Unusual supramolecular assembly and nonlinear optical properties of l-histidinium hydrogen malate

A new nonlinear optical material, l-histidinium hydrogen malate, has been synthesized. The crystal structure was determined at 90 K by single-crystal X-ray diffraction in order to analyze its supramolecular structure. A new building block type has been found. The malate anions form head-to-side infinite chains parallel to [100], via O-H?O interactions, instead of the usual head-to-tail infinite chains found in other hydrogen malate salts. The l-histidine cations form chains parallel to [100] via N-H?O hydrogen bonds, with cations of adjacent chains in anti-parallel way. The compound shows a good optical second-harmonic generation capability with an effective second-order susceptibility estimated to be 0.70 of that for potassium dihydrogen phosphate.     

Molecular structure, vibrational spectra and nonlinear optical properties of l-Valine Hydrobromide: DFT study

FT-IR and Raman spectra of the nonlinear optical material l-Valine Hydrobromide crystal have been recorded and analyzed. The equilibrium geometry, bonding features and the harmonic vibrational wavenumbers of LVB have been calculated with the help of density functional theory (DFT) calculation. The lowering of NH stretching wavenumber indicates the formation of NH?Br hydrogen bonding. The calculated First order hyperpolarizability value shows that LVB is the potential candidate for the NLO applications. The electronic effects and the hydrogen bonding were explained using natural bond orbital analysis.     

Syntheses of enantio-enriched chiral building blocks from l-glutamic acid

Starting from lactone-amide 8, easily derived from l-glutamic acid, enantioselective syntheses of (S)-tetrahydrofuran 2-carboxamide derivative 2 and a protected (S)-3-hydroxypiperidin-2-one (3) are reported. The building block 3 was converted to (2S,3R)-3-hydroxypipecolamide (6) by a three-step procedure. A solvent altered H-bonding capacity leading to a highly chemoselective tosylation of the primary hydroxyl group in the presence of an α-hydroxy-carboxamide was observed.     

Homochiral frameworks derived from magnesium, zinc and copper salts of l-tartaric acid

Five metal-organic frameworks derived from l-tartarate (l-tart=C₄H₄O₆) and the divalent metal ions magnesium, zinc or copper have been prepared and structurally characterized. The frameworks were prepared from the reaction of potassium l-tartrate [KC₄H₅O₆] with the appropriate metal salt in water solutions. The magnesium compound [Mg(l-tart)(H₂O)⊃1.5H₂O], 1, crystallizes as a two-dimensional 6³ sheet structure. The addition of the divergent linker molecules 4,4′-bipyridine (bipy) or trans-1,2-bispyridylethylene (bpe) to systems involving Zn²⁺ and Cu²⁺ results in the formation of the homochiral three-dimensional structures [Zn₂(l-tart)2(biyp)(H₂O)⊃5.25H₂O], 2, [Cu(l-tart)(bipy)⊃2.33H₂O], 3, and [Cu(l-tart)(bpe)⊃8H₂O], 5. Removal of solvent water molecules from 3 resulted in [Cu(l-tart)(bipy)⊃0.2H₂O], 4. Similar experiments on 2 and 5 resulted in breakdown of the frameworks, illustrating the dependence of the stability of these structures on the guest water molecules. This study reports the structures of two new topological types of binodal nets.     

New building blocks for peptide and depsipeptide synthesis: hexafluoroacetone protected l-homoisoserine and d,l-homoisocysteine derivatives

Efficient syntheses of l-homoisoserine and d,l-homoisocysteine derivatives starting from l-malic and d,l-thiomalic acid using hexafluoroacetone as protecting and activating agent are described. The new compounds are interesting building blocks for the preparation of non-natural peptides and depsipeptides as well as for the construction of new GABA derivatives.     

Synthesis of d-gluco-, l-ido-, d-galacto-, and l-altro-configured glycaro-1,5-lactams from tartaric acid

The d-gluco-, l-ido-, d-galacto-, and l-altro-configured glycaro-1,5-lactams 1-4 were prepared from the known tartaric anhydride 5 via the aldehyde 6. These lactams are known (1) or potential (2-4) inhibitors of β-d-glucuronidases and α-l-iduronidases. Olefination of 6 to the (E)- and (Z)-alkenes 7 or 8, followed by reagent or substrate controlled dihydroxylation, lactonization, azidation, reduction, and deprotection led in 10 steps and in overall yields of 11-20% to the title lactams.     

Sulfonate protecting groups. Synthesis of O- and C-methylated inositols: d- and l-ononitol, d- and l-laminitol, mytilitol and scyllo-inositol methyl ether

Syntheses of d- and l-ononitol, d- and l-laminitol, mytilitol and scyllo-inositol methyl ether starting from myo-inositol are described. One or two of the myo-inositol 1,3,5-orthoformate hydroxyl groups were protected as tosylates. These mono or ditosylates served as key intermediates for the preparation of O- and C-methyl inositols. Racemic 2,4-di-O-tosyl-myo-inositol 1,3,5-orthoformate was resolved as its diastereomeric camphanates. Use of sulfonate groups for the protection of inositol hydroxyl groups resulted in substantial improvement in the overall yield of O- and C-methyl inositols.     

Facile synthesis of poly-(l-lysine) dendrimers with a pentaaminecobalt(III) complex at the core

The synthesis of second and third generation dendrimers based on poly-(l-lysine) with a pentaamine cobalt(III) metal complex at the core is described. The synthesis and purification of these dendrimers were facilitated by using the metal complex as the C-terminal protecting group.     

Characterization and release of triptolide-loaded poly (d,l-lactic acid) nanoparticles

Triptolide (TP), which has immunosuppressive effect, anti-neoplastic activity, anti-fertility function and severe toxicities on digestive, urogenital, blood circulatory system, was used as a model drug in this study. TP-loaded poly (d,l-lactic acid) (PLA) nanoparticles were prepared by the modified spontaneous emulsification solvent diffusion method (modified-SESD method). Dynamic light scattering system (DLS), transmission electron microscope (TEM), atomic force microscopy (AFM), differential scanning calorimetry (DSC), X-ray powder diffractometry and Fourier transform infra-red spectroscopy (FT-IR) were employed to characterize the nanoparticles fabricated for size and size distribution, surface morphology, the physical state of drug in nanoparticles, and the interaction between the drug and polymer. Encapsulation efficiency (EE) and the in vitro release of TP in nanoparticles were measured by the reverse phase high-performance liquid chromatography (RP-HPLC). The produced nanoparticles exhibited a narrow size distribution with a mean size of approximately 150 nm and polydispersity index of 0.088. The morphology of the nanoparticles exhibited a fine spherical shape with smooth surfaces without aggregation or adhesion. TP-entrapped in nanoparticles was found in the form of amorphous or semicrystalline. It was found that a weak interaction existed between the drug and polymer. In all experiments, more than 65% of EE were obtained. The in vitro release profile of TP from nanoparticles exhibited a typical biphasic release phenomenon, namely initial burst release and consequently sustained release. In this case, the particle size played an important role for the drug release. The modified-SESD method was a potential and advantage method to produce an ideal polymer nanoparticles for drug delivery system (DDS).     

Control of impurities in l-aspartic acid and l-alanine by high-performance liquid chromatography coupled with a corona charged aerosol detector

In this study a reversed phase ion-pair high-performance liquid chromatography (HPLC) method using charged aerosol detection (CAD) was developed and fully validated for the pharmaceutical quality control of l-aspartic acid (Asp). With a slight modification, the method also allows the evaluation of related substances in l-alanine (Ala). The method enables simultaneous control of related amino acids and of possibly occurring organic acids contaminants. A minimum limit of quantification of 0.03% could be achieved for all occurring related substances. Moreover, the detector sensitivity of the CAD was compared with an evaporative light scattering detector (ELSD). Depending on the analyte the CAD was found to be 3.6–42 times more sensitive than the ELSD. The HPLC method was applied to the purity testing of 8 samples of pharmaceutical grade and reagent grade Asp and of 12 samples of Ala supplied by various manufacturers. Both substances were found to be of high purity (greater than 99.8% for Asp and greater than 99.9% for Ala). Malic acid and Ala were the major impurities in Asp. Asp and glutamic acid (Glu) were the only detectable impurities in Ala.     

Swelling and hydrolytic degradation of poly(d,l-lactic acid) in aqueous solutions

Low molecular weight poly(lactic acid) was synthesized by direct polycondensation of lactic acid. The oligomers were characterized by viscometry, light scattering, and gel permeation chromatography (GPC). The swelling behaviour of tablets made of the above polymer immersed in buffer solutions at 37 °C was studied. In the same experiments, the hydrolytic stability of d,l-PLA was assessed by measuring the weight loss after drying the tablets. In order to inhibit any degradation due to bacteria, formaldehyde was added in the solution as biostatic factor. The effect of an incorporated drug on the swelling behaviour of d,l-PLA tablets was also considered. It was found that the incorporation of drug in d,l-PLA tablets increases their swelling index, probably due to the creation of additional porosity in the specimens or other interaction between drug and polymeric matrix.     

Enatioselective quantitative separation of d- and l-thyroxine by molecularly imprinted micro-solid phase extraction silver fiber coupled with complementary molecularly imprinted polymer-sensor

Thyroxine is a known disease biomarker which demands a highly sensitive and selective technique to measure ultratrace level with enantiodifferentiation of its optical isomers (d- and l-), in real samples. In this work, an approach of hyphenation between molecularly imprinted micro-solid phase extraction and a complementary molecularly imprinted polymer-sensor was adopted for enantioseparation, preconcentration, and analysis of d- and l-thyroxine. In both techniques, the same imprinted polymer, coated on a vinyl functionalized self-assembled monolayer modified silver wire, was used as the respective extraction fiber as well as sensor material. This combination enabled enhanced preconcentration of test analyte substantially so as to achieve the stringent limit [limit of detection: 0.0084 ng mL⁻¹, RSD = 0.81%, S/N = 3 (d-thyroxine); 0.0087 ng mL⁻¹, RSD = 0.63%, S/N = 3 (l-thyroxine)] of clinical detection of thyroid-related diseases, without any problems of non-specific false-positive contribution and cross-reactivity.     

Structure and mechanical properties of poly(l-lactide)/layered silicate nanocomposites

Poly(ε-caprolactone) (PCL) masterbatches with the intercalated and the exfoliated morphology were prepared by ring opening polymerization of ε-caprolactone in the presence of organomodified montmorillonite (MMT) Cloisite 30B. Poly(l-lactide) (PLLA) nanocomposites with Cloisite 30B or PCL masterbatches were prepared by melt blending. The effects of the silicate type, MMT content and the nanocomposite morphology on thermal and mechanical properties of PLLA nanocomposites were examined. The montmorillonite particles in PLLA/Cloisite 30B and PLLA/intercalated masterbatch nanocomposites were intercalated. In contrary to expectations, the exfoliated silicate layers of exfoliated masterbatch were not transferred into the PLLA matrix. Due to a low miscibility of PCL and PLLA, MMT remained in the phase-separated masterbatch domains. The stress-strain characteristics of PLLA nanocomposites, Young modulus E, yield stress σ_y and yield strain ε_y, decreased with increasing MMT concentration, which is associated with the increase in PCL content. The expected stiffening effect of MMT was low due to a low aspect ratio of its particles and was obscured by both plastifying effects of PCL and low PLLA crystallinity. Interestingly, in contrast to the neat PLLA, ductility was enhanced in all PLLA/Cloisite 30B materials and in PLLA/masterbatch nanocomposites with low MMT concentrations.     

Effects of molecular weight and small amounts of d-lactide units on hydrolytic degradation of poly(l-lactic acid)s

Films of poly(l-lactic acid) (PLLA) with different number-average molecular weights (M_n) and d-lactide unit contents (X_d) were made amorphous and the effects of molecular weight and small amounts of d-lactide units on the hydrolytic degradation behavior in phosphate-buffered solution at 37 °C of PLLA were investigated. The degraded films were investigated using gravimetry, gel permeation chromatography, polarimetry, differential scanning calorimetry, X-ray diffractometry, and tensile testing. To exclude the effects of crystallinity on the hydrolytic degradation, the films were made amorphous by melt-quenching. The incorporation of small amounts of d-lactide units drastically enhanced the hydrolytic degradation of PLLA. In the period of 0-32 weeks, the hydrolytic degradation rate constant (k) of PLLA films increased with increasing X_d, while the k values did not depend on M_n. This means that the effects of X_d on the hydrolytic degradation rate of the films are higher than those of M_n. In contrast, in the period of 32-60 weeks neither X_d nor M_n was a crucial parameter to determine k values, probably because in addition to these parameters the differences in the amount of catalytic oligomers accumulated in films and crystallinity affect the hydrolytic degradation behavior of the films. The initially amorphous PLLA films remained amorphous even after the hydrolytic degradation for 60 weeks.     

Degradation studies on segmented polyurethanes prepared with poly (d, l-lactic acid) diol, hexamethylene diisocyanate and different chain extenders

In order to investigate the effect of different chain extenders on degradation properties of segmented polyurethanes (SPUs), three types of segmented polyurethanes (SPU-P, SPU-O and SPU-A) based on poly (d, l-lactic acid) diol, hexamethylene diisocyanate (HDI), were synthesized with three chain extenders: peperazine (PP), 1, 4-butanediol (BDO) and 1, 4-butanediamine (BDA), respectively. Thermogravimetric analysis, activation energy and in vitro degradation were used to characterize the obtained polymers, quantitatively. The results revealed that chain extender played an important role in thermal degradation and biodegradation of polyurethanes. Thermogravimetric analysis and activation energy demonstrated that SPU-O, SPU-P and SPU-A presented best, second and weakest thermostability, respectively, and the thermal degradation mechanism of three SPUs was the same and regarded as a two-stage degradation. Data of hydrolytic degradation of the polymers during 12 weeks indicated that the in vitro degradation stability of SPU-A and SPU-P was similar, but both were better than that of SPU-O. The reason for the differences among three types of SPUs was discussed in this paper.     

Thermal phase transitions of solid chiral N,N′-carbonyl-bis-(l-amino acids) and their methyl and benzyl esters

Compounds derived from different N,N′-carbonyl-bis-(l-amino acids) and their methyl and benzyl esters were synthesized and characterized by elemental analysis, infrared and nuclear magnetic resonance. The amino acids used were valine, leucine, phenylglycine and phenylalanine. All compounds revealed complex thermal behaviour as proved by differential scanning calorimetry, X-ray powder diffractometry and optical birefringence observation by polarizing microscope. Above isotropization temperature N,N′-carbonyl-bis-(l-amino acids) decomposed. The number and kinds of thermal phase transitions of investigated esters vary from a simple phase transition and melting to a complex polymorphism, and strongly depends on molecular structure. One to four phase transitions have been observed upon heating. Phase transition temperatures showed considerable variation with choice of the supstituent on symmetric carbons and therminal carboxylic groups. The results are discussed in terms of the architecture of investigated molecules that hinder mesomorphism.     

Mechanisms involved in stannane generation by aqueous tetrahydroborate(III): Role of acidity and l-cysteine

The role played by acidity (0.01–5 mol L^− 1 HNO₃) and l-cysteine (0.1–0.2 mol L^− 1) in the formation of stannane by reaction of Sn(IV) solution with aqueous tetrahydroborate(III) (0.05–0.2 mol L^− 1), has been investigated by continuous flow hydride generation coupled with atomic absorption spectrometry using a miniature argon–hydrogen diffusion flame as the atomizer. Different mixing sequences and reaction times of the reagents were useful in the identification of those processes which contribute to the generation of stannane in different reaction conditions, both in the absence and in the presence of l-cysteine. The lack of stannane generation at high acidities is due to the formation of Sn substrates and hydridoboron species which are unreactive. The capture of the stannane in solution, following its ionization to SnH₃⁺ from already formed stannane, does not play any role. While the presence of l-cysteine, does not affect the generation efficiency at lower acidities, it expands the optimum range of acidities for stannane generation to higher values. This effect can be addressed to both the buffering capacity of l-cysteine and to the formation of Sn-(l-cysteine) complexes, while the formation of (l-cysteine)–borane complexes do not play a significant role. Formation of Sn-(l-cysteine) complexes also appears to be useful for stabilization of tin solution at low acidities values.     

Complex formation of uranium(VI) with the amino acids l-glycine and l-cysteine: A fluorescence emission and UV–Vis absorption study

The complexation of uranium(VI) with the amino acids l-glycine and l-cysteine has been investigated by time-resolved laser-induced fluorescence spectroscopy (TRLFS) and UV–Vis spectroscopy at a low pH range. The identified 1:1 and 1:2 uranyl-l-glycine complexes fluoresce and have similar absorbance properties. In contrast to the glycine system, uranyl forms two different non-fluorescent 1:1 complexes with l-cysteine, showing individual absorbance properties under the given experimental conditions. The corresponding complex formation constants were calculated using the spectroscopic data.     

Melt spinning of poly(l/d)lactide 96/4: Effects of molecular weight and melt processing on hydrolytic degradation

This study focused on determining the effects of molecular weight on the degradation of polylactide 96l/4d in melt spinning and the effects of melt processing on hydrolytic degradation. Three polylactides with different inherent viscosities were melt spun, and the fibres were studied in vitro. Results showed that during melt spinning high-molecular-weight polylactide degraded because polymer chains were subject to high shear stress and high melt temperatures, whereas a low-molecular-weight polylactide with low melt viscosity was not affected by melt processing. Most degradation occurred during the melting phase in the length of the extruder barrel. Lactide monomer, generated as the polymer degraded in the melt, significantly affected in vitro degradation such that the degradation rate was directly proportional to the lactide concentration of the polymer.     

Plasticization of poly(lactide) with blends of tributyl citrate and low molecular weight poly(d,l-lactide)-b-poly(ethylene glycol) copolymers

Polylactide (PLA) is a potential candidate for the partial replacement of petrochemical polymers because it is biodegradable and produced from annually renewable resources. Characterized by its high tensile strength, unfortunately the brittleness and rigidity limit its applicability. For a great number of applications such as packaging, fibers, films, etc., it is of high interest to formulate new PLA grades with improved flexibility and better impact properties.In order to develop PLA-based biodegradable packaging, the physico-mechanical properties of commercially available PLA should be modified using biodegradable plasticizers. To this end, PLA was melt-mixed with blends of tributyl citrate and more thermally stable low molecular weight block copolymers based on poly(d,l-lactide) and poly(ethylene glycol) of different molecular weights and topologies. The copolymers have been synthesized using a potassium based catalyst which is expected to be non toxic and were characterized by utilization of TGA, GPC and NMR techniques.The effect of the addition of up to 25 wt% plasticizer on the thermo-mechanical properties of PLA was investigated and the results were correlated with particular attention to the relationship between properties and applications.To confirm the safety of the catalyst used for the preparation of the copolymers, in vitro cytotoxicity tests have been carried out using MTS assay and the results show their biocompatibility in the presence of the fibroblast cells.Compost biodegradation experiments carried out using neat and plasticized PLA have shown that the presence of plasticizers accelerates the degradation of the PLA matrix.