Geoff Smith

Dielectric properties of residual water in amorphous lyophilized mixtures of sugar and drug

Dielectric relaxation spectroscopy was used to investigate the properties of residual water in lyophilized formulations of a proprietary tri-phosphate drug containing a sugar (trehalose, lactose or sucrose) or dextran. The dielectric properties of each formulation were determined in the frequency range (0.1 Hz–0.1 MHz) and temperature range (30°C–Tg). The temperature dependence of the relaxation times for all samples showed Arrhenuis behaviour, from which the activation energy was derived. Proton hopping through the hydrogen-bonded network (clusters) of water molecules was suggested as the principle mode of charge transport. Significant differences in dielectric relaxation kinetics and activation energy were observed for the different formulations, which were found to correlate with the amount of monophosphate degradation product.

Low frequency dielectric study on hydrated ovalbumin

Abstract This paper reports the use of low frequency dielectric measurements (0.1 Hz–1 MHz) in the investigation of low hydrated ovalbumin ( 2 O/g ovalbumin), for the purpose of examining the nature of water–protein interactions. Dielectric spectra for each hydration of ovalbumin showed two distinct processes at low and high frequency. The low frequency response showed anomalous dielectric behaviour, in which the real and imaginary permittivities are parallel and increased dramatically toward low frequency. This anomalous dielectric response followed a universal fractional power law of permittivity versus frequency and was ascribed to the quasi-dc percolation of protons through the protein matrix. The high frequency response showed a dielectric loss peak which was modeled by the Davidson–Cole expression, and was ascribed to proton hopping at the scale of the protein macromolecule. A temperature study (253–363 K) on the hydrated ovalbumin revealed that a percolation process was associated with the dielectric response of the hydrated proteins. By calculating fractal dimension from the percolation threshold, it was possible to define a hydration level of 0.08 g/g at which the surface of the protein was saturated by water.

Dielectric Analysis of Phosphorylcholine Head Group Mobility in Egg Lecithin Liposomes

AbstractPurpose. A knowledge of the interfacial properties of lecithin underpins our understanding of many of the physicochemical characteristics of drug delivery systems such as liposomes and lecithin stabilized microemulsions. In order to further this understanding, a high frequency dielectric study of the interfacial properties of egg lecithin liposomes was undertaken. Methods. The effect of temperature, lecithin concentration and probe sonication on the interfacial dielectric properties of liposomal suspensions was investigated by high frequency dielectric relaxation spectroscopy between 0.2–6 GHz. Results. The frequency dependent permittivity of each suspension exhibited a dielectric dispersion centred around 100 MHz, corresponding to the relaxation of zwitterionic head groups. The activation energy for head group reorientation was estimated as ΔH = 6.3 kJ mol−1. There was an increase in extent of inter-head group interactions on increasing the liposome volume fraction, whereas the effect of probe sonication showed that: (i) head groups in both the outer and inner lamellae contribute to the dielectric response; (ii) the head groups may be less restricted in liposomes of high surface curvature with few lamellae; (iii) the high frequency permittivity of the suspension increased on sonication, as a result of a reduction in the amount of (depolarized) interlamellar water following a reduction in the number of lamellae per liposome. Conclusions. Dielectric analysis of the zwitterionic head groups of lecithin therefore provides a means for investigating the surface of lecithin liposomes, and may be used to investigate the effect of drugs and other solutes on membranes.

Fluctuation-noise spectroscopy and a “universal” fitting function of amplitudes of random sequences

A “universal” fitting function has been recognized that makes use of the eigen-coordinates method (Physica A 285 (2000) 547) to accurately describe the distribution of ordered amplitudes within random sequences (taken from a diversity of sources). It is shown that sequences with a discrete structure can be described in terms of specific distributions of relative frequencies with respect to the number of quantum levels. An investigation of these quantum distributions leads to an increase in both sensitivity and selectivity, when attempting the statistical detection of various predominant factors of the hidden signals. The physical meaning of this new function is discussed and a proposal is made as the basis of a new fluctuation-noise spectroscopy, in which the recognized function can be used for the detection of small signals and/or the forerunners of strong signals that are hidden within the random sequences analyzed.

A THz spectroscopy method for quantifying the degree of crystallinity in freeze-dried gelatin/amino acid mixtures: An application for the development of rapidly disintegrating tablets

OBJECTIVE In rapidly disintegrating tablets (RDTs) manufactured by freeze-drying processes, the % crystallinity of the freeze-dried matrix underpins the physical properties such as mechanical strength and dissolution profile. This study examines the feasibility of using terahertz pulsed spectroscopy (TPS) as an off-line tool (in the first instance) for assessing the degree of crystallinity in co-freeze dried amino-acid/gelatine mixtures. MATERIALS AND METHODS Three amino acids (l-alanine, serine and proline) were studied by TPS (in the wave number range of 3-100cm(-1)) both in the pure crystalline form and in the form of a co-freeze-dried matrix with gelatin (in weight fractions of 10:90, 30:70, 50:50, and 70:30). RESULTS In co-freeze dried proline/gelatin matrix no crystallinity was observed using TPS, whereas ~62±1% (n=3) and 72±0.5% (n=3) crystallinity was observed in the co-freeze dried alanine/gelatin matrix (50% w/w and 70% w/w, respectively). Similarly, ~41±0.5% (n=3) and ~80±0.5% (n=3) crystallinity was observed in the serine/gelatin matrix (30% w/w and 50% w/w respectively). CONCLUSION The potential for using TPS as a quantitative in-line tool for determining the degree of crystallinity in a range of complex systems such as RDTs, conventional tablets, sprayed dried micro particles, is suggested from these results.

Effect of acidic and basic pH on Thioflavin T absorbance and fluorescence.

Thioflavin T (ThT) is a fluorescent dye able to enhance significantly its fluorescence quantum yield upon binding to protein amyloids. ThT assay is widely used to detect and quantify amyloids in a variety of conditions, including solutions with different pH levels. In the present work, the effect of acidic and basic pH on the conformation of the ThT molecule and its absorption and fluorescence properties was studied. The results show that both acidic and basic pH decrease significantly the intensity of ThT absorption in the visible region and fluorescence emission intensity. Low pHs induce an immediate “all-or-nothing” decrease in the ThT signal, while in alkaline solutions the ThT signal decreases gradually over time. pH-induced signal quenching is less in the presence of glycerol or protein aggregates. Two different mechanisms are responsible for the ThT signal quenching—the ThT hydroxylation at basic pH and protonation of the nitrogen atom of the dimethylamino group at acidic pH. ThT assays should be carefully carried out at basic or acidic pH as strong pH dependence of ThT could be responsible for misinterpretation and false positive/negative experimental results. The potential unsuitability of ThT as a probe in solutions with high pH (>9) has been shown.

A percolation cluster model of the temperature dependent dielectric properties of hydrated proteins

This study investigates the temperature dependence of the low frequency dielectric properties (0.1 Hz–1 MHz) of hydrated globular proteins (namely, ovalbumin, lysozyme and pepsin). The study aims to reveal the mechanisms of water–protein interaction from the dielectric response of these model proteins. Two principle dielectric responses were observed for each hydrated protein, namely, an anomalous low frequency dispersion and a dielectric loss peak at higher frequency (called the e3 dispersion). The low frequency response conformed to a fractional power low of frequency, while the higher frequency response conformed to a Davidson–Cole model. The strength of both processes reached a maximum at a certain temperature within the experimental temperature range. This temperature is referred to as the percolation threshold (PT) and is thought to be associated with the percolation of protons between hydrogen-bonded water molecules. The relaxation times of the e3 dispersion conformed to Arrhenius behaviour at temperatures below the PT, from which an activation energy (ΔH) could be calculated. This activation energy is thought to be a measure of the concentration of available charged sites through which proton transport is facilitated. The structural fractal dimension in the hydrated protein system was also calculated, and enabled the approximation of the pathway for charge percolation in the protein matrix.

Dielectric analysis as a tool for investigating the lyophilization of proteins

Abstract The use of dielectric analysis to help pharmaceutical scientists to optimize the preservation of therapeutic agents by freeze drying (lyophilization) is yet to be realized fully. The authors provide a brief overview of different approaches to dielectric analysis and highlight (using a few examples) the potential role of this technology in the characterization of lyophilized materials and proteins in particular.

Development of a remote electrode system for monitoring the water content of materials inside a glass vial.

AbstractPurpose: This article explores the use of a remote electrode dielectric measurement system to monitor the water content of hydrated ovalbumin inside a glass vial. Methods: The intrinsic dielectric properties of hydrated ovalbumin were characterized first using conventional parallel plate electrodes. The second stage was to simulate a remote electrode measurement by placing nonconductive, nondispersive polyethylene films between the sample and electrodes. Finally, a study on the dielectric measurement of ovalbumin contained in a 10 ml glass vial was undertaken with the electrodes external to the glass vial. Results: The dielectric behavior of hydrated ovalbumin was characterized by charge transfer (i.e., protons) in the hydrogen bonded network of water molecules in the bulk sample. The mechanism was identified as an anomalous low-frequency dispersion and a dielectric loss peak (ε3). The dielectric relaxation time, τ3, of the ε3 dispersion was especially sensitive to water content. Moreover, a good correlation (R2 = 93%) was observed between relaxation times τ3 obtained from measurements using conventional parallel plate electrodes and the remote electrode system. Conclusions: Dielectric measurements using remote electrodes attached to a glass vial are therefore applicable for the in situ measurement of water content in materials. The application of this technology to the determination of the lyophilization end point is suggested.

Quantification of residual crystallinity in ball milled commercially sourced lactose monohydrate by thermo-analytical techniques and terahertz spectroscopy.

The quantification of crystallinity is necessary in order to be able to control the milling process. The use of thermal analysis for this assessment presents certain challenges, particularly in the case of crystal hydrates. In this study, the residual crystallinity on ball milling of lactose monohydrate (LMH), for periods up to 90min, was evaluated by thermo-analytical techniques (TGA, DSC) and terahertz spectroscopy (THz). In general, the results from one of the DSC analysis and the THz measurements agree showing a monotonous decrease in relative residual crystallinity with milling time (∼80% reduction after 60min milling) and a slight increase at the 90min time point. However, the estimates from TGA and two other methods of analyzing DSC curve do not agree with the former techniques and show variability with significantly higher estimates for crystallinity. It was concluded that, the thermal techniques require more complex treatment of the data in the evaluation of changes in crystallinity of a milled material (in particular to account for the de-vitrification and mutarotation of the material that inevitably occurs during the measurement cycle) while the analysis of THz data is more straightforward, with the measurement having no impact on the native state of the material.