Pyrrolidine ring conformations in prolyl peptides from 13C spin-lattice relaxation times

A method was developed in the framework of a bistable jump model to obtain the pyrrolidine ring conformations in proline peptides from 13C spin-lattice relaxation times. Equations are presented expressing the ring torsions in terms of the 13C spin-lattice relaxation times of the ring carbons. This method was applied to 26 pyrrolidine ring systems and acceptable conformations were obtained.

A Finite Variable Difference Relaxation Scheme for hyperbolic-parabolic equations

Using the framework of a new relaxation system, which converts a nonlinear viscous conservation law into a system of linear convection-diffusion equations with nonlinear source terms, a finite variable difference method is developed for nonlinear hyperbolic-parabolic equations. The basic idea is to formulate a finite volume method with an optimum spatial difference, using the Locally Exact Numerical Scheme (LENS), leading to a Finite Variable Difference Method as introduced by Sakai [Katsuhiro Sakai, A new finite variable difference method with application to locally exact numerical scheme, journal of Computational Physics, 124 (1996) pp. 301-308.], for the linear convection-diffusion equations obtained by using a relaxation system. Source terms are treated with the well-balanced scheme of Jin [Shi Jin, A steady-state capturing method for hyperbolic systems with geometrical source terms, Mathematical Modeling Numerical Analysis, 35 (4) (2001) pp. 631-645]. Bench-mark test problems for scalar and vector conservation laws in one and two dimensions are solved using this new algorithm and the results demonstrate the efficiency of the scheme in capturing the flow features accurately.

Why and when do learning goal orientation and attitude decrease with aging? The role of perceived remaining time and work centrality

We conducted two studies to improve our understanding of why and when older workers are focused on learning. Based on socioemotional selectivity theory, which proposes that goal focus changes with age and the perception of time, we hypothesized and found that older workers perceive their remaining time at work as more limited than younger workers which, in turn, is associated with lower learning goal orientation and a less positive attitude toward learning and development. Furthermore, we hypothesized and found that high work centrality buffers the negative association between age and perceived remaining time, and thus the indirect negative effects of age on learning goal orientation and attitude toward learning and development (through perceived remaining time). These findings suggest that scholars and practitioners should take workers’ perceived remaining time and work centrality into account when examining or stimulating learning activities among aging workers.

Possible mechanism of charge transport and dielectric relaxation in SrO-Bi2O3-B2O3 glasses

Transparent SrBi2B2O7 glasses were prepared via melt-quenching technique and characterized using differential scanning calorimetry and x-ray powder diffraction. The ac conductivities of the glasses were studied as a function of frequency (100 Hz-10 MHz) at different temperatures. The frequency dependence of conductivity has been analyzed using Almond-West expression. The exponent n was nearly unaffected by temperature. Impedance and modulus spectroscopies were employed to further examine the electrical data. Dielectric relaxation exhibited a stretched exponential behavior with a stretching exponent beta independent of temperature. From conductivity analysis we have proposed that the charge transport occurs through the participation of nonbridging oxygen (NBO), which switches positions in a facile manner. The stretched exponential behavior appears to be a direct consequence of the NBO switching mechanism of charge transport.

Vitrification, relaxation and free volume in glycerol-water binary liquid mixture:Spin probe ESR studies

Glass transition and relaxation of the glycerol-water (G-W) binary mixture system have been studied over the glycerol concentration range of 5-85 mol% by using the highly sensitive technique of electron spin resonance (ESR). For the water rich mixture the glass transition,sensed by the dissolved spin probe, arises from the vitrified mesoscopic portion of the binary system. The concentration dependence of the glass transition temperature manifests a closely related molecular level cooperativity in the system. A drastic change in the mesoscopic structure of the system at the critical concentration of 40 mol is confirmed by an estimation of the spin probe effective volume in a temperature range where the tracer reorientation is strongly coupled to the system dynamics.

Characterization of anomalous relaxation using the time-fractional Bloch equation and multiple echo T2 *-weighted magnetic resonance imaging at 7T

PURPOSE To study the utility of fractional calculus in modeling gradient-recalled echo MRI signal decay in the normal human brain. METHODS We solved analytically the extended time-fractional Bloch equations resulting in five model parameters, namely, the amplitude, relaxation rate, order of the time-fractional derivative, frequency shift, and constant offset. Voxel-level temporal fitting of the MRI signal was performed using the classical monoexponential model, a previously developed anomalous relaxation model, and using our extended time-fractional relaxation model. Nine brain regions segmented from multiple echo gradient-recalled echo 7 Tesla MRI data acquired from five participants were then used to investigate the characteristics of the extended time-fractional model parameters. RESULTS We found that the extended time-fractional model is able to fit the experimental data with smaller mean squared error than the classical monoexponential relaxation model and the anomalous relaxation model, which do not account for frequency shift. CONCLUSIONS We were able to fit multiple echo time MRI data with high accuracy using the developed model. Parameters of the model likely capture information on microstructural and susceptibility-induced changes in the human brain.

H-1 and F-19 NMR relaxation time studies in (NH4)(2)ZrF6 superionic conductor

H-1 and F-19 spin-lattice relaxation times in polycrystalline diammonium hexafluorozirconate have been measured in the temperature range of 10-400 K to elucidate the molecular motion of both cation and anion. Interesting features such as translational diffusion at higher temperatures, molecular reorientational motion of both cation and anion groups at intermediate temperatures and quantum rotational tunneling of the ammonium group at lower temperatures have been observed. Nuclear magnetic resonance (NMR) relaxation time results correlate well with the NMR second moment and conductivity studies reported earlier.

Composite scheme using localized relaxation with non-standard finite difference method for hyperbolic conservation laws

Non-standard finite difference methods (NSFDM) introduced by Mickens [Non-standard Finite Difference Models of Differential Equations, World Scientific, Singapore, 1994] are interesting alternatives to the traditional finite difference and finite volume methods. When applied to linear hyperbolic conservation laws, these methods reproduce exact solutions. In this paper, the NSFDM is first extended to hyperbolic systems of conservation laws, by a novel utilization of the decoupled equations using characteristic variables. In the second part of this paper, the NSFDM is studied for its efficacy in application to nonlinear scalar hyperbolic conservation laws. The original NSFDMs introduced by Mickens (1994) were not in conservation form, which is an important feature in capturing discontinuities at the right locations. Mickens [Construction and analysis of a non-standard finite difference scheme for the Burgers–Fisher equations, Journal of Sound and Vibration 257 (4) (2002) 791–797] recently introduced a NSFDM in conservative form. This method captures the shock waves exactly, without any numerical dissipation. In this paper, this algorithm is tested for the case of expansion waves with sonic points and is found to generate unphysical expansion shocks. As a remedy to this defect, we use the strategy of composite schemes [R. Liska, B. Wendroff, Composite schemes for conservation laws, SIAM Journal of Numerical Analysis 35 (6) (1998) 2250–2271] in which the accurate NSFDM is used as the basic scheme and localized relaxation NSFDM is used as the supporting scheme which acts like a filter. Relaxation schemes introduced by Jin and Xin [The relaxation schemes for systems of conservation laws in arbitrary space dimensions, Communications in Pure and Applied Mathematics 48 (1995) 235–276] are based on relaxation systems which replace the nonlinear hyperbolic conservation laws by a semi-linear system with a stiff relaxation term. The relaxation parameter (λ) is chosen locally on the three point stencil of grid which makes the proposed method more efficient. This composite scheme overcomes the problem of unphysical expansion shocks and captures the shock waves with an accuracy better than the upwind relaxation scheme, as demonstrated by the test cases, together with comparisons with popular numerical methods like Roe scheme and ENO schemes.

Dielectric anisotropy and relaxation studies of the homologous series of N-(4-n-alkyloxy-2-hydroxy-benzylidene)-4-carbethoxyaniline

Dielectric properties of the homologous series of newly synthesized nonchiral compounds N-(4-n-alkyloxy-2-hydroxy-benzylidene)-4-carbethoxyaniline, (n = 6, 8, 10, 12) having wide temperature range (∼60°C) smectic A (SmA) phase, have been studied by the impedance spectroscopy in the frequency range of 100 Hz to 1 MHz. Measurements have been carried out for two principal alignments (planar as well as homeotropic) of the SmA phase. Dielectric anisotropy (Δε' = ε'∥ - ε'⊥) for all the members of the series has been found to be negative for the whole temperature range of SmA phase. Magnitude of the dielectric anisotropy (|Δε'|) has been found to decrease with the number of alkyl chains. Relaxation frequencies corresponding to the rotation of the individual molecules about their short axes, lie below 1 MHz and obey the Arrhenius law by which activation energies have been determined. However, the relaxation frequencies corresponding to the rotation of the molecules about their short axes apparently lie above 10 MHz.

Influence of sintering conditions and doping on the dielectric relaxation originating from the surface layer effects in CaCu3Ti4O12 ceramics

Detailed investigations into the dielectric dispersion phenomenon in the giant dielectric constant material CaCu3Ti4O12 (CCTO) around room temperature revealed the existence of two successive dielectric relaxations. In the temperature domain, a new dielectric relaxation was clearly observed around 250K, in addition to the well-investigated dielectric relaxation close to 100K. The effect of sintering and doping (La3+) on the strength of these dielectric relaxations were studied in detail. The sintering temperature as well as its duration was found to have tremendous influence on the dielectric relaxation that was encountered around 250 K. This Maxwell-Wagner (M-W) type of relaxation was found to be originating from the surface layer containing the Cu-rich phase, which was ascribed to the difference in the oxygen content between the surface and the interior of the sample. Interestingly, this particular additional relaxation was not observed in La2/3Cu3Ti4O12, a low dielectric constant member of the CCTO family, in which the segregation of Cu-rich phase on the surface was absent. Indeed the correlation between the new relaxation and the presence of Cu-rich phase in CCTO ceramics was further corroborated by the absence of the same after removing the top and bottom layers. (C) 2007 Elsevier Ltd. All rights reserved.

Influence of Cooling Rate on the Enthalpy Relaxation and Fragility of a Metallic Glass

Structural relaxation behavior of a rapidly quenched (RQ) and a slowly cooled Pd40Cu30Ni10P20 metallic glass was investigated and compared. Differential scanning calorimetry was employed to monitor the relaxation enthalpies at the glass transition temperature, T-g , and the Kolrausch-Williams-Watts (KWW) stretched exponential function was used to describe its variation with annealing time. It was found that the rate of enthalpy recovery is higher in the ribbon, implying that the bulk is more resistant to relaxation at low temperatures of annealing. This was attributed to the possibility of cooling rate affecting the locations where the glasses get trapped within the potential energy landscape. The RQ process traps a larger amount of free volume, resulting in higher fragility, and in turn relaxes at the slightest thermal excitation (annealing). The slowly cooled bulk metallic glass (BMG), on the other hand, entraps lower free volume and has more short-range ordering, hence requiring a large amount of perturbation to access lower energy basins.

Vibrational phase relaxation of O–H stretch in bulk water: Role of large amplitude angular jumps and negative cross-correlations among the forces on the O–H bond

The ultrafast vibrational phase relaxation of O–H stretch in bulk water is investigated in molecular dynamics simulations. The dephasing time (T2) of the O–H stretch in bulk water calculated from the frequency fluctuation time correlation function (Cω(t)) is in the range of 70–80 femtosecond (fs), which is comparable to the characteristic timescale obtained from the vibrational echo peak shift measurements using infrared photon echo [W.P. de Boeij, M.S. Pshenichnikov, D.A. Wiersma, Ann. Rev. Phys. Chem. 49 (1998) 99]. The ultrafast decay of Cω(t) is found to be responsible for the ultrashort T2 in bulk water. Careful analysis reveals the following two interesting reasons for the ultrafast decay of Cω(t). (A) The large amplitude angular jumps of water molecules (within 30–40 fs time duration) provide a large scale contribution to the mean square vibrational frequency fluctuation and gives rise to the rapid spectral diffusion on 100 fs time scale. (B) The projected force, due to all the atoms of the solvent molecules on the oxygen (FO(t)) and hydrogen (FH(t)) atom of the O–H bond exhibit a large negative cross-correlation (NCC). We further find that this NCC is partly responsible for a weak, non-Arrhenius temperature dependence of the dephasing rate.

Subquadratic wavenumber dependence of the structural relaxation of supercooled liquid in the crossover regime

As a liquid is progressively supercooled toward its glass transition temperature, an intriguing weakening of the wavenumber (q) dependence of the structural relaxation time tau(q) in the intermediate-to-large q limit is observed both in experiments and simulation studies. Neither continuous Brownian diffusive dynamics nor discontinuous activated events can alone explain the anomalous wavenumber dependence. Here we use our recently developed theory that unifies the mode coupling theory for continuous dynamics, with the random first order transition theory treatment of activated discontinuous motion as a nucleationlike instanton process to understand the wavenumber dependence of density relaxation. The predicted smooth change in mechanism of relaxation from diffusive to activated, in the crossover regime, is wavevector dependent and appears to be responsible for the observed subquadratic,nalmost linear, q dependence of the relaxation time.