CW Methods Research Articles

Relative Acoustic Transfer Function Estimation in Wireless Acoustic Sensor Networks

In this paper, we present an algorithm to estimate the relative acoustic transfer function RTF of a target source in wireless acoustic sensor networks WASNs. Two well-known methods to estimate the RTF are the covariance subtraction CS method and the covariance whitening CW approach, the latter based on the generalized eigenvalue decomposition. Both methods depend on the use of the noisy correlation matrix, which, in practice, has to be estimated using limited and in WASNs quantized data. The bit rate and the fact that we use limited data records therefore directly affect the accuracy of the estimated RTFs. Therefore, we first theoretically analyze the estimation performance of the two approaches in terms of bit rate. Second, we propose a rate-distribution method by minimizing the power usage and constraining the expected estimation error for both RTF estimators. The optimal rate distributions are found by using convex optimization techniques. The model-based methods, however, are impractical due to the dependence on the true RTFs. We therefore further develop two greedy rate-distribution methods for both approaches. Finally, numerical simulations on synthetic data and real audio recordings show the superiority of the proposed approaches in power usage compared to uniform rate allocation. We find that in order to satisfy the same RTF estimation accuracy, the rate-distributed CW methods consume much less transmission energy than the CS-based methods.

Potential Functional Embedding Theory at the Correlated Wave Function Level. 2. Error Sources and Performance Tests.

Quantum mechanical embedding theories partition a complex system into multiple spatial regions that can use different electronic structure methods within each, to optimize trade-offs between accuracy and cost. The present work incorporates accurate but expensive correlated wave function (CW) methods for a subsystem containing the phenomenon or feature of greatest interest, while self-consistently capturing quantum effects of the surroundings using fast but less accurate density functional theory (DFT) approximations. We recently proposed two embedding methods [for a review, see: Acc. Chem. Res. 2014 , 47 , 2768 ]: density functional embedding theory (DFET) and potential functional embedding theory (PFET). DFET provides a fast but non-self-consistent density-based embedding scheme, whereas PFET offers a more rigorous theoretical framework to perform fully self-consistent, variational CW/DFT calculations [as defined in part 1, CW/DFT means subsystem 1(2) is treated with CW(DFT) methods]. When originally presented, PFET was only tested at the DFT/DFT level of theory as a proof of principle within a planewave (PW) basis. Part 1 of this two-part series demonstrated that PFET can be made to work well with mixed Gaussian type orbital (GTO)/PW bases, as long as optimized GTO bases and consistent electron-ion potentials are employed throughout. Here in part 2 we conduct the first PFET calculations at the CW/DFT level and compare them to DFET and full CW benchmarks. We test the performance of PFET at the CW/DFT level for a variety of types of interactions (hydrogen bonding, metallic, and ionic). By introducing an intermediate CW/DFT embedding scheme denoted DFET/PFET, we show how PFET remedies different types of errors in DFET, serving as a more robust type of embedding theory.

Mechanism of Slow Inactivation in Prokaryotic Voltage-Gated Sodium Channel

Voltage-gated sodium channels (Nav) play a crucial role in electrically excitable cells by mediating the rapid influx of Na+ and thereby initiating the action potential in response to a depolarizing stimulus. In excitable cells, Nav activates and then undergoes fast and slow inactivation. Fast inactivation occurs within the time frame of a single action potential, whereas slow inactivation is produced by prolonged depolarization, mediating the availability of sodium channels over extended period of time. Mutations that alter slow inactivation are associated with several diseases such as hyperkalemic periodic analysis and long-QT syndrome. However, the molecular mechanism of slow inactivation and modulation by drug binding are still not completely known. In order to establish a structural framework of slow inactivation in molecular level, we used the prokaryotic homologue NavSP1 from Silicibacter pomeroyi as a model system. NavSP1 shares very similar overall architecture, activation, inactivation properties, and similar drug binding sites with the eukaryotic homologues and thereby proves to be an excellent model to be studied in native environment. We investigated how channel opening or the activation of voltage sensor initiates slow inactivation by evaluating the changes in conformational dynamics of the channel pore in the presence and absence of the voltage sensor. Our approach combined structural dynamics by EPR spectroscopy (CW and DEER methods) and functional studies by electrophysiological methods. Evaluation of the changes in the conformational dynamics and solvent accessibility in the turret and selectivity filter region show structural rearrangement of the outer pore region during slow inactivation.

Molecular Insights into Allosteric Modulation of GLIC Gating by Membrane Lipids

The pentameric ligand gated ion channels (pLGIC) belong to the cys-loop super family and mediate fast chemical transmission in the central and peripheral nervous system. These channels are targeted by a large number of exogenous and endogenous modulators that include alcohols, anesthetics, neurosteroids, and membrane lipid constituents. Several high resolution structures of various members of the pLGIC family have been solved so far but the structural basis for allosteric modulation remains elusive. Here, using a proton-gated pLGIC from Gloeobacter violaceus (GLIC), we characterize the effects of membrane lipid composition on channel function, structure, and dynamics. Using an integrated approach that involves EPR spectroscopy (CW and DEER methods), X-ray crystallography, and electrophysiological measurements, we provide high-resolution snapshots of the channel bound to these modulators and further describe the changes in channel dynamics that underlie the observed functional effects. Taken together, these findings provide novel insights into this critical yet poorly understood mechanism of lipid modulation in pLGIC.

Copper(II)-Binding Ability of Stereoisomeric cis- and trans-2-Aminocyclohexanecarboxylic Acid–l-Phenylalanine Dipeptides. A Combined CW/Pulsed EPR and DFT Study

With the aim of an improved understanding of the metal-complexation properties of alicyclic β-amino acid stereoisomers, and their peptides, the complex equilibria and modes of coordination with copper(II) of L-phenylalanine (F) derivatives of cis/trans-2-aminocyclohexanecarboxylic acid (c/tACHC), i.e. the dipeptides F-c/tACHC and c/tACHC-F, were investigated by a combination of CW and pulsed EPR methods. For the interpretation of the experimental data, DFT quantum-chemical calculations were carried out. Simulation of a pH-dependent series of room-temperature CW-EPR spectra revealed the presence of EPR-active complexes ([Cu(aqua)](2+), [CuL](+), [CuLH(-1)], [CuLH(-2)](-), and [CuL(2)H(-1)](-)), and an EPR-inactive species ([Cu(2)L(2)H(-3)](-)) in aqueous solutions for all studied cases. [CuLH](2+) was included in the equilibrium model for the c/tACHC-F-copper(II) systems, and [CuL(2)], together with two coordination isomers of [CuL(2)H(-1)](-), were also identified in the F-tACHC-copper(II) system. Comparison of the complexation properties of the diastereomeric ligand pair F-(1S,2R)-ACHC and F-(1R,2S)-ACHC did not reveal significant differences. Considerably lower formation constants were obtained for the trans than for the cis isomers for both the F-c/tACHC and the c/tACHC-F pairs in the case of [CuLH(-1)] involving tridentate coordination by the amino, the deprotonated peptide, and the carboxylate groups. A detailed structural analysis by pulsed EPR methods and DFT calculations indicated that there was no significant destabilization for the complexes of the trans isomers. The lower stability of their complexes was explained by the limitation that only the conformer with donor groups in equatorial-equatorial ring positions can bind to copper(II), whereas both equatorial-axial conformers of the cis isomers are capable of binding. From a consideration of the proton couplings obtained with X-band (1)H HYSCORE, (2)H exchange experiments, and DFT, the thermodynamically most stable cyclohexane ring conformer was assigned for all four [CuLH(-1)] complexes. For the F-cACHC case, the conformer did not match the most stable conformer of the free ligand.

Quantum mechanical modeling of electronic excitations in metal oxides: Magnesia as a prototype

We compare embedded correlated wavefunction (ECW) approaches for predicting excited states within MgO as a prototypical metal oxide. The crystal is partitioned into a cluster treated with CW methods and a background described by various electrostatic or orbital-free-density-functional-theory (DFT)-based embedding potentials. The excited singlet and triplet states are found to be nearly degenerate and of charge-transfer type, consistent with experiment. Although the prediction of excitation energies by ECW theory with an electrostatic description of the background falls slightly short of more expensive Green’s function methods, it is significantly improved over standard DFT or non-embedded CW methods.

Structure and Function of Quinones in Biological Solar Energy Transduction: A Differential Pulse Voltammetry, EPR, and Hyperfine Sublevel Correlation (HYSCORE) Spectroscopy Study of Model Benzoquinones

Quinones are widely used electron transport cofactors in photosynthetic reaction centers. Previous studies have suggested that the structure of the quinone cofactors and the protein interactions or "smart" matrix effects from the surrounding environment govern the redox potential and hence the function of quinones in photosynthesis. In the present study, a series of 1,4-benzoquinone models are examined via differential pulse voltammetry to provide relative redox potentials. In parallel, CW and pulsed EPR methods are used to directly determine the electronic properties of each benzoquinone in aprotic and protic environments. The shifts in the redox potential of the quinones are found to be dependent on the nature of the substituent group and the number of substituent groups on the quinone molecule. Further, we establish a direct correlation between the nature of the substituent group and the change in electronic properties of the benzosemiquinone by analysis of the isotropic and anisotropic components of the electron-nuclear hyperfine interactions observed by CW and pulsed EPR studies, respectively. Examination of an extensive library of model quinones in both aprotic and protic solvents indicates that hydrogen-bonding interactions consistently accentuate the effects of the substituent groups of the benzoquinones. This study provides direct support for the tuning and control of quinone cofactors in biological solar energy transduction through interactions with the surrounding protein matrix.

Time-of-flight range detection using low-frequency intensity modulation of a cw laser diode: application to fiber length measurement

This paper describes a simple optical range detector in the time-of-flight (TOF) category using a cw laser diode emitting at 1.55 µm. The principle of operation combines TOF with frequency-modulated cw methods. The main originality of this method consists in measuring the time delay between two signals without using the time variable. The forward current of the laser diode is modulated by a low-frequency triangular signal. The emitted and received signals are compared, and the amplitude difference gives a readout signal whose peak-to-peak amplitude is proportional to the TOF. An application to optical fiber length measurement is given.

Multilayer modeling of radiative transfer by SLW and CW methods in non-isothermal gaseous medium

Non-isothermal gaseous medium is modeled using the multilayer approach, breaking the one-dimensional system into a series of isothermal layers. Spectral integration of the radiative transfer equation (RTE) is performed for the spectral line weighted-sum-of-gray-gases and cumulative wavenumber approaches to modeling the spectral nature of the gas radiation. An exact analytical solution of RTE is obtained for the layers with both black and gray walls. Predictions show high accuracy, even with surprisingly few layers.

Toward the Goal of Signal Quantification in Contrast-Enhanced Optical Imaging

about the optical scattering coefficient of the tissue. As with endogenous optical imaging, time domain offers the potential to supersede CW for optical molecular imaging applications. The wealth of information contained in the fluorescence excitation and emission TPSFs permits the depth of the fluorophore to be obtained by even simple sonar-like approaches. Once the fluorophore depth is known, the intensity can be normalized to provide relative fluorophore concentration. In addition to the potential of TD to provide more accurate quantitative estimates of fluorophore depth and concentration than CW, TD enables the measurement of the fluoresence lifetime which CW is unable to provide. This permits the discrimination of fluorophores which may have significantly different fluoresence lifetimes, despite having similar emission spectra which challenges discrimination with intensity-based CW methods. Furthermore, it is known that the fluorescence lifetime can be influenced by the local environment of the fluorophore which provides additional physiological information. Time domain optical molecular imaging is the technological basis for the pre-clinical optical molecular imaging device (SAMITM) developed by ART.

Characterization of ultrasound-contrast agents by short-burst excitation

A two-frequency method has been adapted to the use of short ultrasonic bursts for the determination of bubble size. The advantage of a burst method is threefold. The overall energy transfer into the volume of interest is smaller as compared to cw. Second, it allows simple methods for improving spatial resolution. Finally, only one transducer will eventually be required in a pulse-echo mode. The experimental setup is designed to fully compensate for transducer characteristics and sample artifacts such as attenuation through window material and contrast agent solution. This leads to a constant sound pressure exposure for contrast agent in the focal volume. The data acquisition employs the frequency tracking procedure where spectra for narrow-band excition are analyzed for fundamental, second harmonic, and sidebands. A technique of nonconstant imaging frequency is used to avoid ambiguities due to coincidences between higher harmonics of the fundamental and multiples of the sideband frequencies. Measurements with shell-encapsulated microbubbles showed the behavior demonstrated by previous cw methods. Higher harmonics as well as sidebands were present due to the nonlinear dynamics of the contrast agents. [This work was supported in part by USPHS Grant Nos. RO1 DK42290 and 1RO1 HL54201.]

Fourier transform electron spin resonance imaging

Modern Fourier transform (FT) ESR methods have been combined with fast, high power pulsed magnetic field gradients to enable FT-ESR imaging. Spectral—spatial imaging by frequency and phase encoded FT methods are compared with cw methods. The initial phase encoded results are comparable in quality to those from the well-developed cw methods and further improvements which would enhance FT-ESR imaging are noted.

Pulsed NMRON relaxation measurements and thermometric NMR in the quasi-2 dimensional femomagnet: Mn(COOCH 3) 2·4H 2O

The measurement of the field dependence of the nuclear spin-lattice relaxation time of 54Mn in the two manganese sites in the quasi-2 dimensional ferromagnet Mn(COOCH 3) 2·4H 20 obtained by the pulsed NMRON technique is reported. This technique allows the observation in low fields of the higher frequency resonance which previously could not be measured by CW methods. The anomaly in the 54Mn relaxation time observed in the 55Mn level crossing regime is discussed, and the thermometric observation of the field dependence and lice width of the resonance lines from the abundant 55Mn spin systems is reported and related to the 54Mn spin-lattice relaxation behavior.

Electron spin echo and CW perspective in 3D EPR imaging

Three-dimensional spectral-spatial-spatial images are presented for a sample containing six different species in seven compartments. EPR spectra used to construct these images were obtained by the normal CW methods and by electron spin echo detection. Images of the sample when viewed from these two different perspectives are contrasted.

Fluorescence properties of dyes adsorbed to silver islands, investigated by picosecond techniques

The fluorescence properties of dye molecules (rhodamine 6G and erythrosin) adsorbed on pure glass surfaces and on silver islands films are investigated by cw and picosecond time-resolved methods. On pure glass surfaces we observe concentration quenching below a critical intermolecular distance (reduction of the fluorescence power per molecule as well as shortened and non-exponential fluorescence decay). On silver islands films the shortening in fluorescence lifetime is more drastic and is nearly independent of the intermolecular distance. This behavior suggests an electrodynamic interaction between dye monomers and plasmons in the metal particles, modified by a damping influence of dye dimers.

Comparison of LAME analyses for an eight-spin system calibrated by CW and pulsed methods at different larmor frequencies

Although the statistical error estimates of a LAOCOON-3-type analysis of a high-resolution NMR spectrum in the liquid phase have been widely discussed and precise analyses of spectra calibrated by a given method are available, the problem of systematic experimental errors remains. The analysis of the 1H NMR spectra of p-fluorobenzylthiol, calibrated at 100 MHz in a standard way by cw methods and at 90 MHz in the FFT mode under similar conditions of spectral quality, demonstrates that systematic errors need not be significant at the 0.01-Hz confidence level. Correlation coefficients are useful indicators, of course, in addition to standard errors.

A multiple pulse zero crossing NMR technique, and its application to 19F chemical shift measurements in solids

A simple, multiple pulse ’’zero crossing technique’’ for accurately determining the first moment of a solid state NMR spectrum is introduced. This technique was applied to obtain the 19F chemical shift versus pressure curves up to 5 kbar for single crystals of CaF2 (0.29±0.02 ppm/kbar) and BaF2 (0.62±0.05 ppm/kbar). Results at ambient temperature and pressure are also reported for a number of other fluorine compounds. Because of its high data rate, this technique is potentially several orders of magnitude more sensitive than similar cw methods.

Nuclear relaxation and spin exchange in ammonium hexafluorophosphate (NH4PF6)

The three phases of solid ammonium hexafluorophosphate (NH4PF6) have been studied by cw and pulsed NMR methods between 298° and 77°K. The proton and fluorine second moments of 1.8 and 1.9 G2, respectively, at 298°K are consistent with rapid, randomized reorientations of NH4+ and PF6- ions, while the larger values at 77°K require that there be little or no motional narrowing by reorientations of the PF6− ions even though the narrowing by NH4+ reorientations is still present. For both nuclear species the spin-lattice relaxation time (T1) studies of the two low-temperature phases reveal three distinct regions: (1) Between 109° and 110°K their relaxation is exponential with time. (2) Between 105° and 87°K the relaxation is nonexponential, but may be expressed as the sum of two exponential terms. At each temperature the same pair of time constants applies to both nuclear species. (3) At 77°K the relaxation is again exponential, with different T1's for protons and fluorine. The nonexponential behavior in region (2) is attributed to spin exchange between protons and fluorine nuclei. It results from modulation of the H–F dipole-dipole interactions by relatively slow reorientations of the PF6− ions. This interpretation was confirmed by a study of the fluorine T1 of ND4PF6, for which only exponential decays were observed. Activation parameters for the PF6- reorientation were determined from the fluorine T1 in ND4PF6. The parameters depend somewhat upon temperature, being 4.1 ± 0.2 kcal/mole and (3.7 ± 0.5) × 10−16 sec above 130°K and 4.4 ± 0.3 and (5.2 ± 1.5) × 10−17 below. A discontinuity occurs in the T1's at the 192°K phase transition, and in the high temperature NaCl phase the T1's are almost independent of temperature and probably are governed by spin-rotation interactions. DTA experiments indicate that no additional phase transitions occur in NH4PF6 between 300° and 473°K.

Quantitative evaluation of cw methods for measurement of nuclear Zeeman spin-lattice relaxation times in solids

The Provotorov theory of partial saturation in solids is utilized to extract T 1 the Zeeman spin-lattice relaxation time, from on-resonance absorption mode second derivative, and dispersion mode first derivative cw signals, using conventional lock-in methods. Results are found to be in agreement with the usual 90°-τ-90° pulse method.

Studies of 13C 13C coupling constants. IV. The isopropenyl system

13C 13C coupling constants were obtained on ten monolabeled or dilabeled isopropenyl compounds, using repetitive cw methods. The single-bond and double-bond couplings were found to vary over 10 and 14 Hz, respectively, throughout the series. The results were analyzed in terms of INDO calculations, which indicate a relationship between J cc values and −I + substituent character. Both experimental and INDO results are considered in comparison to previously reported data on ethyl and acetyl compounds.