Short-range structure, the role of bismuth and property-structure correlations in bismuth borate glasses.

Bismuth-containing borate glasses, xBi2O3-(1 - x)B2O3, were synthesized in the broad composition range 0.20 ≤ x ≤ 0.80 by melting in Pt crucibles and splat-quenching between two metal blocks. Infrared reflectance spectra, measured in the range 30-5000 cm-1, were transformed into absorption coefficient spectra and then deconvoluted into component bands to probe the glass structure as a function of composition. Integrated intensities of bands above 800 cm-1 were used in combination with mass and charge balance equations to quantify the short-range borate structure in terms of the molar fractions X4m, X4o, X3, X2, X1 and X0 for borate units BØ4-, BØ2O23-, BØ3, BØ2O-, BØO22- and BO33-, where Ø and O- denote bridging and non-bridging oxygen atoms. Borate tetrahedral units were found to be present in both the meta-borate, BØ4-, and ortho-borate, BØ2O23-, forms with BØ4- constituting the dominating tetrahedral species for 0.20 ≤ x ≤ 0.70. The BØ2O23- units prevail at higher Bi2O3 levels (x > 0.7), and coexist with their isomeric triangular borate species BO33- (BØ2O23- ⇌ BO33-). The present IR results for the total molar fraction of borate tetrahedral units, X4 = X4m + X4o, are in very good agreement with reported NMR results for the fraction of boron atoms in four-fold coordination, N4. Besides evaluating X4m and X4o, the present work reports also for the first time the fractions of all types of triangular borate species X3-n with n = 0, 1, 2 and 3. The IR region below 550 cm-1 was found to be dominated by the Bi-O vibrational activity in coexisting ionic (160-230 cm-1) and distorted BiO6 sites (330-365 cm-1 and 475-510 cm-1), a result reflecting the dual role of Bi2O3 as glass-modifier and glass-former oxide. The latter role dominates in glasses exceeding 60 mol% Bi2O3, and is consistent with the extended glass formation in the bismuth-borate system. The structural results were used to calculate the average number of bridging B-Ø bonds per boron center, the average Bi-O and B-O single bond energy, and the atomic packing density of the studied glasses. These properties vary approximately linearly with Bi2O3 content in the three regimes 0.2 ≤ x ≤ 0.4, 0.4 < x ≤ 0.6 and 0.6 < x ≤ 0.83, and contribute collectively to the composition dependence of glass transition temperature.

Transition and post-transition metal ions in borate glasses: Borate ligand speciation, cluster formation, and their effect on glass transition and mechanical properties.

A series of transition and post-transition metal ion (Mn, Cu, Zn, Pb, Bi) binary borate glasses was studied with special consideration of the cations impact on the borate structure, the cations cross-linking capacity, and more generally, structure-property correlations. Infrared (IR) and Raman spectroscopies were used for the structural characterization. These complementary techniques are sensitive to the short-range order as in the differentiation of tetrahedral and trigonal borate units or regarding the number of non-bridging oxygen ions per unit. Moreover, vibrational spectroscopy is also sensitive to the intermediate-range order and to the presence of superstructural units, such as rings and chains, or the combination of rings. In order to clarify band assignments for the various borate entities, examples are given from pure vitreous B2O3 to meta-, pyro-, ortho-, and even overmodified borate glass compositions. For binary metaborate glasses, the impact of the modifier cation on the borate speciation is shown. High field strength cations such as Zn2+ enhance the disproportionation of metaborate to polyborate and pyroborate units. Pb2+ and Bi3+ induce cluster formation, resulting in PbOn- and BiOn-pseudophases. Both lead and bismuth borate glasses show also a tendency to stabilize very large superstructural units in the form of diborate polyanions. Far-IR spectra reflect on the bonding states of modifier cations in glasses. The frequency of the measured cation-site vibration band was used to obtain the average force constant for the metal-oxygen bonding, FM-O. A linear correlation between glass transition temperature (Tg) and FM-O was shown for the metaborate glass series. The mechanical properties of the glasses also correlate with the force constant FM-O, though for cations of similar force constant the fraction of tetrahedral borate units (N4) strongly affects the thermal and mechanical properties. For paramagnetic Cu- and Mn-borate glasses, N4 was determined from the IR spectra after deducing the relative absorption coefficient of boron tetrahedral versus boron trigonal units, α = α4/α3, using NMR literature data of the diamagnetic glasses.

Short-Range Disorder in TeO2 Melt and Glass.

High-resolution X-ray pair distribution functions for molten and glassy TeO2 reveal coordination numbers nTeO ≈ 4. However, distinct from the known α-, ß-, and γ-TeO2 polymorphs, there is considerable short-range disorder such that no clear cutoff distance between bonded and nonbonded interactions exists. We suggest that this is similar to disorder in δ-TeO2 and arises from a broad distribution of asymmetric Te-O-Te bridges, something that we observe becomes increasingly asymmetric with increasing liquid temperature. Such behavior is qualitatively consistent with existing interpretations of Raman scattering spectra, and equivalent to temperature-induced coordination number reduction, for sufficiently large cutoff radii. Therefore, TeO2 contains a distribution of local environments that are, furthermore, temperature dependent, making it distinct from the canonical single-oxide glass formers. Our results are in good agreement with high-level ab initio cluster calculations.

Short-Range Structure, Thermal and Elastic Properties of Binary and Ternary Tellurite Glasses.

Glasses Al2/3O-TeO2, ZnO-TeO2 and R2/3O-ZnO-TeO2 (R = Al, B) were prepared by melting in Pt crucibles and studied for correlations between structure and thermal as well as mechanical properties, whereby the glass composition is varied to tailor the short-range speciation of tellurite, aluminate, and borate groups. The glass structure was studied by Raman and infrared spectroscopy analyses, and the measured properties include glass-transition temperature (Tg), density (ρ), and ultrasonic longitudinal (VL) and transverse (VT) velocities. In addition, atomic packing density (Cg), elastic moduli, and Poisson's ratio (σ) were evaluated from the measured properties. It was found that Al2/3O leads to cross-linked alumino-tellurite networks by strong Te-O-Al bonds, which cause a profound enhancement in Tg. The influence of ZnO and B2/3O on Tg is relatively smaller due to the weaker cross-linking effects of ZnO4 tetrahedra and of Te···O-B bonds. Short-range bonding characteristics, interatomic bonding energy differences, and atomic packing density were found to have a strong effect on VT and mostly on the VL sound velocity. The combined effects of structure and bonding are nicely expressed in the composition dependence of Poisson's ratio; it exhibits decreasing trends with Al2/3O content in the binary and ternary glasses studied here, but increasing trends with ZnO and B2/3O additions in glasses ZnO-TeO2 and B2/3O-ZnO-TeO2, respectively. The results for Poisson's ratio and atomic packing density for the studied glasses were found to fit nicely in the global σ versus Cg correlation established previously for a range of glasses not including tellurites so far. Finally, the sound velocities and Poisson's ratio of pure TeO2 glass were determined for the first time and found to differ markedly from those in the literature for TeO2 glass melted in alumina crucible; this is because the latter glass is highly doped by Al2O3 leached from the alumina crucible.

Laser-Raman and infrared spectroscopic studies of protein conformation in the eggshell of the fish Salmo gairdneri.

Laser-Raman and infrared spectroscopic studies reveal abundant beta-pleated sheet conformation in the eggshell proteins of the fish Salmo gairdneri. This secondary structure is the underlying molecular conformation, dictating the formation of the helicoidal architecture of the eggshell. Disulphide bonds crosslink the eggshell proteins of the fertilized eggs and are apparently found in g-g-g (gauche-gauche-gauche), g-g-t (gauche-gauche-trans) and t-g-t (trans-gauche-trans) conformation. There is no evidence for the existence of free sulphydryls. The tyrosines appear to act as hydrogen-bond acceptors, whereas the aromatic residues phenylalanine and tryptophan are also eggshell protein constituents.

Secondary structure of synthetic peptides derived from the repeating unit of a giant secretory protein from Chironomus tentans.

The secretory proteins of Chironomus tentans larvae, which are used to construct underwater feeding and pupation tubes, assemble into complexes in vitro. Members of a family of 1000 kDa proteins, the spIs, appear to form the fibrous backbone of the assembled complexes. The spIs consist of a core of tandemly repeating units of 60 to 90 amino acids that can be subdivided into two regions: the subrepeat region, made up of short internal repeats, and the constant region, which lacks simple subrepeats. We have synthesized peptides representative of the constant and subrepeat regions of one of the spIs, and have examined their secondary structure using Fourier transform IR and CD spectroscopy. The IR spectrum of the constant peptide indicates that this peptide has alpha-helical regions and beta-turns. The CD spectrum confirms this. The IR spectrum of the subrepeat peptide is similar to that of the poly(Gly)II helix, and also may indicate the presence of beta-turns. The CD spectrum is consistent with this helical structure. Extrapolation of these results to intact spIs is in agreement with secondary structure prediction and modeling studies. Our results indicate that the alpha-helices and poly(Gly)II-like helices are not arranged as coiled-coils, which are often found in fibrous proteins. We suggest that these structural elements may be in an unusual arrangement in the spIs, organized as alternating alpha-helices and poly(Gly)II or collagen-like helices, interspersed with beta-turns.

Spectroscopic studies of Manduca sexta and Sesamia nonagrioides chorion protein structure.

The secondary structure of Manduca sexta and Sesamia nonagrioides chorion proteins has been studied in intact chorions using laser-Raman and Fourier transform infra-red (FTIR) spectroscopy and in a solution containing extracted and reassembled chorion proteins using circular dichroism (CD) spectroscopy. Laser-Raman and IR spectra suggest the predominance of antiparallel beta-pleated sheet structure in intact chorion proteins of both Lepidoptera species. The bands at 1673, 1674 cm-1 (amide I) and 1234-1238 cm-1 (amide III) in the laser-Raman spectra can best be interpreted as resulting from abundant antiparallel beta-pleated sheet structure. Analysis of the amide I band suggests that chorion proteins consist of 60-70% antiparallel beta-pleated sheet and 30-40% beta-turns. Supporting evidence for the prevalence of antiparallel beta-pleated sheet in chorion proteins was supplied using FTIR spectroscopy by the observation of a very intense absorption band at 1635 cm-1 (amide I) and of a weak band at 1530, 1525 cm-1 (amide II) from chorions of both species. Surprisingly, analysis of the CD spectra of extracted and reassembled chorion proteins suggests that, in solution, they retain a regular secondary structure most probably dominated by beta-pleated sheet. We therefore suggest that the prominent regular beta-sheet structure of chorion proteins may exist in solution and dictate the aggregation and polymerization process in vivo.

Laser-Raman and FT-IR spectroscopic studies of peptide-analogues of silkmoth chorion protein segments.

Silkmoth chorion, the proteinaceous major component of the eggshell, with extraordinary mechanical and physiological properties, consists of a complex set of proteins, which have a tripartite structure: a central, evolutionarily conserved, domain and two more variable 'arms'. Peptide-analogues of silkmoth chorion protein central domain segments have been synthesized. Laser-Raman and infrared spectroscopic studies suggest the preponderance of antiparallel beta-pleated sheet structure for these peptides, both in solution and in the solid state.

Structure and optical properties of amorphous lead-germanate films developed by pulsed-laser deposition.

Lead-germanate materials are attractive systems for photonics applications. In this context, amorphous lead-germanate films were grown by pulsed-laser deposition at different substrate temperatures and oxygen pressures using a glassy target of composition 0.4PbO-0.6GeO(2). Optical and infrared measurements showed that the substrate temperature has a strong influence on the optical quality and stability of the deposited films. An accurate characterization of films was achieved by comparing experimental and simulated transmittance spectra in the infrared, and allowed to probe the structural evolution and variations in composition as a function of oxygen pressure. The results showed that the difference in reactivity of lead and germanium toward oxygen in the laser-produced plasma allows for composition adjustments in the lead-germanate films by varying the oxygen pressure in the deposition chamber.