Kinetics of polymer adsorption, desorption and exchange

The aim of the study in this thesis was to gain more insight in the kinetics of polymer adsorption. To this end some well-characterised polymers have been systematically investigated.In the process of polymer adsorption one may distinguish three kinetic contributions: transport to the surface, attachment, and reconformation of the adsorbing and adsorbed chains. In order to assess the role of each of the three contributions it is necessary to measuri the adsorption kinetics under well-defined hydrodynamic conditions. For such measurements the transport (convection and diffusion) can be calculated and therefore it becomes possible to study unambiguously the interfacial processes, i.e., attachment and reconformation.For this study two experimental techniques were used that both fulfil the requirement that the adsorption occurs under well-defined hydrodynamic conditions: reflectometry in a stagnation point flow (chapters 2,3 and 5-7) and a streaming potential method (chapter 4). With both techniques it is possible to follow directly and continuously the buildup of an adsorbed layer. Reflectometry is a relatively new and simple optical technique for the measurement of adsorption on (optically flat) solid surfaces. In a reflectometer a linearly polarised light beam is reflected from the (adsorbing) surface, and the reflected beam is split into its parallel and perpendicular components. The intensity ratio between the two components is continuously measured. This ratio changes upon adsorption, and after calibration the adsorbed amount (mass/area) is obtained. For reflectometry there are only few restrictions on the choice of adsorbate, adsorbent and solvent.The applicability in this study of the streaming potential method is limited to adsorption of uncharged polymers from aqueous solution. For that case, the streaming potential can be related to the hydrodynamic layer thickness of the adsorbed polymer layer. This thickness is mainly determined by loose ends of adsorbed chains, and it is sensitive to very small changes in the adsorbed amount of long chains near saturation. Such small changes occur for desorption of long chains into solvent, so that the streaming potential method is especially suitable for the measurement of the desorption kinetics.In chapter I the aim and scope of this study of this study are explained, and a general introduction to adsorption of polymers is given.Chapter 2 deals with the measurement of adsorption by reflectometry. Using the results of an optical model we discuss the possibilities of the method for measuring the adsorption from dilute solution on a thin film on top of a silicon substrate. For a wide variety of solvents and film materials, a sensitivity can be obtained of the order of 1-2% change in reflectivity per mg/m 2adsorbed, which is quite enough for an accurate determination of the adsorbed amount. By choosing carefully the film thickness and angle of incidence of the light beam, it can be achieved that the reflected intensity varies proportionally with the adsorbed amount, independent of the concentration profile in the adsorbed layer. Under such conditions, the reflectometric signal can be simply converted into the adsorbed amount.In chapter 3 reflectometry is used to investigate the kinetics of adsorption of poly(ethylene oxide) (PEO) from water onto oxidised silicon. For the stagnation point flow the maximum rate of mass transfer of polymer to the surface is calculated. This rate is compared with the observed adsorption rate, and it is concluded that mass transfer is ratelimiting up to or nearly up to saturation, depending on the chain length. Only for long chains ( M >100 kg/mole) near saturation the adsorption rate is lowered by surface processes.In chapter 4 a model is discussed for the desorption rate of polymers into a flow of pure solvent. This model is based on the assumption that near the surface there is a rapid equilibration between free and adsorbed polymer, and that transport of free polymer away from the surface is ratelimiting for the desorption. Due to the shape of the (high affinity) isotherm, the equilibrium concentration of free chains even after a minute desorption is extremely low, so that the transport -and thus the desorptionproceeds slowly. Thus, in spite of the rapid local equilibration, the desorption is slow because of the slow mass transfer. For a logarithmic adsorption isotherm of the polymer (for which the adsorbed amount Γincreases linearly with the log of the concentration c in solution) an explicit expression for the adsorbed amount as a function of time is derived: the desorbed amount increases proportionally with log t. The model predicts that the absolute value of the slope of the (kinetic) desorption curve Γ(log t ) and the (static) adsorption isotherm Γ(log c ) are the same.Using the streaming potential method it is shown in chapter 4 that the above model gives an adequate description of the desorption kinetics in aqueous solutions of PEO on glass, even for high molar mass polymer (M = 847 kg/mole). Again, this shows that the equilibration of adsorbed layers of PEO is rapid as compared to the rate of mass transfer through solution.Chapter 5 describes the adsorption kinetics of polystyrene (PS) from decalin on oxidised silicon. On a bare surface the adsorption rate of PS is limited by mass transfer from solution, like for PEO. For PS, the adsorption rate decreases gradually with increasing coverage. This is due to a decreasing probability of attachment during a collision of a free chain with the (covered) surface. From experiments in which the chain length, the solvent quality and the adsorption energy were varied, the picture arises that the adsorption probability during a collision is the result of a balance between a gain in adsorption energy on the one hand, and repulsive interaction with the adsorbed layer on the other.Exchange between polymers that differ in chain length only is the subject of chapter 6. Displacement of adsorbed short chains of PEO by longer ones in solution is limited only by transport of long chains to the surface. The adsorbed layer is continuously in equilibrium with the solution near the surface. The same conclusion was drawn from the desorption kinetics of this polymer in a flow of pure solvent (chapter 4). For PS also surface processes play a role. During exchange of short by long chains of PS there is a temporary overshoot of short chains in the adsorbed layer. This overshoot may desorb either during adsorption of long chains, or by relaxation of the adsorbed layer. By interrupting the transport of long chains to the surface, this relaxation could also be directly observed. The higher chain stiffness of PS as compared to PEO possibly explains the slower equilibration of adsorbed PS.Finally, we present in chapter 7 some results on the exchange kinetics between three chemically different polymers: polystyrene (PS), poly(butyl methacrylate) (PBMA) and polytetrahydrofuran (PTHF). Displacement of adsorbed layers of the rather stiff polymers PS and PBMA by the very flexible PTHF is limited only by transport of the displacing polymer from the bulk solution. For mutual exchange between the two stiff polymers, surface processes play an important role: the displacer PBMA adsorbs quickly, whereas PS desorbs slowly. Possibly, the slow exchange kinetics is caused by the low mobility of the adsorbed polymers. The displacement rate of PS by PBMA increases considerably after addition of a displacer of low molar mass. The faster exchange kinetics is probably due to the lower binding strength and, consequently higher mobility of the adsorbed polymers

Determination of the Activity of Aminotransferases: Comparison of Two Buffer Systems with and without Supplementary Pyridoxal-5′-phosphate

Peer Reviewe

Mechanism of double-base lesion bypass catalyzed by a Y-family DNA polymerase

As a widely used anticancer drug, cis-diamminedichloroplatinum(II) (cisplatin) reacts with adjacent purine bases in DNA to form predominantly cis-[Pt(NH3)2{d(GpG)-N7(1),-N7(2)}] intrastrand cross-links. Drug resistance, one of the major limitations of cisplatin therapy, is partially due to the inherent ability of human Y-family DNA polymerases to perform translesion synthesis in the presence of DNA-distorting damage such as cisplatin–DNA adducts. To better understand the mechanistic basis of translesion synthesis contributing to cisplatin resistance, this study investigated the bypass of a single, site-specifically placed cisplatin-d(GpG) adduct by a model Y-family DNA polymerase, Sulfolobus solfataricus DNA polymerase IV (Dpo4). Dpo4 was able to bypass this double-base lesion, although, the incorporation efficiency of dCTP opposite the first and second cross-linked guanine bases was decreased by 72- and 860-fold, respectively. Moreover, the fidelity at the lesion decreased up to two orders of magnitude. The cisplatin-d(GpG) adduct affected six downstream nucleotide incorporations, but interestingly the fidelity was essentially unaltered. Biphasic kinetic analysis supported a universal kinetic mechanism for the bypass of DNA lesions catalyzed by various translesion DNA polymerases. In conclusion, if human Y-family DNA polymerases adhere to this bypass mechanism, then translesion synthesis by these error-prone enzymes is likely accountable for cisplatin resistance observed in cancer patients

Grafted block complex coacervate core micelles and their effect on protein adsorption on silica and polystyrene

We have studied the formation and the stability of grafted block complex coacervate core micelles (C3Ms) in solution and the influence of grafted block C3M coatings on the adsorption of the proteins β-lactoglobulin, bovine serum albumin, and lysozyme. The C3Ms consist of a grafted block copolymer PAA21-b-PAPEO14 (poly(acrylic acid)-b-poly(acrylate methoxy poly(ethylene oxide)), with a negatively charged PAA block and a neutral PAPEO block and a positively charged homopolymer P2MVPI (poly(N-methyl 2-vinyl pyridinium iodide). In solution, these C3Ms partly disintegrate at salt concentrations between 50 and 100 mM NaCl. Adsorption of C3Ms and proteins has been studied with fixed-angle optical reflectometry, at salt concentrations ranging from 1 to 100 mM NaCl. In comparison with the adsorption of PAA21-b-PAPEO14 alone adsorption of C3Ms significantly increases the amount of PAA21-b-PAPEO14 on the surface. This results in a higher surface density of PEO chains. The stability of the C3M coatings and their influence on protein adsorption are determined by the composition and the stability of the C3Ms in solution. A C3M-PAPEO14/P2MVPI43 coating strongly suppresses the adsorption of all proteins on silica and polystyrene. The reduction of protein adsorption is the highest at 100 mM NaCl (>90%). The adsorbed C3M-PAPEO14/P2MVPI43 layer is partly removed from the surface upon exposure to an excess of β-lactoglobulin solution, due to formation of soluble aggregates consisting of β-lactoglobulin and P2MVPI43. In contrast, C3M-PAPEO14/P2MVPI228 which has a fivefold longer cationic block enhances adsorption of the negatively charged proteins on both surfaces at salt concentrations above 1 mM NaCl. A single PAA21-b-PAPEO14 layer causes only a moderate reduction of protein adsorption

Enhanced antitumor efficacy of cisplatin in combination with HemoHIM in tumor-bearing mice

<p>Abstract</p> <p>Background</p> <p>Although cisplatin is one of the most effective chemotherapeutic agents, cisplatin alone does not achieve a satisfactory therapeutic outcome. Also cisplatin accumulation shows toxicity to normal tissues. In this study, we examined the possibility of HemoHIM both to enhance anticancer effect with cisplatin and to reduce the side effects of cisplatin in melanoma-bearing mice.</p> <p>Methods</p> <p>HemoHIM was prepared by adding the ethanol-insoluble fraction to the total water extract of a mixture of 3 edible herbs, Angelica Radix, Cnidium Rhizoma and Paeonia Radix. Anticancer effects of HemoHIM with cisplatin were evaluated in melanoma-bearing mice. We used a Cr⁵¹-release assay to measure the activity of NK/Tc cell and ELISA to evaluate the production of cytokines.</p> <p>Results</p> <p>In melanoma-bearing mice, cisplatin (4 mg/kg B.W.) reduced the size and weight of the solid tumors, and HemoHIM supplementation with cisplatin enhanced the decrease of both the tumor size (p < 0.1) and weight (p < 0.1). HemoHIM itself did not inhibit melanoma cell growth <it>in vitro</it>, and did not disturb the effects of cisplatin <it>in vitro</it>. However HemoHIM administration enhanced both NK cell and Tc cell activity in mice. Interestingly, HemoHIM increased the proportion of NK cells in the spleen. In melanoma-bearing mice treated with cisplatin, HemoHIM administration also increased the activity of NK cells and Tc cells and the IL-2 and IFN-γ secretion from splenocytes, which seemed to contribute to the enhanced efficacy of cisplatin by HemoHIM. Also, HemoHIM reduced nephrotoxicity as seen by tubular cell of kidney destruction.</p> <p>Conclusion</p> <p>HemoHIM may be a beneficial supplement during cisplatin chemotherapy for enhancing the anti-tumor efficacy and reducing the toxicity of cisplatin.</p

Grafted ionomer complexes and their effect on protein adsorption on silica and polysulfone surfaces

We have studied the formation and the stability of ionomer complexes from grafted copolymers (GICs) in solution and the influence of GIC coatings on the adsorption of the proteins β-lactoglobulin (β-lac), bovine serum albumin (BSA), and lysozyme (Lsz) on silica and polysulfone. The GICs consist of the grafted copolymer PAA28-co-PAPEO22 {poly(acrylic acid)-co-poly[acrylate methoxy poly(ethylene oxide)]} with negatively charged AA and neutral APEO groups, and the positively charged homopolymers: P2MVPI43 [poly(N-methyl 2-vinyl pyridinium iodide)] and PAH∙HCl160 [poly(allylamine hydrochloride)]. In solution, these aggregates are characterized by means of dynamic and static light scattering. They appear to be assemblies with hydrodynamic radii of 8 nm (GIC-PAPEO22/P2MVPI43) and 22 nm (GIC-PAPEO22/PAH∙HCl160), respectively. The GICs partly disintegrate in solution at salt concentrations above 10 mM NaCl. Adsorption of GICs and proteins has been studied with fixed angle optical reflectometry at salt concentrations ranging from 1 to 50 mM NaCl. Adsorption of GICs results in high density PEO side chains on the surface. Higher densities were obtained for GICs consisting of PAH∙HCl160 (1.6 ÷ 1.9 chains/nm2) than of P2MVPI43 (0.6 ÷ 1.5 chains/nm2). Both GIC coatings strongly suppress adsorption of all proteins on silica (>90%); however, reduction of protein adsorption on polysulfone depends on the composition of the coating and the type of protein. We observed a moderate reduction of β-lac and Lsz adsorption (>60%). Adsorption of BSA on the GIC-PAPEO22/P2MVPI43 coating is moderately reduced, but on the GIC-PAPEO22/PAH∙HCl160 coating it is enhanced