T. Alan Hatton

Protein Separations Using Colloidal Magnetic Nanoparticles

Phospholipid‐coated colloidal magnetic nanoparticles with mean magnetite core size of 8 nm are shown to be effective ion exchange media for the recovery and separation of proteins from protein mixtures. These particles have high adsorptive capacities (up to 1200 mg protein/mL adsorbent, an order of magnitude larger than the best commercially available adsorbents) and exhibit none of the diffusional resistances offered by conventional porous ion exchange media. Protein‐laden particles are readily recovered from the feed solution using high‐gradient magnetic filtration.

ON THE SIZE AND SHAPE OF SELF-ASSEMBLED MICELLES

Equilibrium size and shape distributions of self-assembled micelles are investigated using lattice Monte Carlo simulation techniques. The micellar size distributions are shown to include a Gaussian peak of spherical micelles, in combination with an exponential tail of cylindrical micelles.

Cell death from Bursting bubbles : Role of cell attachment to rising bubbles in sparged reactors

Bursting bubbles are thought to be the dominant cause of cell death in sparged animal or insect cell cultures. Cells that die during the bubble burst can come from three sources: cells suspended near the bubble; cells trapped in the bubble lamella; and cells that attached to the rising bubble. This article examines cell attachment to rising bubbles using a model in which cell attachment depends on cell radius, bubble radius, and cell-bubble attachment time. For bubble columns over 1 m in height and without protective additives, the model predicts significant attachment for 0.5- to 3-mm radius bubbles, but no significant attachment in the presence of protective additives. For bubble columns over 10 cm in height, and without protective additives, the model predicts significant attachment for 50- to 100-micron radius bubbles, but not all protective additives prevent attachment for these bubbles. The model is consistent with three sets of published data and with our experimental results. Using hybridoma cells, serum-free medium with antifoam, and 1.60 +/- 0.05 mm (standard error) radius bubbles, we measured death rates consistent with cell attachment to rising bubbles, as predicted by the model. With 1.40 +/- 0.05 mm (SE) radius bubbles and either 0.1% w/v Pluronic-F68 or 0.1% w/v methylcellulose added to the medium, we measured death rates consistent with no significant cell attachment to rising bubbles, as predicted by the model.

Stochastic dynamics simulation of surfactant self-assembly

The self-assembly of short amphiphilic molecules of type A2B2 (A=hydrophilic, B=hydrophobic) is investigated using Stochastic Dynamics simulations with a scalar frictional coefficient. Equilibrium properties were calculated and explained in the context of existing thermodynamic theories. Spherical micelles are observed to form and the effect of temperature and total surfactant concentration on the structural properties are investigated. Above the critical micelle concentration, a decline in the free surfactant concentration is observed, contrary to existing theories dealing with micelle formation. The observed behavior can be explained in terms of excluded volume effects. Excellent agreement is obtained between the resulting predictions and the simulation results over the entire concentration range.

Aqueous magnetic fluids stabilized by surfactant bilayers

Surfactant bilayer-stabilized aqueous magnetic fluids have been characterized in terms of their magnetic properties, particle size and size distributions, and surfactant bilayer structure. Thermal analysis showed the existence of two distinct populations of surfactants on the particle surface and possible interpenetration between the tails of the primary and secondary surfactants, which was confirmed in a small angle neutron scattering study using contrast matching.

Fluorescence-Based Thermometry:Principles and Applications

236 Introduction 236 1. A Photophysical Description of Fluorescence 237 2. Temperature-Dependent Fluorescent Phenomena 239 2.1 Quantum Yield 240 2.2 Peak Wavelength 241 2.3 Fluorescence Emission From Molecular Complexes 242 3. Fluorescent Probes for Temperature Sensing 246 4. Photophysical Behavior of Bipyrenyl Fluorophores 249 4.1 Fluorescent Behavior of Bipyrenyl Fluorophores in Dodecane 250 4.1.1 Spectral Characteristics 250 4.1.2 Ground-State Conformation 253 4.2 Thermochromic Behavior of Excimer Emission 256 5. Implementation of Fluorescence-Based Thermometry 261 5.1 Previous Work 262 5.2 General Design of Fluorescent Thermometry 263 5.2.1 Common Excitation and Detector Design Considerations 264 5.2.2 The Excitation System 264 5.2.3 The Detector 267 5.3 Dimensionality of Data 270 5.4 One-Dimensional Implementation 270 5.4.1 One-Dimensional Experimental Example 272 5.5 Two-Dimensional Implementation 274 5.5.1 Excitation System 276 5.5.2 Detection System 276 5.6 Three-Dimensional Implementation 279 6. Conclusions and Future Outlook 281

Pluronic block copolymers and Pluronic poly(acrylic acid) microgels in oral delivery of megestrol acetate

Several Pluronic‐based formulations were studied in‐vitro and in a rat model with respect to the release and bioavailability of megestrol acetate (MA) after oral administration. It was demonstrated that an aqueous, micellar formulation comprising a mixture of a hydrophobic (L61) and a hydrophilic (F127) Pluronic copolymer, significantly enhanced the bioavailability of MA administered orally at relatively low doses (1–7 mg kg−1). Pluronic‐based microgels (spherical gel particles of sub‐millimetre size) were introduced as MA vehicles. The microgels comprised a cross‐linked network of poly(acrylic acid) onto which the Pluronic chains were covalently attached. Microgels of Pluronic L92 and poly(acrylic acid) fabricated into tablet dosage forms exhibited dramatically lowered MA initial burst release. The MA release was pH‐dependent owing to the pH sensitivity of the microgel swelling, with the drug retained by the microgel at pH 1.8 and released slowly at pH 6.8. In the rat model, a significant increase in MA bioavailability was observed when the microgel‐formulated MA was administered orally at a high dose of 10 mg kg−1, owing to the enhanced retention of the microgel. The study of the microgel passage through the gastrointestinal tract demonstrated the microgel retention characteristic of a very high molecular weight polymer and the absence of any systemic absorption of the polymer.

Protein Refolding by Reversed Micelles Utilizing Solid-Liquid Extraction Technique

This article reports that a reversed micellar solution is useful for refolding proteins directly from a solid source. The solubilization of denatured RNase A, which had been prepared by reprecipitation from the denaturant protein solution, into reversed micelles formulated with sodium di-2-ethylhexyl sulfosuccinate (AOT) has been investigated by a solid-liquid extraction system. This method is an alternative to the ordinary protein extraction in reversed micelles based on the liquid-liquid extraction. The solid-liquid extraction method was found to facilitate the solubilization of denatured proteins more efficiently in the reversed micellar media than the ordinary phase transfer method of liquid extraction. The refolding of denatured RNase A entrapped in reversed micelles was attained by adding a redox reagent (reduced and oxidized glutathion). Enzymatic activity of RNase A was gradually recovered with time in the reversed micelles. The denatured RNase A was completely refolded within 30 h. In addition, the efficiency of protein refolding was enhanced when reversed micelles were applied to denatured RNase A containing a higher protein concentration that, in the case of aqueous media, would lead to protein aggregation. The solid-liquid extraction technique using reversed micelles affords better scale-up advantages in the direct refolding process of insoluble protein aggregates.

Dynamics of self-assembled surfactant systems

The dynamics of self-assembling systems were investigated for the model amphiphile A2B2 using stochastic dynamic simulations. Temperature jump computer “experiments” were performed and the evolution of the system to its new equilibrium state monitored. The results were interpreted based on the Aniansson–Wall theory of micellar kinetics. The transient behavior predicted using the Aniansson–Wall theory agrees well with the simulated data, particularly at short times. At long times, deviations are observed which may be ascribed to errors in estimating the dissociation rate and number density of aggregates in the all important micelle-depleted zone. The micellar dissociation constant was calculated from independent tagging simulations. The amphiphile exit rate constant was calculated at different temperatures from which an activation energy associated with the removal of a surfactant chain from a micelle was found to be of order 10–15 kT and to be independent of the friction coefficient. Finally the Helmholtz...

General reptation and scaling of 2d athermal polymers on close-packed lattices

A new treatment of the general reptation algorithm, introducing the notion of a transition state, is presented for close-packed lattices. Dynamic and static properties of short N-bead polymer systems on the two-dimensional triangular lattice are obtained from Monte Carlo simulations using this algorithm for systems ranging from dilute to concentrated conditions. Extrapolation of the simulation results to long chain lengths is found to be consistent with simple asymptotic power law scaling relations for 〈Rg2〉, the mean-square dimensions of the polymer; their relaxation time, τR; and the center of mass self-diffusion coefficient, Dc.m.. Empirical formulas of the form 〈RSAW2〉/〈R2〉=αX+exp(−βX) are proposed for the mean-square dimensions, 〈R2〉, of athermal polymers as a function of the scaled density X=(N−1)dν−1ρ; where ρ is the polymer segment density; ν is the power law exponent for the mean dimensions, 〈RSAW2〉, of dilute chains of the same length (N−1); and d is the dimensionality of the system. The fitted ...