functions coordinately with other EGF-like domains while tethered to the precursor backbone. Finally, localization of PEGF-1 immunoreactivity occurred only in cell populations of the mouse previously demonstrated as sites for EGF/EGF precursor, which suggests that PEGF-1 is exclusively a domain of the EGF precursor.

Optimization of Ormosil Films for Optical Sensor Applications

Recent work has indicated that Ormosil films, fabricated from organically modified precursors, produce better sensor performance for some specific applications, compared to films fabricated from conventional sol-gel precursors such as TEOS or TMOS. This paper aims to compare film properties and sensor behavior for films fabricated from tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) silica precursors and both methyltrimethoxysilane (MTMS) and methyltriethoxysilane (MTES) organically modified precursors. Microstructural differences, for example, porosity changes due to the different precursor backbone structures, are interrogated by monitoring oxygen gas and aqueous-phase sensor response. Oxygen sensing using these films is enabled by incorporating in the films an oxygen-sensitive ruthenium dye whose fluorescence is quenched in the presence of oxygen. Film properties such as thickness, thickness stabilization time, as well as sensor response, are discussed in terms of relative hydrolysis and condensation behavior for the different precursors. Film hydrophobicity, an issue which has been identified as being of crucial importance for optimum dissolved oxygen sensor response, is discussed and contact angle measurements are used to investigate the degree of hydrophobicity for different film types. The main motivation for this work is film optimization for optical gas-phase and dissolved oxygen sensors.

Post‐Translational Heterocyclic Backbone Modifications in the 43‐Peptide Antibiotic Microcin B17

Microcin B17 (McB17), the first known gyrase inhibitor of peptidic nature, is produced by ribosomal synthesis and post-translational modification of the 69-residue precursor protein by an Escherichia coli strain. To elucidate the chemical structure of the mature 43-residue peptide antibiotic, fermentation and purification protocols were established and optimized which allowed the isolation and purification of substantial amounts of highly pure McB17 (non-labelled, 15N-labelled and 13C/15N-labelled peptide. By ultraviolet-absorption spectroscopy. HPLC-electrospray mass spectrometry and GC-mass spectrometry, amino acid analysis, protein sequencing, and, in particular, multidimensional NMR, we could demonstrate and unequivocally prove that the enzymic modification of the precursor backbone at Gly-Cys and Gly-Ser segments leads to the formation of 2-aminomethylthiazole-4-carboxylic acid and 2-aminomethyloxazole-4-carboxylic acid, respectively. In addition, two bicyclic modifications 2-(2-aminomethyloxazolyl)thiazole-4-carboxylic acid and 2-(2-aminomethylthiazolyl)oxazole-4-carboxylic acid were found that consist of directly linked thiazole and oxazole rings derived from one Gly-Ser-Cys and one Gly-Cys-Ser segment. Analogous to the thiazole and oxazole rings found in antitumor peptides of microbial and marine origin, these heteroaromatic ring systems of McB17 presumably play an important role in its gyrase-inhibiting activity, e.g. interacting with the DNA to trap the covalent protein-DNA intermediate of the breakage-reunion reaction of the gyrase.

Thermal imidization behavior of aromatic poly(amic dialkyl ester) precursors derived from biphenyltetracarboxylic dianhydride

Poly(amic dialkyl ester) precursors of high performance poly(4,4′-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA) and poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) were synthesized and their thermal imidizations were investigated isothermally and nonisothermally by thermogravimetry. All precursors showed a two-step imidization behavior, fast imidization at early stage and slow imidization at intermediate and later stage. Kinetic parameters were estimated from the conversion versus time plots with the first-order rate equation. The imidization behaviors observed were interpreted with considering the Tg variation, the natures of alkyl leaving group and precursor backbone, and the flexibility of polymer chain. In addition, the onset temperature of imidization and Tg of precursors were estimated from the imidization kinetic data.

Blood group antigens in normal and neoplastic urothelium.

The ABO and Lewis blood group antigens are cell surface carbohydrate determinants formed by the sequential addition of saccharides to precursor backbone structures of membrane lipids and proteins. Suppression of normally active glycosyltransferases results in the absence of antigens that are normally expressed. ABH antigen deletion in malignant and premalignant urothelium has been extensively evaluated; it appears to correlate with significantly higher rates of tumor recurrence and disease progression. However, we have recently shown that the ABH blood group system is differentially expressed in the normal urothelium of secretors in contrast to nonsecretor individuals. The normal urothelium of nonsecretors does not express A, B or H determinants; therefore, deletion of ABH antigens can only be ascertained in secretor individuals. Although nonsecretors only comprise 22-24% of the population, this observation mandates a reevaluation of earlier studies. Deletion of A, B or H antigens is noted in carcinoma in situ (CIS), and in invasive and metastatic transitional cell carcinoma (TCC) of secretor individuals. Increased synthesis or activation of normally quiescent glycosyltransferases in bladder tumors can result in the expression of aberrant tumor-associated blood group antigens. Immunohistochemical analysis has demonstrated that Lewis X (Le(x)) is not detected in normal adult urothelium except for occasional umbrella cells. However, papillomas, CIS and TCC expressed Le(x) in 84% of cases, regardless of grade, stage, blood type or secretor status of the individual. The presence of Le(x)-positive cells in bladder lavage specimens enhanced the detection of urothelial tumor cells, correctly identifying bladder tumors in 253/293 (86%) cases compared to a 63% sensitivity for cytology alone.(ABSTRACT TRUNCATED AT 250 WORDS)