Home About us Contact
|
Home About us Contact | ||
|
|
||
Opposite Polarity (opposite + polarity)
Selected AbstractsMeasured and calculated CD spectra of G-quartets stacked with the same or opposite polarities,,CHIRALITY, Issue 3-4 2008Donald M. Gray Abstract Circular dichroism (CD) spectroscopy is widely used to characterize the structures of DNA G-quadruplexes. CD bands at 200,300 nm have been empirically related to G-quadruplexes having parallel or antiparallel sugar-phosphate backbones. We propose that a more fundamental interpretation of the origin of the CD bands is in the stacking interactions of neighboring G-quartets, which can have the same or opposing polarities of hydrogen bond acceptors and donors. From an empirical summation of CD spectra of the d(G)5 G-quadruplex and of the thrombin binding aptamer that have neighboring G-quartets with the same and opposite polarities, respectively, the spectra of aptamers selected by the Ff gene 5 protein (g5p) appear to arise from a combination of the two types of polarities of neighboring G-quartets. The aptamer CD spectra resemble the spectrum of d(G3T4G3), in which two adjacent quartets have the same and two have opposite polarities. Quantum-chemical spectral calculations were performed using a matrix method, based on guanine chromophores oriented as in d(G3T4G3). The calculations show that the two types of G-quartet stacks have CD spectra with features resembling experimental spectra of the corresponding types of G-quadruplexes. Chirality, 2008. © 2007 Wiley-Liss, Inc. [source] Analysis of b -value calculations in diffusion weighted and diffusion tensor imagingCONCEPTS IN MAGNETIC RESONANCE, Issue 1 2005Daniel Güllmar Abstract Diffusion weighted imaging has opened new diagnostic possibilities by using microscopic diffusion of water molecules as a means of image contrast. The directional dependence of diffusion has led to the development of diffusion tensor imaging, which allows us to characterize microscopic tissue geometry. The link between the measured NMR signal and the self-diffusion tensor is established by the so-called b matrices that depend on the gradient's direction, strength, and timing. However, in the calculation of b -matrix elements, the influence of imaging gradients on each element of the b matrix is often neglected. This may cause errors, which in turn leads to an incorrect extraction of diffusion coefficients. In cases where the imaging gradients are high (high spatial resolution), these errors may be substantial. Using a generic pulsed gradient spin-echo (PGSE) imaging sequence, the effects of neglecting the imaging gradients on the b -matrix calculation are demonstrated. By measuring an isotropic phantom with this sequence it can be analytically as well as experimentally shown that large deviations in single b -matrix elements are generated. These deviations are obtained by applying the diffusion weighting in the readout direction of the imaging dimension in combination with relatively large imaging gradients. The systematic errors can be avoided by a full b -matrix calculation considering all the gradients of the sequence or by generating cross-term free signals using the geometric average of two diffusion weighted images with opposite polarity. The importance of calculating the exact b matrices by the proposed methods is based on the fact that more precise diffusion parameters are obtained for extracting correct property maps, such as fractional anisotropy, volume ratio, or conductivity tensor maps. © 2005 Wiley Periodicals, Inc. Concepts Magn Reson Part A 25A: 53,66, 2005 [source] Direct Observation of Inversion Domain Boundaries of GaN on c -Sapphire at Sub-ångstrom Resolution,ADVANCED MATERIALS, Issue 11 2008Fude Liu Inversion domain boundaries (IDBs) of GaN are studied by a high-resolution technique. The IDB separates adjacent domains of opposite polarity. The image shows a GaN IDB in the [bar;2110] projection. The theoretical IDB structure fits the experimentally obtained structure well. The inset is an image acquired from a very thin region on the right side of the IDB. It can indicate the polarity of GaN directly. [source] Entrainment by an Extracellular AC Stimulus in a Computational Model of Cardiac TissueJOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 10 2001JASON M. MEUNIER B.S. Sinusoidal Stimulation of Cardiac Sheet.Introduction: Cardiac tissue can be entrained when subjected to sinusoidal stimuli, often responding with action potentials sustained for the duration of the stimulus. To investigate mechanisms responsible for both entrainment and extended action potential duration, computer simulations of a two-dimensional grid of cardiac cells subjected to sinusoidal extracellular stimulation were performed. Methods and Results: The tissue is represented as a bidomain with unequal anisotropy ratios. Cardiac membrane dynamics are governed by a modified Beeler-Reuter model. The stimulus, delivered by a bipolar electrode, has a duration of 750 to 1,000 msec, an amplitude range of 800 to 3,200 ,A/cm, and a frequency range of 10 to 60 Hz. The applied stimuli create virtual electrode polarization (VEP) throughout the sheet. The simulations demonstrate that periodic extracellular stimulation results in entrainment of the tissue. This phase-locking of the membrane potential to the stimulus is dependent on the location in the sheet and the magnitude of the stimulus. Near the electrodes, the oscillations are 1:1 or 1:2 phase-locked; at the middle of the sheet, the oscillations are 1:2 or 1:4 phase-locked and occur on the extended plateau of an action potential. The 1:2 behavior near the electrodes is due to periodic change in the voltage gradient between VEP of opposite polarity; at the middle of the sheet, it is due to spread of electrotonic current following the collision of a propagating wave with refractory tissue. Conclusion: The simulations suggest that formation of VEP in cardiac tissue subjected to periodic extracellular stimulation is of paramount importance to tissue entrainment and formation of an extended oscillatory action potential plateau. [source] Origin of the conformation dependence of protein charge-state distributions in electrospray ionization mass spectrometryJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 1 2003Rita Grandori Abstract The influence of tertiary structure on the electrospray ionization mass spectra of proteins is a well known and broadly exploited phenomenon. However, the underlying mechanism is not well understood. This paper discusses the bases and the implications of the two current hypotheses (solvent accessibility and Coulombic repulsions), pointing out the remaining open questions. Evidence reported here supports a third hypothesis, i.e. that intramolecular interactions in folded proteins play a key role in determining the observed charge-state distributions. It is proposed that native protein structures stabilize to a large extent pre-existing charges of the opposite polarity to the net charge of the ion, preventing their neutralization during the electrospray process. Thus, the higher charge states of unfolded conformations, relative to the folded structure, would not derive from a more extensive ionization of the former, but rather from a higher content of neutralizing charges in the latter. This interpretation allows several other problematic observations to be explained, including the different shapes of the spectra of folded and unfolded proteins, the discrepancies between observed and predicted gas-phase reactivity of protein ions and the apparent inconsistency of positive- and negative-ion mode results. Copyright © 2003 John Wiley & Sons, Ltd. [source] Use of the Rotation Vector in Brownian Dynamics Simulation of Transient Electro-Optical PropertiesMACROMOLECULAR THEORY AND SIMULATIONS, Issue 1 2009Tom Richard Evensen Abstract We have recently developed a new singularity-free algorithm for Brownian dynamics simulation of free rotational diffusion. The algorithm is rigorously derived from kinetic theory and makes use of the Cartesian components of the rotation vector as the generalized coordinates describing angular orientation. Here, we report on the application of this new algorithm in Brownian dynamics simulations of transient electro-optical properties. This work serves two main purposes. Firstly, it demonstrates the integrity of the new algorithm for BD-simulations of the most common transient electro-optic experiments. Secondly, it provides new insight into the performance of the new algorithm compared to algorithms that make use of the Euler angles. We study the transient electrically induced birefringence in dilute solutions of rigid particles with anisotropic polarization tensor in response to external electric field pulses. The use of both one single electric pulse and two electric pulses with opposite polarity are being analyzed. We document that the new singularity-free algorithm performs flawlessly. We find that, for these types of systems, the new singularity-free algorithm, in general, outperforms similar algorithms based on the Euler angles. In a wider perspective, the most important aspect of this work is that it serves as an important reference for future development of efficient BD-algorithms for studies of more complex systems. These systems include polymers consisting of rigid segments with single-segment translational,rotational coupling, segment,segment fluid-dynamic interactions and holonomic constraints. [source] Imaging the changes in renal T1 induced by the inhalation of pure oxygen: A feasibility studyMAGNETIC RESONANCE IN MEDICINE, Issue 4 2002Richard A. Jones Abstract The effect of the inhalation of pure oxygen on the kidney was evaluated by measuring monoexponential T1 and T relaxation times in nine volunteers using a multiple-shot turbo spin echo and multiple echo gradient echo sequences, respectively. The T1 of the renal cortex decreased significantly when breathing pure oxygen as compared to normoxia (from 882 ± 59 to 829 ± 70 msec, P < 0.05), while that of the renal medulla was unchanged. No significant changes were seen in the T of either compartment. Dynamic imaging using an inversion recovery sequence with an optimized inversion time typically produced signal changes of 20% in the renal cortex. Studies to assess if oxygen-induced changes in flow contributed to this effect showed that the flow contribution was not significant. Although longer inversion times (880 ms) produced optimal contrast, acceptable contrast was also obtained at shorter inversion times (450 msec) in the renal cortex, spleen, and lung, with the latter being of opposite polarity to the other two tissues, implying a shorter parenchymal T1 than previously reported in the literature. The results are consistent with oxygen acting as an intravascular contrast agent which induces a shortening of T1 in the arterial blood volume. Magn Reson Med 47:728,735, 2002. © 2002 Wiley-Liss, Inc. [source] Primary structure of a novel subunit in ba3 -cytochrome oxidase from thermus thermophilusPROTEIN SCIENCE, Issue 11 2000Tewfik Soulimane Abstract The ba3 -type cytochrome c oxidase from Thermus thermophilus is known as a two subunit enzyme. Deduced from the crystal structure of this enzyme, we discovered the presence of an additional transmembrane helix "subunit IIa" spanning the membrane. The hydrophobic N-terminally blocked protein was isolated in high yield using high-performance liquid chromatography. Its complete amino acid sequence was determined by a combination of automated Edman degradation of both the deformylated and the cyanogen bromide cleaved protein and automated C-terminal sequencing of the native protein. The molecular mass of 3,794 Da as determined by MALDI-MS and by ESI requires the N-terminal methionine to be formylated and is in good agreement with the value calculated from the formylmethionine containing sequence (3,766.5 Da + 28 Da = 3,794.5 Da). This subunit consits of 34 residues forming one helix across the membrane (Lys5-Ala34), which corresponds in space to the first transmembrane helix of subunit II of the cytochrome c oxidases from Paracoccus denitrificans and bovine heart, however, with opposite polarity. It is 35% identical to subunit IV of the ba3 -cytochrome oxidase from Natronobacterium pharaonis. The open reading frame encoding this new subunit IIa (cbaD) is located upstream of cbaB in the same operon as the genes for subunit I (cbaA) and subunit II (cbaB). [source] Detailed ECG Analysis of Atrial Repolarization in HumansANNALS OF NONINVASIVE ELECTROCARDIOLOGY, Issue 1 2009Fredrik Holmqvist M.D., Ph.D. Introduction: Data on human atrial repolarization are scarce since the QRS complex normally obscures its ECG trace. In the present study, consecutive patients with third-degree AV block were studied to better describe the human Ta wave. Methods and Results: Forty patients (mean age 75 years, 17 men) were included. All anti-arrhythmic drugs were discontinued before ECG recording. Standard 12-lead ECGs were recorded, transformed to orthogonal leads and studied using signal-averaged P wave analysis. The average P wave duration was 124 ± 16 ms. The PTa duration was 449 ± 55 ms (corrected PTa 512 ± 60 ms) and the Ta duration (P wave end to Ta wave end) was 323 ± 56 ms. The polarity of the Ta wave was opposite to that of the P wave in all leads. The Ta peaks were located at 196 ± 55 ms in Lead Y, 216 ± 50 ms in Lead X, and 335 ± 92 in Lead Z. No correlation was found between P wave duration and Ta duration, or between Ta peak amplitude and Ta duration. The morphology of the Ta wave was similar regardless of the interatrial conduction. Conclusions: The Ta wave has the opposite polarity, and the duration is generally two to three times that, of the P wave. Although the Ta peak may occasionally be located in the PQ interval during normal AV conduction, it is unlikely that enough information can be obtained from analysis of this segment to differentiate normal from abnormal atrial repolarization. Hence, an algorithm for QRST cancellation during sinus rhythm is needed to further improve analysis. [source] Center to limb variation of penumbral Stokes V profilesASTRONOMISCHE NACHRICHTEN, Issue 6 2010M. Franz Abstract We investigated the horizontal and the vertical component of the Evershed flow (EF). To this end, we computed average Stokes V profiles for various velocity classes in penumbrae at different heliocentric angles. Our results show that for blueshifted profiles an additional lobe with the same polarity as the spot is present in the blue side of the average Stokes V profile. The amplitude of the additional lobe grows with increasing blueshift and with increasing heliocentric angle. For small redshifts, the profiles show an additional lobe with the opposite polarity as the spot on the red side of the average Stokes V profile. Even at disk center, the original polarity of the average Stokes V profile is reversed for strong redshifts. The transition between the different types of Stokes V profiles is continuous and indicates that not only the vertical, but also the horizontal EF is a magnetized stream of plasma in a magnetic background field (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Seed plants of Fiji: an ecological analysisBIOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 3 2006MICHAEL HEADS An annotated list of indigenous Fijian seed plant genera is presented and comprises 484 genera and 1315 species in 137 families. The relative diversity of the largest families and genera in Fiji is indicated and compared with floras in New Caledonia and the Upper Watut Valley, Papua New Guinea. Differences and similarities appear to be due to biogeographical/phylogenetic factors rather than ecological differences or means of dispersal. Generic diversity for the seed plants as a whole is greatest between 0,100 m and decreases monotonically with altitude. However, in the largest family, Orchidaceae, maximum diversity occurs between 200,400 m. Fifty percent of the families are recorded from shore habitat. Twenty-seven percent of the families and 80 species occur in or around mangrove, where the most diverse families are Orchidaceae, Rubiaceae, and the legumes. Some of the mangrove-associate species are pantropical or Indo-Pacific but most are locally or regionally endemic. Fifty-six percent of the Fijian families are recorded on limestone. Twenty-nine species are restricted to limestone and 12 species usually occur on limestone. The importance of calcium in reducing the effects of salinity is emphasized and 39 species are recorded from both mangrove and limestone. A plagiotropic habit occurs in 38 species which occur on limestone or around beaches, and 20 of these are Pacific endemics. Genera restricted to higher altitudes include many present elsewhere in Melanesia but absent from Australia despite suitable habitat there, again indicating the importance of biogeographical and historical factors. Altitudinal anomalies in Fiji taxa are cited and include 7 anomalously high records from northern Viti Levu, a site of major uplift, and 22 anomalously low altitudinal records in the Lau Group, a site of subsidence. It is suggested that the Fijian flora has not been derived from immigrants from Asia, but has evolved more or less in situ. Taxa would have survived as metapopulations on the individually ephemeral volcanic islands always found at oceanic subduction zones and hot spots, and the atolls which characterize areas of subsidence. The complex geology of Fiji is determined by its position between two subduction zones of opposite polarity, the Vanuatu and Tonga Trenches, in what is currently a region of transform faulting. The large islands comprise fragments of island arcs that have amalgamated and welded together. There has been considerable uplift as well as subsidence in the islands and it is suggested that both these processes have had drastic effects on the altitudinal range of the taxa. Limestone and mangrove floras could have provided a widespread, diverse ancestral species pool from which freshwater swamp forest, lowland rainforest, dry forest, secondary forest, thickets, and montane forest have been derived during phases of uplift. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 407,431. [source] | |||||||||||||||||||||||||