Hydration Forces Between Lipid Bilayers: A Theoretical Overview and a Look on Methods Exploring Dehydration.

Although, many biological systems fulfil their functions under the condition of excess hydration, the behaviour of bound water as well as the processes accompanying dehydration are nevertheless important to investigate. Dehydration can be a result of applied mechanical pressure, lowered humidity or cryogenic conditions. The effort required to dehydrate a lipid membrane at relatively low degree of hydration can be described by a disjoining pressure which is called hydration pressure or hydration force. This force is short-ranging (a few nm) and is usually considered to be independent of other surface forces, such as ionic or undulation forces. Different theories were developed to explain hydration forces that are usually not consistent with each other and which are also partially in conflict with experimental or numerical data.Over the last decades it has been more and more realised that one experimental method alone is not capable of providing much new insight into the world of such hydration forces. Therefore, research requires the comparison of results obtained from the different methods. This chapter thus deals with an overview on the theory of hydration forces, ranging from polarisation theory to protrusion forces, and presents a selection of experimental techniques appropriate for their characterisation, such as X-ray diffraction, atomic force microscopy and even calorimetry.

In-situ spectroscopic investigation of ultrasonic assisted unfolding and aggregation of insulin.

It is well-known that fibrillogenesis of proteins can be influenced by diverse external parameters, such as temperature, pressure, agitation or chemical agents. The present preliminary study suggests that ultrasonic excitation at moderate intensities has a significant influence on the unfolding and aggregation behaviour of insulin. Irradiation with an average sound intensity of even as low as 70mW/cm(2) leads to a lowering of the unfolding and aggregation temperature up to 7K. The effect could be explained by an increase of the aggregation kinetics due to ultrasonically induced acoustic micro-streaming in the insulin solution that most probably enhances the aggregation rate. The clear and remarkable effect at relatively low sound intensities offers interesting options for further applications of ultrasound in biophysics and biochemistry. On the other hand, a process that causes a change of kinetics equivalent to 7K also gives a warning signal concerning the safety of those medical ultrasonic devices that work in this intensity range.

The influence of correlated protein-water volume fluctuations on the apparent compressibility of proteins determined by ultrasonic velocimetry.

The elasticity of proteins, expressed by the compressibility, is potentially one of the most important properties of proteins because of the close relationship with its functionality. The compressibility of solutions can be determined by measurements of sound velocity and density. These quantities are related by the Newton-Laplace equation. In order to interpret the apparent compressibility of solutes in highly dilute solutions, it is required to consider the relation between compressibility and sound velocity of the solution using an appropriate reference system. The classical approach usually gives too small values for the apparent compressibility when compared with other methods. We show that the difference can partially be explained if the correlated volume fluctuations of the solvent are taken into consideration. A special attention is given to the compressibility of proteins. Finally, the present paper is not intended to replace established approaches, but it wants to create awareness that the classical mixing rules refer to ideal gasses assuming uncorrelated volume fluctuations and that a considerable part of the hydration effects could be explained by correlated volume fluctuations.

A compact device for simultaneous dielectric spectroscopy and microgravimetric analysis under controlled humidity.

Water plays a key role in the functioning of natural and synthetic molecular systems. Despite several hydration studies, different techniques are employed individually for monitoring different physical features such as kinetics, dynamics, and absorption. This study describes a compact hydration cell that enables simultaneous dielectric relaxation spectroscopy (DRS) and mass loss/uptake measurements in thin organic layers under controlled humidity conditions and in a wide temperature range. This approach enabled us to correlate the physical quantities obtained during the same experiment by complementary techniques. To demonstrate the performance of this device, a 200 nm thick poly(methyl methacrylate) (PMMA) layer was measured at various relative humidity levels (0%-75%), temperatures (25-75 °C), and frequencies (DRS: 0.1 Hz-1 MHz) to study how hydration and dehydration processes affect its molecular dynamics. The results show the capability of this setup to study the changes in the PMMA film regarding the kinetics and molecular dynamics upon variation of the water content.

Modeling the dose dependence of the vis-absorption spectrum of EBT3 GafChromic™ films.

PURPOSE: The aim of this work was to model the dose dependence of the darkening of GafChromic™ EBT3 films by combining the optical properties of the polydiacetylene polymer phases, and a modified version of the single-hit model, which will take the stick-like shape of the monomer microcrystals into account. Second, a comparison is made between the quantification of the film darkening by flatbed scanning and by UV-vis absorption spectroscopy. METHOD: GafChromicTM EBT3 films were irradiated with a 6 MV photon beam at dose levels between 0 and 50 Gy. The radiation-induced darkening of the films is quantified by a flatbed scanner, and by UV-vis absorption spectroscopy in the wavelength range of 220-750 nm. From the UV-vis absorption spectra, the contribution of each polymer phase to the absorbance was deduced. Next, the dose dependence of the polymer content is described by a modified single-hit model where the size distribution of polymerizable centers is approximated by way of the size distribution of the monomer microcrystals in the film. RESULTS: The absorption properties of the film can be accurately quantified by UV-vis spectroscopy for dose levels between 0 and 10 Gy. Over 10 Gy, the absorption spectrum saturates due to the limited sensitivity of the spectrometer. The modified single-hit model was successful in describing the increasing polymer concentration with radiation dose, using a log-normal distribution for the length of the stick-like monomer microcrystals. The dose dependence of the polymer content, deduced from the UV-vis absorption spectrum, differs from that of the flatbed scanning method and is more sensitive to changes in dose. CONCLUSION: The dose dependence of the polymer concentration can be modeled by taking into account the distribution of active centers using the microstructure of the active layer for dose levels between 0 and 10 Gy. The dissimilar dose dependence of the polymer concentration and the absorbance must be accounted for when modeling darkening from the kinetics of the photopolymerization reaction.

Peculiarity of aqueous solutions of 2,2,2-trifluoroethanol.

Aqueous solutions of 2,2,2-trifluoroethanol appear to show a structural transition at alcohol mole fraction equal to x(TFE) = 0.05, which can be concluded from a discontinuity of the speed of sound. At the same concentration, a discontinuity was observed in the parameters of the long-living component of the positron annihilation spectrum. Moreover, the partial molar volumes of components show transition-like behavior in the range of low solute contents, which is significantly different from nonsubstituted ethanol. The peculiarities of the low concentration system correlate with minor infrared spectra changes assigned to a mode composed of the CH(2) bending and CF(3) stretching internal vibrations being sensitive to polarity of the hydration shell surrounding the solute. The majority of the spectral changes arise from a gradual shift of the equilibrium between trans ↔ gauche isomers when the composition of the solution is changing. A possible explanation for the peculiar behavior of the system is a thermodynamic equilibrium between hydrated monomers and dimers at that respective mole number.

Fiber optic SPR biosensing of DNA hybridization and DNA-protein interactions.

In this paper we present a fiber optic surface plasmon resonance (SPR) sensor as a reusable, cost-effective and label free biosensor for measuring DNA hybridization and DNA-protein interactions. This is the first paper that combines the concept of a fiber-based SPR system with DNA aptamer bioreceptors. The fibers were sputtered with a 50nm gold layer which was then covered with a protein repulsive self-assembled monolayer of mixed polyethylene glycol (PEG). Streptavidin was attached to the PEG's carboxyl groups to serve as a versatile binding element for biotinylated ssDNA. The ssDNA coated SPR fibers were first evaluated as a nucleic acid biosensor through a DNA-DNA hybridization assay for a random 37-mer ssDNA. This single stranded DNA showed a 15 nucleotides overlap with the receptor ssDNA on the SPR fiber. A linear calibration curve was observed in 0.5-5 microM range. A negative control test did not reveal any significant non-specific binding, and the biosensor was easily regenerated. In a second assay the fiber optic SPR biosensor was functionalized with ssDNA aptamers against human immunoglobulin E. Limits of detection (2nM) and quantification (6nM) in the low nanomolar range were observed. The presented biosensor was not only useful for DNA and protein quantification purposes, but also to reveal the binding kinetics occurring at the sensor surface. The dissociation constant between aptamer and hIgE was equal to 30.9+/-2.9nM. The observed kinetics fully comply with most data from the literature and were also confirmed by own control measurements.

The sound velocity in ideal liquid mixtures from thermal volume fluctuations.

The selection of the correct mixing rule for sound velocity in ideal liquid mixtures determines the interpretation of the sound velocity in real mixtures. This is especially important for the determination of apparent properties of solutes, such as their apparent compressibility. There are different approaches reported in the literature, and this article presents a new derivation of the mixing rule based on statistical mechanics. It is shown that the correlation of volume fluctuations between adjacent components has a crucial influence on the ideal mixing rule.