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Browsing Health and Biomedical Sciences by Author "Edwin, Nadia J."
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Item Determination of Particle Size Distributions, Molecular Weight Distributions, Swelling, Conformation, and Morphology of Dilute Suspensions of Cross-linked Polymeric Nanoparticles Via Size-exclusion Chromatography/Differential Viscometry(2014) Edwin, Nadia J.Size-exclusion chromatography (SEC), coupled with differential viscometry detection (SEC/DV), is applied to the dilute suspension characterization of solvent-swollen cross-linked polymeric nanoparticles (PNPs). Cross-linked, unimolecular polymeric nanoparticles in the 5–50 nm weight-average diameter (dw) range were prepared by batch and semibatch microemulsion polymerization techniques and isolated. SEC and SEC/DV characterization techniques yield, based on the principle of universal calibration, a wealth of information regarding the structural attributes of PNPs, including apparent and absolute molecular weight distributions, apparent and absolute molecular weight averages, peak and weight-average particle diameters, particle size distributions in both the solvent-swollen and solvent-free states, particle conformation (shape), and an estimate of the volumetric swell factor. These structural parameters are critical to understanding PNP performance, and all are obtained in a single rapid chromatographic experiment, when conducted under conditions where universal calibration applies. Particle sizes determined under such conditions are in excellent agreement with those obtained by dynamic light scattering, transmission electron microscopy, hydrodynamic chromatography, and SEC/static light scattering (SEC/SLS). In addition, Mark–Houwink exponents of approximately zero were found across the molecular weight and size distribution of many of these tightly cross-linked PNPs, which is consistent with a spherical particle conformation in these dilute suspensions. The SEC/DV methods are especially valuable to characterize the diameter, volume swell factor, and suspension conformation of small (4–5 nm dw) PNPs.Item Dynamics of Poly(styrenesulfonate) Sodium Salt in Aqueous Solution(2006) Edwin, Nadia J.The diffusion of poly(styrenesulfonate) sodium salt (NaPSS) was investigated using dialysis dynamic light scattering (DLS) and dialysis fluorescence photobleaching recovery (FPR). Never-dried or “virgin” NaPSS was synthesized directly from 4-styrenesulfonic sodium salt to achieve 100% sulfonation. Upon reducing the ionic strength directly in the DLS cell by dialysis, a clean sample developed clearly distinct fast and slow modes that were first identified as an extraordinary phase in low-salt solutions of poly-l-lysine by Lin, Lee, and Schurr [Biopolymers1978, 17, 1041]. This result complements published polyelectrolyte investigations in a high-dielectric constant organic solvent, and also studies where the degree of ionization was tuned, which confirms that hydrophobic patches along the polymer chain are not required for the extraordinary behavior. The fast mode dominated even at low ionic strength, with scattering amplitude exceeding 70% of the total. For the virgin NaPSS sample in the dialysis cell, there is no convincing evidence of a slow mode at high salt (≥200 mM NaCl). The appearance of distinct slow and fast modes proved reversible upon removing and adding salt by dialysis, without any other perturbation save restoration of the concentration by dialysis centrifugation. This suggests that the behavior represents a thermodynamically equilibrated state. Dialysis FPR measurements of aqueous solutions of a commercial NaPSS that was labeled with fluoresceinamine (LNaPSS) showed no obvious long-range ordering. A reversible decrease in the optical tracer self-diffusion coefficient of LNaPSS as salt is dialyzed out of the solution is instead attributed to chain expansion. Comparison of FPR and DLS on a mixed LNaPSS/NaPSS sample suggests that the residence time of a chain in temporal aggregates [Schmitz et al. Biopolymers1984, 23, 1637] that are thought to be responsible for the DLS slow mode is shorter than the FPR time scale.Item Elucidating the Kinetics of β-amyloid Fibril Formation(American Chemical Society, 2005) Edwin, Nadia J.The formation of β-Amyloid peptide (Aβ1-40) aggregates was monitored by dynamic light scattering. Various sizes of materials may be present throughout the aggregation process, but small scatterers are difficult to detect in the presence of large ones. Fluorescence photobleaching recovery studies on 5-carboxyfluorescein-labeled Aβ1-40 peptide solutions readily confirmed the presence of large and small species simultaneously. The effects of dye substitution on the aggregation behavior of Aβ1-40 peptide are subtle, but should not prevent further investigations by fluorescence photobleaching recovery or other fluorescence methods.Item Fluorescence Photobleaching Recovery: A Primer(Springer, 2008) Edwin, Nadia J.Item β-amyloid Protein Aggregation(Humana Press, 2007) Edwin, Nadia J.The β-amyloid peptide aggregates via a nucleation pathway where micellar aggregates propagate to form oligomers (protofibrils), which then polymerize into insoluble fibrils. This fibrillogenic process has been linked to the pathogenesis associated with Alzheimer’s disease. One purpose of this chapter is to provide a protocol for reliably producing monomeric Aβas a starting point for physical and biological studies. Many research groups have used organic solvents to disaggregate pre-seeded Aβ in an attempt to acquire monomeric starting materials. Others have used instrumental techniques such as size exclusion chromatography to isolate monomer, structural intermediates, and fibrils and study their affects on A β nucleation. This chapter discusses a modified method of A βpreparation using organic solvents followed by dissolution into aqueous phosphate buffer systems that renders monomeric A β starting solutions for kinetic experiments. Additionally, this chapter details a number of physical techniques such as scanning force microscopy, circular dichroism spectroscopy, transmission electron microscopy, fluorescence spectroscopy, fluorescence photobleaching recovery, and dynamic light scattering, together with physiological techniques such as cell viability assays to characterize Aβ nucleation, aggregation, and fibrillization and the potential biological activity of the various A βparticles.