Effect of the electric field on the apparent mobility of large DNA fragments in agarose gels

The electrophoresis of a series of DNA fragments ranging in size from 0.5 to 12 kilobase pairs, has been studied as a function of agarose gel concentration and electric field strength. The apparent mobility of all fragments decreased with decreasing electric field strength and with increasing gel concentration. When extrapolated to zero electric field strength and zero agarose concentration, the apparent mobility of all DNA fragments extrapolated to a common value (2.0 ± 0.1) × 10^?4 cm²/V s. The square roots of the retardation coefficients of the various fragments were found to be linearly related to the root-mean-square radii of gyration of the fragments, as predicted by pore-size distribution theory. As predicted by reptation theory, the molecular weights of the various fragments were found to be linearly related to the reciprocal of the apparent mobilities. An equation is given for estimating the apparent pore size of agarose gels between 0.25 and 1.5% in concentration.     

High-frequency dielectric study of side-chain dynamics in poly(lysine) aqueous solutions

The high-frequency dielectric properties of poly(lysine) of different chirality in aqueous solutions have been measured in the frequency range from 1 MHz to 1.8 GHz. The dielectric spectra show the existence of relatively small dielectric dispersions at around 100 MHz that have been attributed to internal motion in the polymer chain, due to side-chain polar groups. Our results indicate that the local structure of the chain and its possibility to undergo a conformational transition induced by pH does not modify the main feature of the side-chain dynamics, the dielectric strength being largely proportional to the concentration of charged groups. A similar behavior has been found in poly(alpha-glutamate) and in poly(gamma-glutamate) aqueous solutions, where the dielectric parameters appear to be related to the change in the charge density on the main chain, rather than to the accompanying conformational helix-coil transition.     

Influence of electric field on the unsteady dispersion coefficient in couple-stress flow

The influence of electric field on unsteady convective diffusion in couple stress flow is studied using a time dependent dispersion model. The electric field, arising as a body couple in the governing equations, is shown to increase the axial dispersion coefficient. This is useful in the control of haemolysis caused by artificial organs implanted or extracorporeal. The contribution of pure convection in the dispersion of concentration is singled out and investigated in detail. The results obtained are compared with those in the absence of electric field and some important conclusions are drawn.     

Effect of ionic strength on the diffusion coefficient of chondroitin sulfate and heparin measured by quasielastic light scattering

The normal modes for a mixture of charged macromolecules and electrolyte solution are calculated. We derive a generalized Debye relaxation time and the apparent diffusion coefficient of the macroion, which is shown to increase from its Stokes value, obtained in excess of added salt, with decreasing ionic strength. We test our result with experimental data for macromolecules with different charge densities: heparin and chondroitin sulfate. Besides, we show for this latter molecule that while the diffusion coefficient is increased, the scattered intensity is decreased but not by the same factor. Our results are compared with other theories developed in quasielastic light scattering.     

The role of apparent diffusion coefficient values in characterization of solid focal liver lesions:a prospective and comparative clinical study

We evaluated and compared the diagnostic accuracy(DA) of apparent diffusion coefficient(ADC) values with that of lesion-to-liver ADC ratios in the characterization of solid focal liver lesions(FLLs).This prospective study was approved by the Institutional Human Ethics Board,after waiving written informed consent.Diffusion-weighted imaging and other routine magnetic resonance imaging were performed on 142 consecutive patients with suspected liver disease.The mean ADC values and lesion-to-liver ADC ratios were compared between benign and malignant solid FLLs.Receiver operating characteristic analysis was performed.The study participants included 46 patients(28 men,18 women;mean age,52.5 years) with 57 solid FLLs(32 malignant and 25 benign FLLs).The mean ADC values and ADC ratios of benign solid FLLs were significantly higher than those of malignant lesions(P0.01).The difference between the area under the receiver operating characteristic curve of the ADC values(0.699) and ADC ratios(0.752) was not significant.Our study suggests that the DA of the ADC ratio is not significantly higher than that of ADC in characterizing solid FLLs.     

Continuous optical scanning in polyacrylamide gel electrophoresis:estimation of the apparent diffusion coefficient of beta-lactoglobulin B.

The continuous scanning apparatus developed by Catsimpoolas was applied to an analysis of the concentration profiles of a protein, β-lactoglobulin B, while it was subjected to polyacrylamide gel electrophoresis (PAGE) in a multiphasic buffer system. Continuous optical scanning in PAGE permitted reliable estimation of the standard deviation of the concentration profile (σ), the relationship between σ² and time, and the apparent diffusion coefficient, D′, derived from σ², as the current density varied from 2 to 9 mA/cm², protein load varied from 250 to 900 μg/cm², and the ionic strength varied from 0.015 to 0.065 m. Under these conditions, D′ was linearly related to current density and protein load. Further, log (D′) was linearly related to gel concentration (%T) ranging from 6 to 14%. However, D′ was nonlinearly related to ionic strength. Due primarily to the ionic strength factor, the apparent diffusion coefficient of protein in gels appeared to be approximately 10-fold larger than under the conditions of high ionic strength conventionally used in sedimentation and diffusion studies. Extrapolation of D′ to 0% T, zero protein load, zero current density, and “infinite” ionic strength (assuming noninteraction of these factors), as well as correction for viscosity and temperature, yielded an estimated free-diffusion coefficient, D_20,w, of 3.1 × 10^?7 cm²/s, which is compatible with previously reported values. These studies indicate that the optimal resolution obtained by PAGE will be considerably lower than that predicted theoretically on the basis of free-diffusion coefficients, and suggest that electrostatic interaction between the proteins and/or deformation of voltage gradient and pH within the protein zones may contribute significantly to band spreading.     

Nuclear magnetic resonance (NMR) measurement of the apparent diffusion coefficient (ADC) of tissue water and its relationship to cell volume changes in pathological states

Diffusion-weighted nuclear magnetic resonance (NMR) imaging (DWI) is sensitive to the random translational motion of water molecules due to Brownian motion. Although the mechanism is still not completely understood, the cellular swelling that accompanies cell membrane depolarization results in a reduction in the net displacement of diffusing water molecules and thus a concomitant reduction in the apparent diffusion coefficient (ADC) of tissue water. Cerebral regions of reduced ADC appear hyperintense in a DWI and this technique has been used extensively to study acute stroke. In addition to cerebral ischemia, reductions in the ADC of cerebral water have been observed following cortical spreading depression, ischemic depolarizations (IDs), transient ischemic attack (TIA), status epilepticus, and hypoglycemia. Although the mechanism responsible for initiating membrane depolarization varies in each case, the ensuing cell volume changes follow a similar pattern. Water ADC values are also affected by the presence and orientation of barriers to translational motion (such as cell membranes and myelin fibers) and thus NMR measures of anisotropic diffusion are sensitive to more chronic pathological states where the integrity of these structures is modified by disease. Both theoretical prediction and experimental evidence suggest that the ADC of tissue water is related to the volume fraction of the interstitial space via the electrical conductivity of the tissue. The implication is that acute neurological disorders that exhibit electrical conductivity changes should also exhibit ADC changes that are detectable by DWI. A qualitative correlation between electrical conductivity and the ADC of water has been demonstrated in a number of animal model studies and the results indicate that reduced ADC values are associated with reductions in the extracellular volume fraction and increased extracellular tortuosity. The close relationship between ADC changes and cell volume changes in various pathological states suggests that NMR measurements are also sensitive to chemical communication between cells through the extracellular space (i.e., extrasynaptic or volume transmission, VT).     

Effect of a pulsating electric field on ECR heating in the CERA-RX(C) X-ray generator

3D particle-in-cell plasma simulations for the field configurations implemented in the CERA-RX(C) ECR X-ray generator (2.45 GHz) have been conducted. Dependences of the energy spectra of electrons incident on the target electrode on the amplitude and frequency of pulsations of the electric field in a megahertz range are derived. The simulation data are compared with the results of the full-scale experiment.     

Effect of poly(ethylene glycol) on phospholipid hydration and polarity of the external phase

The hydration properties of phosphatidylcholine (PC)/water dispersions on the addition of poly(ethylene glycol) were studied by means of ²H-NMR. The quadrupole splittings and their temperature dependences correspond to measurements of PC/water dispersions at low water content. It is concluded that the bound water is partly extracted by poly(ethylene glycol) but the binding properties of the water in the inner hydration shell of about five water molecules are not changed. The ability of some phospholipid/water dispersions to undergo phase transitions to nonlamellar structures upon dehydration is discussed. Dipalmitoylphosphatidylcholine (DPPC) and egg phosphatidylcholine do not form nonlamellar structures on addition of purified poly(ethylene glycol), as was demonstrated by means of ³¹P-NMR. Poly(ethylene glycol) decreases the polarity of the aqueous phase and the partition of hydrophobic molecules between the membrane and the external phase is changed. This was demonstrated using the excimer fluorescence of pyrene in a ghost suspension. It is suggested that the changes in polarity and hydration on the addition of poly(ethylene glycol) can contribute to the alterations in the membrane surface observed under conditions of membrane contact and fusion.     

Effect of intraparticle diffusion resistance on apparent stability of immobilized enzymes

The effect of the intraparticle diffusion resistance on the apparent stability of the immobilized enzyme suffering from the first-order deactivation has been studied quantitatively. A general expression for the relationship between the decreasing observed enzymatic reaction rate and the intrinsic enzyme deactivation rate has been introduced. The method to estimate the intrinsic deactivation rate constant also has been proposed. Using the invertases immobilized on a anion-exchange resin, the theory proposed in this work has been verified experimentally.     

Two-dimensional/ATR infrared correlation spectroscopic study on water diffusion in a poly(epsilon-caprolactone) matrix

In the present contribution, two-dimensional ATR-FTIR spectroscopy was used to study the diffusion of water in poly(epsilon-caprolactone) (PCL). In the spectral region of the nu(OH) stretching vibration of water, four absorption bands (3635, 3560, 3411, and 3220 cm(-1)) can be identified. The higher wavenumber band pair at 3635 and 3560 cm(-1) is assigned to the antisymmetric and symmetric OH stretching vibrations, respectively, of water which is partially hydrogen-bonded to the carbonyl groups of PCL, whereas the lower frequency band pair at 3411 (antisymmetric) and 3220 cm(-1) (symmetric) is attributed to the OH stretching vibrations of bulk water which is fully hydrogen-bonded to other water molecules. From the asynchronous map of a 2D correlation analysis of spectra recorded during the diffusion of water into PCL, it was concluded that during the diffusion process the water molecules first penetrate into the free volume (microvoids) of the PCL matrix or are molecularly dispersed in the polymer matrix and then form hydrogen bonds with the C=O groups of the polymer.     

Helix-helix transitions in DNA: fibre X-ray study of the particular cases poly(dG-dC) · poly(dG-dC) and poly(dA) · 2poly(dT)

The helix-helix transitions which occur in poly(dG-dC) · poly(dG-dC) and in poly (dG-m⁵dC) · poly(dG-m⁵dC) are commonly assumed to be changes between the right-handed A- or B-DNA double helices and the left-handed Z-DNA structure. The mechanisms for such transconformations are highly improbable, especially when they are supposed to be active in long polynucleotide chains organised in semicrystalline fibres. The present alternative possibility assumes that rather than the Z-DNA it is a right-handed double helix (S-DNA) which actually takes part in these form transitions. Two molecular models of this S form, in good agreement with X-ray measurements, are proposed. They present alternating C(2′)-endo and C(3′)-endo sugar puckering like the “alternating B-DNA” put forward some years ago. Dihedral angles, sets of atomic coordinates and stereo views of the two S-DNA structures are given, together with curves of calculated diffracted intensities. Furthermore, we question the possibility of obtaining semicrystalline fibres with triple helices of poly(dA) · 2poly(dT) in a way which renders X-ray diffraction efficient. It is suggested that, up to now, only double helices of poly(dA) · poly(dT) can actually be observed by fibre X-ray diffraction measurements. Received: 30 March 1999 / Revised version: 30 June 1999 / Accepted: 30 June 1999     

Apparent diffusion coefficient in the aging mouse brain: a magnetic resonance imaging study

Novel magnetic resonance imaging sequences have and still continue to play an increasing role in neuroimaging and neuroscience. Among these techniques, diffusion-weighted imaging (DWI) has revolutionized the diagnosis and management of diseases such as stroke, neoplastic disease and inflammation. However, the effects of aging on diffusion are yet to be determined. To establish reference values for future experimental mouse studies we tested the hypothesis that absolute apparent diffusion coefficients (ADC) of the normal brain change with age. A total of 41 healthy mice were examined by T2-weighted imaging and DWI. For each animal ADC frequency histograms (i) of the whole brain were calculated on a voxel-by-voxel basis and region-of-interest (ROI) measurements (ii) performed and related to the animals' age. The mean entire brain ADC of mice <3 months was 0.715(+/-0.016) x 10(-3) mm2/s, no significant difference to mice aged 4 to 5 months (0.736(+/-0.040) x 10(-3) mm2/s) or animals older than 9 months 0.736(+/-0.020) x 10(-3) mm2/s. Mean whole brain ADCs showed a trend towards lower values with aging but both methods (i + ii) did not reveal a significant correlation with age. ROI measurements in predefined areas: 0.723(+/-0.057) x 10(-3) mm2/s in the parietal lobe, 0.659(+/-0.037) x 10(-3) mm2/s in the striatum and 0.679(+/-0.056) x 10(-3) mm2/s in the temporal lobe. With advancing age, we observed minimal diffusion changes in the whole mouse brain as well as in three ROIs by determination of ADCs. According to our data ADCs remain nearly constant during the aging process of the brain with a small but statistically non-significant trend towards a decreased diffusion in older animals.     

The measurement of the rotational diffusion coefficient of bovine plasma fibronectin by electric birefringence technique

Although the conformation of fibronectin has been widely investigated by various techniques, there has not yet been any determination of its rotational diffusion coefficient. We report here this determination by the transient electric birefringence study of solutions of bovine plasma fibronectin at physiological ionic strength. The solutions showed a positive birefringence. A linear relationship was observed between the intensity of the birefringence at equilibrium and the square of the electric field within the range of fields applied (up to 12.5 kV.cm-1). The field-independent decay of the induced birefringence was described by a single exponential with a relaxation time of 0.76 (+/- 0.08) microsecond at 23 degrees C. This establishes fibronectin in solution as a globally rigid structure with a rotational diffusion coefficient, at 20 degrees C, of 202,000 s-1. This result allows the first rigorous determination of the low-resolution structure of fibronectin. It is important to notice that the analysis combines only results obtained in physiological conditions on native molecules and follows a strict hydrodynamic interpretation. The conclusion of this work is that a hollow sphere of about 20 nm external diameter can be proposed as a model for the three-dimensional structure of the fibronectin molecule in solution. This new model suggests the fibronectin could have the structure of a carrier protein.     

Measurement of the diffusion coefficient of ethanol in agarose gel beads: A reproducibility study

Summary The diffusion coefficient of ethanol in a 4 w/w% agarose gel at 25°C was measured, using the methods of unsteady-state diffusion into, and out of, gel beads dispersed in a solution of finite volume. The results (8.0 – 9.5×10^–6 cm²/s) agreed well with available theory. The results of method out were more reproducible than method in, but the standard deviation was in no case lower than 3.3%.