Heterogeneity of water at the phospholipid membrane interface

Femtosecond infrared (IR) two-color pump-probe experiments were used to investigate the nonlinear response of the D2O stretching vibration in weakly hydrated dimyristoyl-phosphatidylcholine (DMPC) membrane fragments. The vibrational lifetime is comparable to or longer than that in bulk D2O and is frequency dependent, as it decreases with increasing probe frequency. Also, the lifetime increases when the water content of the sample is lowered. The measured lifetimes range between 903 and 390 fs. A long-lived spectral feature grows in following the excitation and is attributed to photoinduced D-bond breaking. The photoproduct spectrum differs from the steady state difference Fourier transform infrared (FTIR) spectrum, showing that the full thermalization of the excitation energy happens on a much longer time scale than the time interval considered (12 ps). Further evidence of the inhomogeneous character of the water residing in the polar region of the bilayer comes from the spectral anisotropy. The water molecules absorbing on the low frequency side of the absorption band show no decay at all of the anisotropy, while an ultrafast partial decay appears when the high frequency side of the spectrum is probed. The overall behavior differs remarkably from that observed with similar experiments in bulk water and in water segregated in inverse micelles. In weakly hydrated phospholipid membranes, water molecules are present mostly as isolated species, prevalently involved in strong, rigid, and persistent hydrogen bonds with the polar groups of the bilayer molecules. This specific character appears to have a direct effect on the structural stability and thermal properties of the membrane.     

Diffusion of condensable vapors in single adsorbent particles measured via concentration-swing frequency response

A concentration-swing frequency response method is extended to examine mass transfer mechanisms and the concentration dependence of mass transfer rates for adsorption of condensable vapors in single adsorbent particles. The adsorption kinetics of water and hexane in BPL activated carbon and the adsorption of water in silica gel are determined at several different concentrations. The mechanism that best describes the adsorption of water in BPL activated carbon is nanopore diffusion. The diffusivity of water in BPL activated carbon has a clear minimum at approximately P/Po = 0.5, and the concentration dependence of the diffusion data are not described well by the Darken relationship. Both nanopore diffusion and the Glueckauf linear driving force models can be used to describe the diffusion of hexane in BPL activated carbon for the pressure range studied, and the dependence of the diffusivity on concentration can be described approximately using the Darken relationship. However, the diffusion of water in silica gel cannot be described by the nanopore diffusion model and is best characterized by the Glueckauf linear driving force model. The results illustrate the ability of concentration-swing frequency response to accurately determine adsorption rate mechanisms and quantify the complex adsorption kinetics of condensable vapors using small quantities of adsorbent.     

A Piezoelectric crystal detector for continuous monitoring of water evolved during thermal decomposition reactions

A method for the continuous monitoring of water vapour evolved during thermal decomposition reactions is developed based on the reversible adsorption of water on a bare piezoelectric crystal with nickel electrodes. Gaseous decomposition products formed in the furnace chamber of the thermoanalytical equipment are collected and transported to the detector cell by the carrier gas. A linear relationship exists between the decrease in vibrational frequency of the crystal and the concentration of water in the carrier gas up to 900 mug/l. H(2)O [1200 ppm (v/v)]. The signal curve of the piezoelectric water monitor can be recorded simultaneously along with the TG, DTG and DTA curves and used to identify the decomposition step(s) in which water was formed. CO, CO(2) and low molecular weight hydrocarbons do not interfere. Condensable organic crack products can be removed by a suitable pretrap.     

Broadband dielectric and resistivity spectroscopy of WO(3) x H(2)O in the range of 10(3)-10(10) Hz: particle size effect

The interest in studying the electrical properties of WO(3) x H(2)O powders is made absolutely necessary because their infrared modulation properties depend on their morphologies and electronic populations. Broadband dielectric and resistivity spectra of WO(3) x H(2)O powders were recorded in a frequency range of 10(3)-10(10) Hz at temperatures varying between 200 and 300 K. Complex resistivity and permittivity diagrams have permitted thermal behavior of both dc-conductivity and permittivity to be obtained. A dielectric relaxation is found, attributed to water molecules motions. The role of the powder morphology has been investigated on two types of compounds: the first one being constituted by nanometric particles and the second by micrometric particles. Strong differences are observed in the thermal behaviors of the dc-conductivities (activation energies). Particle size effect is evidenced, giving rise to stronger electron localization on the nanometric particles. The permittivity values and the dynamical behavior of the structural water are also influenced by the particle size effect. A strong interaction between moving polarons and water molecules has been determined.     

Lipid phase transition in saccharide-coated cholate-containing liposomes: coupling to the surrounding matrix

We performed FTIR measurements on cholate-containing liposomes (CCL) embedded in saccharide (trehalose or sucrose) matrixes with different contents of residual water. We obtained information on the CCL phase transition following the thermal evolution (310-70 K) of the IR spectrum of the carbonyl moieties of phospholipids in the frequency range 4225-4550 cm(-1). Furthermore, we simultaneously followed the thermal evolution of the water association band, which gave information on the behavior of the surrounding water-saccharide matrix. The analysis revealed a small sub-band of the water association band present in CCL but not in cholate-free liposomes, the thermal evolution of which is tightly coupled to that of the spectrum of the carbonyl moieties of phospholipids. We suggest that this band arises from water molecules, which are inserted within the lipidic structure, in the region located at the border between the hydrophilic and the hydrophobic moieties of phospholipids in the presence of cholic acid. Such water molecules could be responsible for the peculiar flexibility and hydrophilicity of CCL. Following Giuffrida et al. (J. Phys. Chem. B 2003, 107, 13211-13217), we also performed a Spectra Distance analysis, which enabled us to detect an overall liposomes-matrix structural coupling.     

Micro-droplet detection and characterization using thermal responses

We suggest a novel method to detect droplets and determine the protein content of droplets in microfluidic system using the 3ω method, which is a powerful tool to easily detect thermal response changes with a simple device. By measuring the thermal response of droplets and a carrying flow in real time, water droplets in an oleic acid carrying flow can be detected, and the concentration of bovine serum albumin in droplets can be estimated. This method is expected to increase the practicality and power of droplet-based microfluidic systems.     

Thermal or Mechanical Stimuli‐Induced Photoluminescence Color Change of a Molecular Assembly Composed of an Amphiphilic Anthracene Derivative in Water

Molecular assemblies that change photoluminescence color in response to thermal or mechanical stimulation without dissociation into the monomeric states in water are described herein. A dumbbell‐shaped amphiphilic compound forms micellar molecular assemblies in water and exhibits yellow photoluminescence derived from excimer formation of the luminescent core, which contains a 2,6‐diethynylanthracene moiety. Annealing of the aqueous solution induces a photoluminescence color change from yellow to green (λ_{em, max}=558→525 nm). The same photoluminescence color change is also achieved by rubbing the yellow‐photoluminescence‐emitting molecular assemblies adsorbed on glass substrates with cotton wool in water. The observed green photoluminescence is ascribed to micelles that are distinct from the yellow‐photoluminescence‐emitting micelles, on the basis of transmission electron microscopy observations, atomic force microscopy observations, and dynamic light scattering measurements. We examined the relationship between the structure of the molecular assemblies and the photophysical properties of the anthracene derivative in water before and after thermal or mechanical stimulation and concluded that thermal or mechanical stimuli‐induced slight changes of the molecular‐assembled structures in the micelles result in the change in the photoluminescence color from yellow to green in water.     

Shear response of nanoconfined water on muscovite mica: role of cations

By monitoring the thermal noise of a vertically oriented micromechanical force sensor, we detect the viscoelastic response to shear for water in a subnanometer confinement. Measurements in pure water as well as under acidic and high-ionic-strength conditions relate this response to the effect of surface-adsorbed cations, which, because of their hydration, act as pinning centers restricting the mobility of the confined water molecules.     

The Hydrated Excess Proton in the Zundel Cation H5O2+: The Role of Ultrafast Solvent Fluctuations

The nature of the excess proton in liquid water has remained elusive after decades of extensive research. In view of ultrafast structural fluctuations of bulk water scrambling the structural motifs of excess protons in water, we selectively probe prototypical protonated water solvates in acetonitrile on the femtosecond time scale. Focusing on the Zundel cation H₅O₂⁺ prepared in room‐temperature acetonitrile, we unravel the distinct character of its vibrational absorption continuum and separate it from OH stretching and bending excitations in transient pump‐probe spectra. The infrared absorption continuum originates from a strong ultrafast frequency modulation of the H⁺ transfer vibration and its combination and overtones. Vibrational lifetimes of H₅O₂⁺ are found to be in the sub‐100 fs range, much shorter than those of unprotonated water. Theoretical results support a picture of proton hydration where fluctuating electrical interactions with the solvent and stochastic thermal excitations of low‐frequency modes continuously modify the proton binding site while affecting its motions.     

Hydration and temperature interdependence of protein picosecond dynamics

We investigate the nature of the solvent motions giving rise to the rapid temperature dependence of protein picoseconds motions at 220 K, often referred to as the protein dynamical transition. The interdependence of picoseconds dynamics on hydration and temperature is examined using terahertz time domain spectroscopy to measure the complex permittivity in the 0.2-2.0 THz range for myoglobin. Both the real and imaginary parts of the permittivity over the frequency range measured have a strong temperature dependence at >0.27 h (g water per g protein), however the permittivity change is strongest for frequencies <1 THz. The temperature dependence of the real part of the permittivity is not consistent with the relaxational response of the bound water, and may reflect the low frequency protein structural vibrations slaved to the solvent excitations. The hydration necessary to observe the dynamical transition is found to be frequency dependent, with a critical hydration of 0.19 h for frequencies >1 THz, and 0.27 h for frequencies <1 THz. The data are consistent with the dynamical transition solvent fluctuations requiring only clusters of ~5 water molecules, whereas the enhancement of lowest frequency motions requires a fully spanning water network.     

The Principles of Micro-Thermal Analysis and its Application to the Study of Macromolecules

A newly developed Micro-Thermal Analyzer affords images based on thermal properties such as thermal conductivity, thermal diffusivity, and permits localized thermal analyses on samples of a square micrometer area by combining the imaging ability of the atomic force microscope and the thermal characterization ability of temperature-modulated differential scanning calorimetry. Since thermal penetration depth depends on frequency, one can obtain depth profiles of thermal conductivity and thermal diffusivity by varying the modulation frequency. Also, the analyzer can be used to characterize phase-transition temperatures, such as glass and melting transitions, of small sample regions with a precision of about ±3 K. Heating rates can be varied between 1 and 1500 K min^–1. Modulation frequencies can be applied in the range from 2 to 100 kHz. We applied this new type of instrument to characterize microscopic thermal and structural properties of various polymer systems. The operation principles of the instrument are described, application examples are presented, and the future of the technique is discussed.This revised version was published online in November 2005 with corrections to the Cover Date.     

Determination of competitive adsorption isotherms applying the nonlinear frequency response method: Part II. Experimental demonstration

This paper demonstrates an experimental application of the nonlinear frequency response (FR) method extension to determine adsorption isotherms of binary mixtures. This method, based on the analysis of the response of a chromatographic column subjected to the sinusoidal inlet concentration changes, is shown to be an alternative for isotherm determination. The critical issue related to the successful application of the method is to reach experimentally the low frequency asymptotic behaviour of the corresponding frequency response functions (FRFs). Although, there are different possibilities to perform periodical inlet concentration changes, in this paper only simultaneous changes for both components were considered. The adsorption of phenol and 2-phenylethanol on octadecyl silica was analyzed experimentally using a mixture of methanol and water as a solvent. Parameters of competitive isotherms were also estimated for comparison using the classical perturbation method. Despite certain differences between competitive isotherms estimated with the two methods that were found, the obtained results show the potential of the nonlinear FR method for measuring competitive isotherms.     

Chemical thermodynamics

The curing of composition containing epoxy bond is complicated chemical and technological process where under temperature and pressure conditions a change of its structure occurs. The structure changes are possible to know by thermal methods as DSC, DTA, etc. and also by measurement of dielectrical response under the low frequency electrical field.     

Microthermal analysis of rubber-polyaniline core-shell microparticles using frequency-dependent thermal responses

Alternating current (ac) thermal microscopy and microthermal analysis have been utilized for the investigation in the surface thermal conductivity imaging and local thermal analysis (LTA) of polybutadiene-polyaniline core-shell microparticles. The significant variances of thermal conductivity and stiffness between rubber and conducting polymer revealed the remarkable responses in the microthermal analysis. The depth-dependent thermal microscopy controlled by the heating frequency distinguished the rubber core, which was buried under few micron thickness of polyaniline out layer. Local thermal analysis also demonstrated the heat penetration-dependent sensor response from the rigid polyaniline shell to soft polybutadiene core. These experimental results confirmed the core-shell structure of these microparticle materials, as well as the continuous conducting phase of polyaniline.     

Effect of ammonia in the ion-chromatographic determination of trace anions and optimization of analytical conditions

The ion-chromatographic determination of traces of anions in the fluids of a thermal power plant is examined with an on-line automated system. Ammonia in the sample affects the analytical response for the Dionex AS4-A and AS2 anion-separation columns, because of the variable efficiency of the preconcentration. This effect can be corrected for, without sample pretreatment, by evaluating the response surface of the systems. The use of the more sensitive AS4-A column requires the chloride peak to be shifted away from the “water dip” in the chromatograms. Simplex optimization of the eluent enabled this to be done for samples with ammonia. Real utilization of AS4-A columns in however inhibited if samples contain organic acids because their peaks are not well resolved from that of chloride under the conditions adopted.     

Reversible Control by Light of the High‐Spin Low‐Spin Elastic Interface inside the Bistable Region of a Robust Spin‐Transition Single Crystal

By using a weak modulated laser intensity we have succeeded in reversibly controlling the dynamics of the spin‐crossover (SC) single crystal [{Fe(NCSe)(py)₂}₂(m‐bpypz)] inside the thermal hysteresis. The experiment could be repeated several times with a reproducible response of the high‐spin low‐spin interface and without crystal damage. In‐depth investigations as a function of the amplitude and frequency of the excitation brought to light the existence of a cut‐off frequency ca. 1.5 Hz. The results not only document the applicability of SC materials as actuators, memory devices, or switches, but also open a new avenue for the reversible photo‐control of the spin transition inside the thermal hysteresis.     

Estimation of single solute adsorption isotherms applying the nonlinear frequency response method using non-optimal frequencies

In order to estimate single solute adsorption isotherms, the nonlinear frequency response (FR) of a chromatographic column is analyzed experimentally and evaluated using the concept of higher order frequency response functions (FRFs) based on the Volterra series and generalized Fourier transform. In this case study, it has been investigated the adsorption of ethyl benzoate on octadecyl silica from a mixture of methanol and water (60:40) as a solvent. Experiments are performed using a standard gradient HPLC unit. For estimation of adsorption isotherms by the nonlinear FR method the column inlet concentration is changed in a nearly sine waveform around several steady-state concentrations. Using this method the first three local derivatives of a single solute adsorption isotherm are estimated from the low frequency asymptotes of the corresponding functions, i.e. the phase and first order derivative of the FRFs. For an accurate estimation of isotherm coefficients periodical experiments should be preformed for frequencies below a certain critical frequency. This is the frequency needed for approaching the low frequency asymptotic behaviour of the corresponding functions close enough, so that errors due to the non-feasibility of experiments with zero frequency can be neglected. Unfortunately, depending on the properties of the system, it can happen (as for the system investigated here) that experiments for the critical frequency would be too long and cannot be realized. In order to study the loss of accuracy of the nonlinear FR method, when it is applied for non-optimal frequencies, experiments are performed for frequencies approximately one order of magnitude higher than the critical frequency required to evaluate the FRF phases. The obtained isotherm model coefficients are compared with the ones estimated using conventional frontal analysis as a reference method. The isotherms determined by two methods are similar, however a closer look reveals that peaks predicted under overloading conditions differ.     

Rheological properties of protein-surfactant based gels

Water-based protein-surfactant gels, formed by mixing bovine serum albumin (BSA) and sodium dodecyl sulfate in water, were investigated by rheological methods. The measurements were performed for many different protein-to-surfactant ratios as a function of the applied frequency, stress, or strain, as well as by changing the temperature, in the range between 15 and 65 degrees C. The rheological behavior of the gels as a function of applied frequency is interpreted in terms of the overlapping of at least two viscoelastic relaxation processes. The rheological results indicate the presence of thermal transitions from essentially viscous to mainly elastic regimes, in analogy with the thermal gelation processes observed in polymer solutions. The thermal gelation threshold in the present system is modulated by the protein/surfactant ratio. Differential scanning calorimetry measurements were also performed to determine whether thermal gelation is somehow concomitant to protein denaturation. The results indicate that the thermal denaturation of BSA in protein-surfactant based gels occurs at slightly higher temperatures than in the bulk. Scanning electron microscopy indicates the occurrence in the gel structure of globules formed by the arrangement of fibrils.     

Picosecond acoustic transmission measurements. I. Transient grating generation and detection of acoustic responses in thin metal films

The technique of impulsive stimulated thermal scattering is extended to backside measurement of acoustic wave packets that have propagated through thin metal films following their generation by pulsed optical excitation, heating, and thermal expansion at the front side. The acoustic transmission measurement at the backside substantially isolates the acoustic responses from thermal and electronic responses of the metal film that often dominate acoustic reflection signals measured from the front side, and permits straightforward measurement of the acoustic response generated by optical excitation at a substrate-thin film interface. It can thus better distinguish among different factors that limit the bandwidth of the acoustic wave packet, an issue of concern in the measurement of high frequency responses. The paper that follows demonstrates the application of the backside measurement to a study of high frequency structural relaxation in the glass-forming liquid glycerol.