DNA interstrand crosslinks (ICLs) are a potent type of DNA damage that arise as a consequence of normal cell metabolism. By covalently linking opposing strands of the double helix, ICLs block essential DNA transactions such as replication, transcription, and recombination. If unrepaired, or incorrectly repaired, ICLs can lead to gross genome instability and cell death. This cytotoxicity has been exploited in the clinic, where ICL inducing drugs are among the oldest and most widely prescribed anti-cancer therapies. However, acquired resistance is a significant limitation of these drugs, and the mechanism by which this occurs remains largely elusive. In order to develop more effective ICL-based therapies, it is imperative to first fully elucidate how healthy cells respond to and repair ICLs. Moreover, better understanding ICL repair mechanisms is necessary to fully unravel the complex DNA repair networks that govern genomic integrity, and understand the physiology of diseases such as Fanconi Anemia, which result from the inability to efficiently repair ICL lesions. Multiple mechanisms of ICL repair exist, and repair pathway choice is primarily determined by the phase of the cell cycle. In proliferating cells, the ICL repair occurs during S-phase, and in a process termed “replication coupled repair” (RCR). In contrast, slowly or non-dividing cells rely on an alternative modality of repair called “replication independent repair” (RIR). RIR is critical for homeostasis and survival in quiescent healthy cells that (for example, neurons) and in cycling cells deficient for replication coupled repair proteins (i.e. Fanconi Anemia cells). Despite its importance, little is known about RIR. This is due, in part, to the fact that ICL repair has been primarily studied in systems, such as cultured cells, that favor RCR and are therefore bias against RIR. More recently, non-replicating Xenopus cell-free extracts has emerged as a powerful system to study RIR. This system faithfully recapitulates RIR and has been instrumental in identifying DNA polymerase kappa (Pol κ) and the eukaryotic sliding clamp, proliferating cell nuclear antigen (PCNA), as two critical RIR factors. However, other important RIR factors are yet to be identified. ICL repair is unique among DNA repair pathways as it harnesses proteins from diverse DNA repair pathways including, Base Excision Repair (BER), Nucleotide Excision Repair (NER), Mismatch Repair (MMR), and Double Strand Break Repair (DSBR). Chapter 1 provides an overview of these pathways including the types of DNA damage that each pathway responds to, key steps of the repair process, and the corresponding proteins that are involved. This chapter provides context for the rest of the thesis in which I explore the contribution of multiple DNA repair proteins on the repair of ICL lesions. In Chapter 2, I detail our studies assessing the contribution of the MMR machinery to RIR. We show that the mismatch repair sensor, MutS complex (MSH2-MSH6), is critical for ICL recognition, and the stepwise recruitment of other MMR proteins including MutL (MLH1-PMS2) and EXO1. In this chapter, I also investigate how ICL structure influences repair. I find that more distorting ICLs use an MMR-dependent ICL repair mechanism, while less distorting ICLs are repaired MMR-independently (see also Appendix A), or not repaired at all. Appendix B further explores the contribution of the MMR pathway on ICL repair in mammalian cells. Finally, in Appendix C and D we provide further evidence that RIR is fundamentally distinct from replication coupled ICL repair, as depletion of key RCR proteins from our extracts yields no phenotype. I summarize all of these findings in Chapter 3, and discuss their implications to the DNA repair field as well as the clinic, where crosslinker drugs remain a mainstay in the treatment of cancer.

Thesis (Ph.D)--Biomedical Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Elliot Chaikof; Committee Member: Marc Levenston; Committee Member: Robert Nerem; Committee Member: Vincent Conticello; Committee Member: Yadong Wang. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (Ph.D.) -- McMaster University, 2001. / Includes bibliographical references. Also available via World Wide Web.

This thesis presents the first reported grafted PPE - polycaprolactone-g-PPE; the first PPE based sensing model: biotinylated grafted PPE/streptavidin coated sphere; the first photocrosslinkable PPE ¨C allyloxy PPE; and the new mechanism which demonstrates morphology control on a single molecular level

As a low cost natural polymer, starch is widely used in paper, food, adhesive, and many other industries. In order to improve the performance of starch, crosslinking is often conducted either in the processes of starch modification or during the application processes. Many crosslinkers have been developed in the past for crosslinking starch. Ammonium zirconium carbonate (AZC) is one of the common crosslinkers for crosslinking starch in aqueous solutions, having been widely used as a starch crosslinking agent in paper surface coating for more than 20 years. However, the mechanisms of starch crosslinking with AZC have not been well studied. In order to optimize the crosslinking chemistry of starch and find new paths for the utilization of starch in papermaking, a better understanding of the starch crosslinking mechanism is necessary. This thesis focuses on the fundamental study of starch crosslinking in an aqueous solution and its applications in fiber surface grafting, filler modification, and starch nanoparticle formation. Particularly, the thesis contains three major parts: (1) Mechanism study of starch crosslinking induced by AZC: In this thesis, the crosslinking (or gelation) kinetics of starch/AZC blends were investigated by using rheological measurements. The evolution of viscoelastic properties of AZC solutions and the AZC-starch blends was characterized. It was found that for both AZC self-crosslinking and AZC-starch co-crosslinking, the initial bond formation rate and the gel strength had a strong power law relationship with the concentrations of both AZC and starch. It is suggested that the development of the crosslinking network is highly dependent on the AZC concentration, while the starch concentration effect is less significant. It was determined that the activation energy of AZC self-crosslinking was approximately 145-151 kJ/mol, while the activation energy of AZC-starch co-crosslinking was 139 kJ/mol. (2) Fiber and filler modifications with starch and crosslinkers: Besides reacting with starch, AZC can react with cellulose which also contains hydroxyl groups. Theoretically, it is possible to use AZC as a crosslinker / coupling agent to graft starch onto cellulose fibers. It is believed that the grafted starch on fiber surfaces can improve the fiber bonding capability. In this thesis, a facile method to graft starch onto cellulose fiber surfaces through the hydrogen bond formation among cellulose, starch and AZC was developed. Compared with the paper sheets made of fibers with an industry refining level (420 ml CSF), the paper sheets made of fibers with a much lower refining degree but with grafted starch showed higher paper strengths, including the tensile strength, stiffness and z direction tensile; meanwhile, a faster drainage rate during web formation could also be achieved. Not only can the fiber-fiber bonding be improved by grafting starch onto fiber surfaces, but the filler-fiber bonding can also be improved if starch can be effectively coated on the filler surface. This concept has been supported by the early studies. In this thesis, the effects of the crosslinking of starch in the filler modification for the papermaking application were also studied. (3) Mechanism of starch nanoparticle formation during extrusion with crosslinkers: It was reported that starch crosslinking could facilitate the reduction of starch particle size during reactive extrusion. However, the mechanism of the particle size reduction by starch crosslinking was not illustrated. The reason that the crosslinking can cause the particle size reduction of starch during extrusion is fundamentally interesting. In this thesis, the mechanism of starch particle size reduction during extrusion with and without crosslinkers was investigated by identifying the contributions of thermal and mechanical effects. The effects of extrusion conditions, including temperature, screw speed, torque, starch water content and crosslinker addition, on the particle size were studied. It was found that the addition of crosslinkers could significantly increase the shear force (torque), and consequently facilitate the reduction of the particle size. The results indicate that for extrusion without a crosslinker, the starch particle size decreased with the increase of temperature. At 100 degree Celsius, the starch particles with a size of 300 nm could be obtained. With the addition of appropriate crosslinkers (glyoxal), the starch particle size could be reduced to around 160 nm, even at a lower extrusion temperature of 75 degree Celsius .

Natural gas feeds often contain contaminants such as CO₂, H₂S, H₂O, and small hydrocarbons. Carbon dioxide is a major contaminant reducing the heating value of the gas and causing pipeline corrosion, so CO₂ level should be lowered to below 2% to meet the United States pipeline specifications. Membrane separation technology can be advantageous over cryogenic distillation and amine adsorption in terms of cost and efficiency. The key hurdle to overcome in polymeric membrane separation technology is improvement in selectivity, productivity, and durability without introducing significant additional cost. The ultimate goal of this study is to analyze effects due to polymerization conditions and imidization methods on properties of 1,3-propanediol monoesterified crosslinkable polyimide (PDMC). Hillock, Omole, Ward, and Ma did work on PDMC synthesis; however, variability of polymer properties remains a challenge that must be overcome for industrial implementation of PDMC material. First, reaction temperature and reaction time of polymerization prior to imidization were considered as key conditions to affect molecular weight, crosslinkability and transport properties of polymer. Batches with controlled reaction temperature and time were prepared, and properties of each dense film were measured and optimized in terms of permeability, selectivity, and plasticization suppression. Second, imidization methods for PDMC were also studied. There are mainly two kinds of Imidization: chemical Imidization and thermal Imidization. Surprisingly, thermally imidized PDMC showed 70% higher permeability than chemically imidized samples with minimal acrifice in selectivity. At high reaction temperature during the thermal imidization, transamidation can occur. It is believed that the transamidation led to more randomized sequence distribution in the thermally imidized samples. We thus hypothesize that the higher permeability of the thermally imidized PDMC results from greater uniformity of the sequence distribution, as compared to the chemically imidized sample that does not experience high temperature during imidization. XRD, DSC, DMA, and permeation instruments checked and supported this hypothesis. FTIR, TGA, and NMR ruled out the possibility of an alternate hypothesis related to side reaction. Finally, effects of aggressive feed conditions on both chemically imidized PDMC and thermally imidized PDMC dense film were examined. The aggressive feed conditions include high CO₂ partial pressure, operating temperatures, and exposure to high feed pressure. Testing aggressive feed conditions for dense film should be pursued before pursuing hollow fiber applications, to decouple effects on the basic material from those on the more complex asymmetric morphology. This study enables understanding of the disparity between various previous researchers’ selectivity and permeability values. The work shows clearly that polymerization conditions and imidization methods must be specified and controlled to achieve consistently desirable polymer properties. In addition, for batch scale-up and development to a hollow fiber, this fundamental study should enable production of high molecular weight PDMC with good fiber spinnability and defect-free structure.

Thesis (M.S.)--Rutgers University, 2007. / "Graduate Program in Biomedical Engineering." Includes bibliographical references (p. 80-85).

Poly(vinyl chloride)(PVC) wire and cable insulation has poor thermal stability, causing the plasticizer to separate from the PVC chain and produce an oily residue, lowering the tensile elongation at break and thus increasing brittleness. We have added 4 wt.% of three different types of cross-linking agents and antioxidants, as well as mixtures of both, to improve the thermal stability of the plasticizer and tensile properties of PVC after thermal exposure. We performed tensile tests, tribological tests, profilometry, scanning electron microscopy(SEM) and water absorption determination before and after thermal exposure at 136 ℃ for 1 week. After adding the agents, elongation at break increased by 10 to 20 % while the wear rate and water absorption were lower than for the control sample. Less voids are seen in the SEM images after adding these two kinds of agents. The thermal resistance of the PVC cable insulation is best enhanced by combinations of cross-linking agents and antioxidants.

Submitted by Jean Medeiros (jeanletras@uepb.edu.br) on 2016-03-17T16:57:36Z No. of bitstreams: 1 PDF - Silvana Cartaxo da Costa.pdf: 2220034 bytes, checksum: fa6fe0b10616cb9ed70f24ac4dc15d62 (MD5) / Approved for entry into archive by Secta BC (secta.csu.bc@uepb.edu.br) on 2016-07-22T15:01:05Z (GMT) No. of bitstreams: 1 PDF - Silvana Cartaxo da Costa.pdf: 2220034 bytes, checksum: fa6fe0b10616cb9ed70f24ac4dc15d62 (MD5) / Approved for entry into archive by Secta BC (secta.csu.bc@uepb.edu.br) on 2016-07-22T15:01:16Z (GMT) No. of bitstreams: 1 PDF - Silvana Cartaxo da Costa.pdf: 2220034 bytes, checksum: fa6fe0b10616cb9ed70f24ac4dc15d62 (MD5) / Made available in DSpace on 2016-07-22T15:01:16Z (GMT). No. of bitstreams: 1 PDF - Silvana Cartaxo da Costa.pdf: 2220034 bytes, checksum: fa6fe0b10616cb9ed70f24ac4dc15d62 (MD5) Previous issue date: 2014-05-23 / The development of a colon-specific delivery system using polymeric microparticles has received great attention in the pharmaceutical field. An interesting group of polymers with potential properties in this area are the hemicellulose. Xylan is a hemicellulose that has the ability to pass through the digestive tract unchanged and its complex structure requires enzymes that are produced specifically by the human colonic microflora, which makes it an interesting raw material in the production of target drug delivery systems. The microparticulate systems can be developed by various techniques. The interfacial crosslinking polymerization is one of the major techniques to produce polysaccharide based microparticles. However, the use of highly toxic crosslinkers often makes the use of this technique limited. The sodium trimetaphosphate (TSTP), a low toxic crosslinking agent, has no adverse effects reported on human beings. The aim of this study was to develop xylan microparticles using sodium trimetaphosphate. The microparticles were characterized by optical microscopy, SEM, XRD and FT -IR. The influence of different parameters on the diameter of the microparticles was analyzed during their development. Toxicity studies against Artemia saline Leach were made to compare the microparticles produced with terephthaloyl chloride and sodium trimetaphosphate. Xylan microparticles showed to be spherical shape, well individualized and with a smooth surface. All different parameters influenced the in size of the microparticles. The FT-IR spectrum of microparticles was similar to xylan, but with the presence of the peak at 1258 cm -1 , which is typical of phosphate ester bonds, that can be attributed to the bond between TSTP and xylan during the crosslinking process. The xylan microparticles produced in this work showed no toxicity at the concentration studied. It be concluded that TSTP was able to produce xylan microparticles with well defined physicochemical characteristics and low toxicity. / O desenvolvimento de um sistema de liberação cólon-específica utilizando micropartículas poliméricas têm recebido grande atenção no campo farmacêutico. Um grupo interessante de polímeros com potenciais propriedades nessa área são as hemiceluloses. A xilana é uma hemicelulose que tem a capacidade de passar através do trato digestivo inalterada e sua complexa estrutura requer enzimas que são produzidas especificamente pela microflora colônica humana, o que a torna uma interessante matéria-prima na área produção de sistemas de liberação de fármacos. Os sistemas microparticulados podem ser desenvolvidos por várias técnicas. A reticulação polimérica interfacial é uma das principais técnicas para produção de micropartículas à base de polissacarídeos. Porém o uso de reticulantes de alta toxicidade muitas vezes torna o uso desta técnica limitada. O trimetafosfato de sódio (TSTP) é um reticulante de baixa toxicidade, sem efeitos adversos relatados em seres humanos. Esse trabalho teve como objetivo desenvolver micropartículas de xilana utilizando TSTP. As micropartículas foram caracterizadas por microscopia óptica, MEV, DRX e FT-IR. Estudos de toxicidade frente à artemia salina Leach foram feitos para comparar as micropartículas produzidas com cloridrato de tereftaloíla e trimetafosfato de sódio. As micropartículas de xilana apresentaram forma esférica, bem individualizada e com superfície bem definida. Todos os diferentes parâmetros influenciaram no tamanho das micropartículas. O espectro de FT-IR das micropartículas foi semelhante ao da xilana, porém com a presença de um pico em 1258 cm -1 , que é típico de ligações éster-fosfato, que pode ser atribuído a ligação entre TSTP e a xilana durante o processo de reticulação. As micropartículas de xilana produzidas neste trabalho não apresentaram toxicidade na concentração estudada. Podemos concluir que o TSTP foi capaz de produzir micropartículas de xilana com cracterísticas fisico-químicas bem definidas e de baixa toxicidade.

Recombinant synthesis of elastin-mimetic proteins has been employed for several decades, however, long-term biocompatibility and biostability of such proteins was not fully defined. We present virtually crosslinked elastin-mimetic proteins which exhibit exceptional biocompatibility and long-term biostability over a period of at least seven months. This report is the first evidence of a non-chemically or ionically crosslinked system that exhibits long-term in vivo stability. Although, physically crosslinked protein-based materials possess a number of advantages over their chemically crosslinked counterparts, physical crosslinks and the related domains so formed may be deformed or damaged at applied stresses lower than those required to disrupt covalent crosslinks. In this regard, we have synthesized a new class of recombinant elastin-mimetic triblock copolymer capable of both physical and chemical crosslinking. We have demonstrated that chemical crosslinking provides an independent mechanism for control of protein mechanical responses. Specifically, elastic modulus was enhanced and creep strain reduced through the addition of chemical crosslinking sites. A number of reports have described the design of synthetic genes, which encode elastin-like proteins for bacterial expression in Escherichia coli. Although advantages with this expression system exist, significant limitations including the lack of eukaryotic post-translational systems, the tendency to sequester mammalian proteins into inclusion bodies, difficult purification protocols, and endotoxin contamination have been noted. We demonstrate the expression of a recombinant elastin-mimetic protein from P. pastoris. A novel synthetic strategy, monomer library concatamerization, was utilized in designing non-repetitive elastin genes for highly repetitive protein sequences. It is likely that this strategy will be useful for creating large, repetitive genes for a variety of expression systems in order to more closely approach the genetic diversity inherent to native DNA sequences. All told, elastin-based protein polymers are a promising class of material characterized by high degree of biocompatibility, excellent biostability, and a tunable range of mechanical properties from plastic to elastic. A variety of options facilitate the processing of these biopolymers into chemically crosslinked or non-crosslinked gels, films, or nanofibers for any of a number of implant applications including structural components of artificial organs and engineered living tissues, carriers for controlled drug release, or biocompatible surface coatings.