Browsing by Subject "hydrolysis"
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Item Catalysts for the hydrolysis of thiophosphate triesters(Texas A&M University, 2005-02-17) Picot, AlexandreThe degradation of phosphate triesters is efficiently catalyzed by organophosphate hydrolases (OPH). While a number of recent studies have focused on optimizing the rate of hydrolysis observed with the native enzyme, no dinuclear complexes that mimic the function of OPH have been reported or investigated. Our present research focuses on the synthesis of dinuclear metal complexes and on the study of their catalytic abilities. An important aspect of this research concerns the investigation of the coordination chemistry of dinuclear ligands designed to hold two metal cations in well defined positions. The ability of the different complexes to catalyze the degradation of thiophosphate triester is presented. Out of several complexes studied, ortho-metallated Pd (II) complexes have been found to display the highest catalytic activity for the hydrolysis of parathion.Item Constitutive modeling for biodegradable polymers for application in endovascular stents(Texas A&M University, 2008-10-10) da Silva Soares, Joao FilipePercutaneous transluminal balloon angioplasty followed by drug-eluting stent implantation has been of great benefit in coronary applications, whereas in peripheral applications, success rates remain low. Analysis of healing patterns in successful deployments shows that six months after implantation the artery has reorganized itself to accommodate the increase in caliber and there is no purpose for the stent to remain, potentially provoking inflammation and foreign body reaction. Thus, a fully biodegradable polymeric stent that fulfills the mission and steps away is of great benefit. Biodegradable polymers have a widespread usage in the biomedical field, such as sutures, scaffolds and implants. Degradation refers to bond scission process that breaks polymeric chains down to oligomers and monomers. Extensive degradation leads to erosion, which is the process of mass loss from the polymer bulk. The prevailing mechanism of biodegradation of aliphatic polyesters (the main class of biodegradable polymers used in biomedical applications) is random scission by passive hydrolysis and results in molecular weight reduction and softening. In order to understand the applicability and efficacy of biodegradable polymers, a two pronged approach involving experiments and theory is necessary. A constitutive model involving degradation and its impact on mechanical properties was developed through an extension of a material which response depends on the history of the motion and on a scalar parameter reflecting the local extent of degradation and depreciates the mechanical properties. A rate equation describing the chain scission process confers characteristics of stress relaxation, creep and hysteresis to the material, arising due to the entropy-producing nature of degradation and markedly different from their viscoelastic counterparts. Several initial and boundary value problems such as inflation and extension of cylinders were solved and the impacts of the constitutive model analyzed. In vitro degradation of poly(L-lactic acid) fibers under tensile load was performed and degradation and reduction in mechanical properties was dependent on the mechanical environment. Mechanical testing of degraded fibers allowed the proper choice of constitutive model and its evolution. Analysis of real stent geometries was made possible with the constitutive model integration into finite element setting and stent deformation patterns in response to pressurization changed dramatically as degradation proceeded.Item Development and Characterization of Novel Biomaterials for Fabrication of Multilayered Vascular Grafts(2013-12-05) Dempsey, David KandellCurrent synthetic alternatives to autologous grafts have often failed in small diameter applications (<6 mm) due to their thrombogenicity and compliance mismatch of native vasculature. No known synthetic material is capable of providing a non-thrombogenic inner layer that promotes endothelial cell (EC) interactions while also providing sufficient compliance and burst pressure for long term success in vivo. We have developed a multilayer design with an inner thromboresistant poly(ethylene glycol) (PEG) hydrogel based on Scl2-2 proteins designed to promote rapid in situ endothelialization. The bioactive component is reinforced with an electrospun segmented polyurethane (SPU) layer with tunable mechanical properties to withstand physiological loading conditions. The modulating of electrospun parameters to influence graft architecture coupled with the tunability of SPU mechanical performance is expected to give rise to improved graft biomechanical properties. Unfortunately, this advantage was found to be limited to only one property at a time. To this end, we have developed a novel semi-IPN approach to expand the range of possible graft compliance and burst pressures in order to simultaneously achieve biomechanical properties that exceed autologous veins. In addition to matching biomechanical properties, probability of long term success is also dependent on the grafts retention of perfomance despite cell-mediated attack. Typically, in the case poly(ether urethanes) (PEUs) and poly(carbonate urethanes) (PCUs), oxidative stability is the primary focus in the development of biostable SPUs. We have characterized the biostability of several commercially available polyurethanes while simultaneously evaluating the predictive capabilities of two main in vitro test methods to optimize our graft?s design. Despite ensured long term performance through optimizing biostability, a permanent scaffold prevents vasoactivity, a key function of vasculature. We have taken a tissue engineering approach to restorate vasoactivity by evaluating a novel aromatic biodegradable poly(ester urethane) (PEsU) for the reinforcing layer of a tissue engineering vascular graft (TEVG). This PEsU was expected to degrade into safe byproducts given its design based on glycolic acid and ethylene glycol. Following characterization, the PEsU was determined to be a strong potential reinforcing layer of a potential TEVG design. In summary, we have improved small diameter grafts through a multilayer design approach. Our graft demonstrated favorable initial fabrication feasibility, promising in vivo testing, biomechanical properties exceeding autologous veins, and strong oxidative stability. Overall, we have optimized the reinforcing layer through improvement of biomechanical properties via modulated material chemistry, optimized biostability, and identification of a biodegradable component expected to allow for restored vasoactivity.Item Fundamental study of structural features affecting enzymatic hydrolysis of lignocellulosic biomass(Texas A&M University, 2006-10-30) Zhu, LiLignocellulose is a promising and valuable alternative energy source. Native lignocellulosic biomass has limited accessibility to cellulase enzyme due to structural features; therefore, pretreatment is an essential prerequisite to make biomass accessible and reactive by altering its structural features. The effects of substrate concentration, addition of cellobiase, enzyme loading, and structural features on biomass digestibility were explored. The addition of supplemental cellobiase to the enzyme complex greatly increased the initial rate and ultimate extent of biomass hydrolysis by converting the strong inhibitor, cellobiose, to glucose. A low substrate concentration (10 g/L) was employed to prevent end-product inhibition by cellobiose and glucose. The rate and extent of biomass hydrolysis significantly depend on enzyme loading and structural features resulting from pretreatment, thus the hydrolysis and pretreatment processes are intimately coupled because of structural features. Model lignocelluloses with various structural features were hydrolyzed with a variety of cellulase loadings for 1, 6, and 72 h. Glucan, xylan, and total sugar conversions at 1, 6, and 72 h were linearly proportional to the logarithm of cellulase loadings from approximately 10% to 90% conversion, indicating that the simplified HCH-1 model is valid for predicting lignocellulose digestibility. Carbohydrate conversions at a given time versus the natural logarithm of cellulase loadings were plotted to obtain the slopes and intercepts which were correlated to structural features (lignin content, acetyl content, cellulose crystallinity, and carbohydrate content) by both parametric and nonparametric regression models. The predictive ability of the models was evaluated by a variety of biomass (corn stover, bagasse, and rice straw) treated with lime, dilute acid, ammonia fiber explosion (AFEX), and aqueous ammonia. The measured slopes, intercepts, and carbohydrate conversions at 1, 6, and 72 h were compared to the values predicted by the parametric and nonparametric models. The smaller mean square error (MSE) in the parametric models indicates more satisfactorily predictive ability than the nonparametric models. The agreement between the measured and predicted values shows that lignin content, acetyl content, and cellulose crystallinity are key factors that determine biomass digestibility, and that biomass digestibility can be predicted over a wide range of cellulase loadings using the simplified HCH-1 model.Item Lime pretreatment and enzymatic hydrolysis of corn stover(Texas A&M University, 2005-08-29) Kim, Se HoonRenewable energy sources, such as lignocellulosic biomass, are environmentally friendly because they emit less pollution without contributing net carbon dioxide to the atmosphere. Among lignocellulosic biomass, corn stover is a very useful feedstock to economically produce environmentally friendly biofuels. Corn stover was pretreated with an excess of calcium hydroxide (0.5 g Ca(OH)2/g raw biomass) in non-oxidative and oxidative conditions at 25, 35, 45, and 55oC. The optimal condition is 55oC for 4 weeks with aeration, determined by yields of glucan and xylan. The overall yields of glucose (g glucan hydrolyzed/100 g original glucan) and xylose (g xylan hydrolyzed/100 g original xylan) were 91.3 and 51.8 at 15 FPU/g cellulose, respectively. Furthermore, when considering the dissolved fragments of glucan and xylan in the pretreatment liquors, the overall yields of glucose and xylose were 93.2 and 79.5 at 15 FPU/g cellulose, respectively. The pretreatment liquor has no inhibitory effect on ethanol fermentation using Saccharomyces cerevisiae D5A. At the recommended condition, only 0.073 g Ca(OH)2 was consumed per g of raw corn stover. Under extensive delignification conditions, 87.5% of the initial lignin was removed. Extensive delignfication required oxidative treatment and additional lime consumption. Deacetylation quickly reached a plateau within 1 week. Delignification highly depended on temperature and the presence of oxygen. Lignin and hemicellulose were selectively removed, but cellulose was not affected by lime pretreatment in mild temperatures (25 ?? 55oC). The delignification kinetic models of corn stover were empirically determined by three simultaneous first-order reactions. The activation energies for the oxidative delignification were estimated as 50.15 and 54.21 kJ/mol in the bulk and residual phases, respectively. Crystallinity slightly increased with delignification because amorphous components (lignin, hemicellulose) were removed. However, the increased crystallinity did not negatively affect the 3-d sugar yield of enzyme hydrolysis. Oxidative lime pretreatment lowered the acetyl and lignin contents to obtain high digestibility, regardless of crystallinity. The enzymatic digestibility of lime-treated biomass was affected by the change of structural features (acetylation, lignification, and crystallization) resulting from the treatment. The non-linear models for 3-d hydrolysis yields of glucan and xylan were empirically established as a function of the residual lignin fraction for the corn stover pretreated with lime and air.