Thermal degradation mechanisms of wood under inert and oxidative environments using DAEM methods.

The pyrolytic behavior of wood is investigated under inert and oxidative conditions. The TGA experiment is given a temperature variation from 323 to 1173 K by setting the heating rate between 5 and 40 K/min. The results of DTG curves show that the hemicellulose shoulder peak for birch is more visible under inert atmosphere due to the higher content of reactive xylan-based hemicellulose (mannan-based for pine). When oxygen presents, thermal reactivity of biomass (especially the cellulose) is greatly enhanced due to the acceleration of mass loss in the first stage, and complex reactions occur simultaneously in the second stage when char and lignin oxidize. A new kinetic model is employed for biomass pyrolysis, namely the distributed activation energy model (DAEM). Under inert atmosphere, the distributed activation energy for the two species is found to be increased from 180 to 220 kJ/mol at the solid conversion of 10-85% with the high correlation coefficient. Under oxidative atmosphere, the distributed activation energy is about 175-235 kJ/mol at the solid conversion of 10-65% and 300-770 kJ/mol at the solid conversion of 70-95% with the low correlation coefficient (below 0.90). Comparatively, the activation energy obtained from established global kinetic model is correspondingly lower than that from DAEM under both inert and oxidative environments, giving relatively higher correlation coefficient (more than 0.96). The results imply that the DAEM is not suitable for oxidative pyrolysis of biomass (especially for the second mass loss stage in air), but it could represent the intrinsic mechanism of thermal decomposition of wood under nitrogen better than global kinetic model when it is applicable.

The pyrolytic degradation of wood-derived lignin from pulping process.

Lignin is a key component in the biomass with a complex polymeric structure of the phenyl-C(3) alkyl units. The kraft lignin from the wood pulping process is tested in TG-FTIR and Py-GC-MS. The samples are pyrolyzed in TGA coupled with FTIR from 30 to 900 degrees C at the heating rate of 20 and 40K/min. The evolution of phenolic compounds in the initial pyrolysis stage of lignin is determined by FTIR, while the second stage is mainly attributed to the production of the low molecular weight species. A bench-scale fast pyrolysis unit is employed to investigate the effect of temperature on the product yield and composition. It is found that the guaiacol-type and syringol-type compounds as the primary products of lignin pyrolysis are predominant in bio-oil, acting as the significant precursors for the formation of the derivatives such as the phenol-, cresol- and catechol-types. A series of free-radical chain-reactions, concerning the cracking of different side-chain structures and the methoxy groups on aromatic ring, are proposed to demonstrate the formation pathways for the typical compounds in bio-oil by closely relating lignin structure to the pyrolytic mechanisms. The methoxy group (-OCH(3)) is suggested to work as an important source for the formation of the small volatile species (CO, CO(2) and CH(4)) through the relevant free radical coupling reactions.

The mechanism for thermal decomposition of cellulose and its main products.

Experiment is performed to investigate the mechanism of the cellulose pyrolysis and the formation of the main products. The evolution of the gaseous products is examined by the 3-D FTIR spectrogram at the heating rate of 5-60 K/min. A pyrolysis unit, composed of fluidized bed reactor, carbon filter, vapour condensing system and gas storage, is employed to investigate the products of the cellulose pyrolysis under different temperatures (430-730 degrees C) and residence time (0.44-1.32 s). The composition in the bio-oil is characterized by GC-MS while the gases sample is analyzed by GC. The effects of temperature and residence time on the main products in bio-oil (LG, 5-HMF, FF, HAA, HA and PA) are examined thoroughly. Furthermore the possible routes for the formation of the products are developed from the direct conversion of cellulose molecules and the secondary reactions of the fragments. It is found that the formation of CO is enhanced with elevated temperature and residence time, while slight change is observed for the yield of CO(2).

PsrA is a positive transcriptional regulator of the type III secretion system in Pseudomonas aeruginosa.

The type III secretion system (TTSS) of Pseudomonas aeruginosa is induced in vivo upon contact with eukaryotic cells and in vitro by calcium depletion in culture medium. We have observed a previously identified protein, PsrA, necessary for full activation of TTSS gene expression in P. aeruginosa. Electrophoretic mobility shift assays showed that recombinant PsrA could bind to the exsCEBA promoter region. A mutant with a deletion in the psrA gene was constructed. Using transcriptional fusions, we demonstrated that PsrA is required for the full activation of transcription of the TTSS regulatory operon exsCEBA and effector exoS, although the deletion mutant still responded to calcium depletion, to serum, and to host cell contact. The psrA mutant showed a marked decrease in the secretion of the type III effectors and weak resistance to phagocyte-like PLB-985 cells. The defect in TTSS transcription and secretion in the psrA mutant could be complemented by expression in trans of psrA. PsrA was previously identified as a transcriptional activator of RpoS, a central regulator during stationary phase. We confirmed with our strain that RpoS has a negative effect on TTSS gene expression. Taken altogether, these results suggest that PsrA is a newly identified activator that is involved in the expression of the TTSS by enhancing the exsCEBA transcriptional level.

Gas-filled liposomes as echocardiographic contrast agents in rabbits with myocardial infarcts.

RATIONALE AND OBJECTIVES: The authors evaluated gas-filled liposomes as echocardiographic contrast agents in rabbits with myocardial infarcts. METHODS: Ten rabbits underwent ligation of the left anterior descending coronary artery. Five animals underwent echocardiography before and after production of myocardial infarct (MI) and four animals had post-MI imaging only. In either case, images were obtained before and after injection of a single dose of 1 mL of gas-filed liposomes. Three radiologists blinded to clinical information reviewed the pre- and postcontrast images and assessed endomyocardial border definition, wall motion, confidence levels for normal versus abnormal wall motion and visualization of papillary muscle and mitral valve. RESULTS: Postcontrast scans showed significant improvement (P < .05) in endomyocardial border definition, visualization of wall motion, papillary muscle and mitral valve as well as increased reader confidence level. CONCLUSIONS: These results are encouraging and suggest that gas-filled liposomes may be a useful contrast agent for echocardiography.

Manganese-based liposomes. Comparative approaches.

RATIONALE AND OBJECTIVES: The purpose of this study was to further develop and compare manganese-based liposomes prepared by two different approaches wherein a manganese ion was entrapped within the internal aqueous space of the vesicles or into the bilayer surface via membrane bound complexes. METHODS: Small unilamellar liposomes (SUVs) were prepared entrapping manganese chloride. Alkylated complexes of manganese were prepared and also incorporated into SUVs. The two different manganese-based liposomes were compared for in-vitro relaxivity, stability, toxicity, and in-vivo imaging in rats with liver tumors. RESULTS: Liposomes entrapping manganese had a concentration-dependent change in relaxivity that was maximal at a several-fold molar excess of phospholipid relative to manganese ion. Liposomes bearing membrane-bound complexes showed relaxivity inversely proportional to vesicle size. In-vivo imaging showed greater and more specific hepatic enhancement with manganese liposomes bearing alkylated complexes than those entrapping manganese ion. CONCLUSIONS: Correlation effects likely explain the increased relaxivity of manganese entrapped in phospholipid vesicles. Greater efficacy, however, is afforded by liposomes bearing alkylated complexes.

Status of liposomes as MR contrast agents.

Recent work on the development of liposomal magnetic resonance (MR) contrast agents has yielded structures with higher overall relaxivity than that of other nanoparticles of similar diameter. Liposomes incorporating membrane-bound complexes of manganase ("memsomes") produce greater hepatic enhancement per micromole of metal ion than either ferrite particles or paramagnetic chelates. Memsomes also hold promise for targeting of sites outside the liver. Work is in progress to take these agents into clinical trials.

Nitrogen-filled liposomes as a vascular US contrast agent: preliminary evaluation.

Liposomes with a mean diameter of 1-2 microns were made to entrap nitrogen gas and tested as an ultrasound (US) contrast agent. The gas-filled liposomes, or Aerosomes (ImaRx Pharmaceutical, Tucson) were tested in vitro for size, stability, reflectivity, and acoustic characterization, and were tested in vivo for acute toxicity in mice and for cardiac imaging in rabbits after intravenous injection. Aerosomes have much greater reflectivity and higher attenuation than do standard liposomes and retain their acoustic properties after storage in aqueous media for several months. The interpolated median lethal dose of Aerosomes is approximately 2.5 mmol of lipid per kilogram, and the imaging dose is under 5 mumol of lipid per kilogram, yielding a potential therapeutic index of over 500 to 1. Postcontrast US images showed sustained enhancement of all four cardiac chambers as well as enhancement in the aorta, vena cava, and hepatic veins. Aerosomes hold promise as a contrast agent for cardiac and blood-pool imaging. Further work is in progress to characterize and develop this novel US contrast agent.

Liposomes as MR contrast agents: pros and cons.

Liposomes are generally thought of as being useful for entrapping drugs within their internal aqueous space. When used with MR contrast agents, this has the drawback that water flux across the membrane bilayer is limiting to contrast enhancement. This can be partially overcome by making the liposomes very small, such that surface area is relatively great compared to internal volume, thereby facilitating water exchange. Alternatively the membranes can be designed to be permeable to water but this may render the vesicles unstable in serum. Another approach is to incorporate the contrast agents into the lipid bilayer. By designing novel complexes of manganese with the ligands incorporated onto dual acyl chains, we have achieved R1 and R2 values of over 20/mmol sec-1 or more than five times higher than Gd-DTPA. Hepatic metastasis detection is significantly improved in rats at doses of only 10 mumol/kg manganese. Membrane-bound manganese complexes function as highly effective liver imaging agents and merit further study for development as agents to undergo human clinical trials.