Cross-relaxation Terms Research Articles

Examination of Solvent Interactions with Trp-Cage in 1,1,1,3,3,3-Hexafluoro-2-propanol-water at 298 K through MD Simulations and Intermolecular Nuclear Overhauser Effects.

MD simulations of the peptide Trp-cage dissolved in 28% hexafluoroisopropanol (HFIP)-water have been carried out at 298 K with the goal of exploring peptide hydrogen-solvent fluorine nuclear spin cross-relaxation. The work was motivated by the observation that most experimental fluoroalcohol-peptide cross-relaxation terms at 298 K are small, both positive and negative, and not always well predicted from simulations. The cross-relaxation terms for hydrogens of the caged tryptophan residue of Trp-cage are substantially negative, a result consistent with simulations. It was concluded that hexafluoroisopropanol interactions near this part of the peptide are particularly long-lived. While both HFIP and water are present in all regions of the simulation box, the composition of the solvent mixture is not homogeneous throughout the system. HFIP generally accumulates near the peptide surface, while water molecules are preferentially found in regions that are more than 1.5 nm from the surface of the peptide. However, some water remains in higher-than-expected amounts in the solvent layer surrounding 6Trp, 9Asp, Ser13, and Ser14 residues in the helical region of Trp-cage. As observed in simulations of this system at 278 K, HFIP molecules aggregate into clusters that continually form and re-form. Translational diffusion of both HFIP and water appears to be slowed near the surface of the peptide with reduction in diffusion near the 6Trp residue 2- to 3-fold larger than calculated for solvent interactions with other regions of Trp-cage.

Revisiting paramagnetic relaxation enhancements in slowly rotating systems: how long is the long range?

Cross-relaxation terms in paramagnetic systems that reorient rigidly with slow tumbling times can increase the effective longitudinal relaxation rates of protons of more than 1 order of magnitude. This is evaluated by simulating the time evolution of the nuclear magnetization using a complete relaxation rate-matrix approach. The calculations show that the Solomon dependence of the paramagnetic relaxation rates on the metal-proton distance (as ) can be incorrect for protons farther than 15 Å from the metal and thus can cause sizable errors in -derived distance restraints used, for instance, for protein structure determination. Furthermore, the chemical exchange of these protons with bulk water protons can enhance the relaxation rate of the solvent protons by far more than expected from the paramagnetic Solomon equation. Therefore, it may contribute significantly to the water proton relaxation rates measured at magnetic resonance imaging (MRI) magnetic fields in the presence of slow-rotating nanoparticles containing paramagnetic ions and a large number of exchangeable surface protons.

Highly Efficient, Compact Tm3+:RE2O3 (RE = Y, Lu, Sc) Sesquioxide Lasers Based on Thermal Guiding

Cubic sesquioxides, RE2O3, where RE = Y, Lu or Sc, are attractive host crystals for thulium (Tm3+) doping. A comparison of the spectroscopic properties of Tm3+:RE2 O3 crystals in terms of transition cross sections and cross-relaxation (CR) efficiency required for efficient upconversion pumping is presented. Thermo-optic properties of Tm3+:RE2O3 crystals (thermal lensing, fractional heat loading, and thermo-optic coefficients) are described. The positive thermal lens, broadband emission, and efficient CR of the Tm:RE2O3 crystals enable the development of compact, highly efficient and power-scalable lasers operating above 2 μm, based on thermal guiding. Nowadays, Tm:Lu2O3 microchip lasers are capable of generating nearly 5 W of output power at ∼2.06 μm with a slope efficiency $ \eta $ of 67% and in a rod geometry—up to 47.5 W with $ \eta $ of 59%. For multiwatt output at even longer wavelengths around 2.15 μm, Tm:Sc2O3 is an interesting candidate.

Irreversible thermodynamics of uniform ferromagnets with spin accumulation: Bulk and interface dynamics

Using ideas from Landau's Fermi-liquid theory, we apply irreversible thermodynamics to conducting and insulating ferromagnets with magnetic variables $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{M}$ for the quantization axis and for the spin accumulation $\stackrel{P\vec}{m}$ of the nonequilibrium excitations; thus the total magnetization is taken to be $\stackrel{P\vec}{\mathcal{M}}=\stackrel{P\vec}{M}+\stackrel{P\vec}{m}$. The resulting theory closely corresponds to the theory of Silsbee et al. [Silsbee, Janossy, and Monod, Phys. Rev. B 19, 4382 (1979)]. For the bulk, in addition to confirming the usual Landau-Lifshitz equation for $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{M}$ and a Bloch-like equation for $\stackrel{P\vec}{m}$ (with a nonuniform precession term), there are two related cross-relaxation terms between the transverse parts of the nonequilibrium $\stackrel{P\vec}{m}$ and $\stackrel{P\vec}{M}$. Unlike the s-d model, where in a field $\stackrel{P\vec}{H}$ the equilibrium magnetizations ${\stackrel{P\vec}{M}}_{s}$ and ${\stackrel{P\vec}{M}}_{d}$ are both nonzero, for this m-M model in a field $\stackrel{P\vec}{H}$, only the equilibrium magnetization $\stackrel{P\vec}{M}$ is nonzero. For the interface, the boundary condition for $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{M}$ is given by micromagnetics, and that for $\stackrel{P\vec}{m}$ is given by irreversible thermodynamics, where the current of transverse spins crossing the interface is proportional to the discontinuity in the transverse part of the vector spin chemical potential. $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{M}, \stackrel{P\vec}{m}$, and $\stackrel{P\vec}{H}$ are coupled; in the decoupled approximation, we find the wave vectors for the modes of $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{M}$ and the transverse $\stackrel{P\vec}{m}$. We discuss reciprocity between spin pumping ($\stackrel{P\vec}{\mathcal{M}}$ driven out of the ferromagnet) and spin transfer torque ($\stackrel{P\vec}{\mathcal{M}}$ driven into the ferromagnet).

The Conducting Spin-Crossover Compound Combining Fe(II) Cation Complex with TCNQ in a Fractional Reduction State.

The radical anion salt [Fe{HC(pz)3}2](TCNQ)3 demonstrates conductivity and spin-crossover (SCO) transition associated with Fe(II) complex cation subsystem. It was synthesized and structurally characterized at temperatures 100, 300, 400, and 450 K. The compound demonstrates unusual for 7,7,8,8,-tetracyanoquinodimethane (TCNQ)-based salts quasi-two-dimensional conductivity. Pronounced changes of the in-plane direct-current resistivity and intensity of the electron paramagnetic resonance (EPR) signal, originated from TCNQ subsystem, precede the SCO transition at the midpoint T* = 445 K. The boltzmannian growth of the total magnetic response and structural changes in the vicinity of T* uniquely show that half [Fe{HC(pz)3}2] cations exist in high-spin state. Robust broadening of the EPR signal triggered by the SCO transition is interpreted in terms of cross relaxation between the TCNQ and Fe(II) spin subsystems.

Further Efforts Toward a Molecular Dynamics Force Field for Simulations of Peptides in 40% Trifluoroethanol–Water

A set of force field parameters for trifluoroethanol (TFE) was shown in earlier work from this lab to give a good accounting of system density, translational diffusion coefficients, and the solvent fluorine-solute hydrogen NMR cross-relaxation parameter (∑HF) for acetate dissolved in 40% TFE-water. It has since been found that this parameter set performs poorly when used to predict ∑HF for interactions of TFE with the hydrogens of an octapeptide, [val(5)]angiotensin. In the present work, adjustment of nonbonded force field parameters for interactions of fluorine with hydrogen was explored with the goal of improving these predictions. Six sets of TFE parameters were examined. When used in conjunction with the TIP5PE water model, all gave values for system density, translational diffusion coefficients, and ∑HF for acetate-TFE interactions that were similar to experimental results. Increasing the Lennard-Jones σHF for fluorine-hydrogen interactions by 10% led to calculated solvent-solute cross-relaxation parameters that are in better agreement with experiment for many of the peptide hydrogens. Changing the Lennard-Jones εHF parameters for the same interactions had little effect on calculated ∑HF values. There was no discernible influence of the TFE model used on the radius of gyration of the peptide or on the backbone conformational angles of the peptide, implying that the conformational properties of the peptide are not strongly influenced by changes in the force field description of TFE in 40% TFE-water. A recent parameter set for TFE proposed by Vymetal and Vondrasek (2014), which reproduces well various physical properties of neat TFE and TFE-water mixtures, was shown to predict cross-relaxation terms for this system poorly.

Investigation of Ethanol–Peptide and Water–Peptide Interactions through Intermolecular Nuclear Overhauser Effects and Molecular Dynamics Simulations

Molecular dynamics simulations have been used to explore solvent-solute intermolecular nuclear Overhauser effects (NOEs) on NMR (nuclear magnetic resonance) signals of [val5]angiotensin dissolved in 35% ethanol-water (v/v). Consideration of chemical shift, coupling constant and intramolecular NOE data suggest that conformations of the peptide are adequately sampled by simulations of up to 0.6 μs duration. Calculated cross relaxation terms at 0 and 25 °C are compared to experimental values and to terms predicted using a particulate model of the solvent. Many calculated solvent NOEs are in agreement with experimental results; disagreements are particularly striking for hydrogens of the Phe8 residue of the peptide. Calculations show that individual molecules of either solvent component can spend many ns in association with the peptide but dipolar interactions within such a complex account for only a few percent of an observed cross relaxation rate. Most parts of the peptide interact selectively with ethanol. Diffusion of both solvent components is slowed when they are close to the peptide. Solvent-solute cross relaxation terms for acetic acid in the same solvent obtained from simulations agree with experiment. Preferential interactions of solvent molecules with acetic acid are largely absent, as are effects of this solute on solvent diffusion rates.

Luminescence properties of Y2O3:Bi3+, Ln3+ (Ln=Sm, Eu, Dy, Er, Ho) and the sensitization of Ln3+ by Bi3+

The spectroscopic characterization of yttria, singly and doubly doped with Ln3+ (Ln=Sm, Eu, Dy, Er, Ho) and Bi3+ ions, is performed through excitation spectra, emission spectra and decay time measurements. The obtained spectroscopic data clearly indicate that energy transfer takes place from Bi3+ to Ln3+ ions. The energy transfer efficiency of Bi3+→Ln3+ and quantum efficiency of Ln3+ were calculated. Upon excitation of 370nm (Bi3+ excitation band), the quantum efficiency of Ln3+ varies from ∼4% to ∼44%. The energy transfer efficiency increases continuously with increasing Ln3+ concentrations, whereas the variation of the quantum efficiency of Ln3+ is complicated. The quantum efficiency of Ln3+ is discussed in terms of electron transfer and cross relaxation.

Investigation of Methanol–Peptide Nuclear Overhauser Effects through Molecular Dynamics Simulations

Intermolecular nuclear Overhauser effects (NOEs) produced by interactions of methanol with [val(5)]angiotensin in 25% methanol-water at 0 °C were examined through molecular dynamics (MD) simulations and compared to experimental results. Calculated average (3)J(NHCαH) spin coupling constants, conformation-sensitive chemical shift changes, and intramolecular (1)H-(1)H NOEs indicated that peptide conformations present over the course of simulation trajectories of 100-300 ns are likely similar to those present in the experimental system. Calculated cross-relaxation terms for the methanol-peptide interactions showed the same trends as corresponding experimental data but were about a factor of 3 too large. The lack of agreement between observed and calculated cross-relaxation terms probably has origins in characteristics of the simulations that lead to overestimation of translational diffusion coefficients of the system components. Simulations confirmed the heterogeneity of the methanol-water solvent at the molecular level, with clusters of methanol and water molecules changing their size and composition on a subpicosecond time scale. Most peptide hydrogens are preferentially solvated by interactions with methanol molecules. Simulations suggest that diffusion of water and methanol molecules near the peptide is slowed as these species approach the peptide backbone.

Role of modifier oxide in emission spectra and kinetics of Er–Ho codoped Na 2SO 4–MO–P 2O 5 glasses

The glasses of the composition 19Na 2SO 4–20MO–60P 2O 5: 1.0Ho 2O 3/1.0Er 2O 3 (M = Mg, Ca, and Ba) have been synthesized. Optical absorption and fluorescence spectra (in the spectral range 350–2100 nm were studied at ambient temperature. The spectra were characterized using Judd–Ofelt theory. From the luminescence spectra, various radiative properties like transition probability A, branching ratio β and the radiative life time τ for blue (B), green (G) and red (R) emission levels of these glasses have been evaluated. The energy transfer between the two rare earth ions (Ho 3+ and Er 3+) in co-doped Na 2SO 4–MO–P 2O 5 glass systems in the visible and NIR regions has also been investigated. Highest intensity, the highest quantum efficiency and maximum energy transfer with low phonon losses of B, G, and R lines has been observed in BaO mixed glasses. The reasons for such higher values of these parameters have been discussed in the light of varying field strengths at the rare earths ion site due to replacement of one modifier oxide with the other. The enhanced intensity of NIR emission (at 2.0 μm) has also been discussed in terms of cross relaxation of Er 3+ ions from 4I 13/2 level to 5I 7 of Ho 3+ ions.

Interactions of Nonprotic Organic Solvents with [Val5]angiotensin in Water

Intermolecular solvent-solute nuclear Overhauser effects have been used to explore interactions of the organic component of acetonitrile-water, acetone-water, and dimethyl sulfoxide-water mixtures with the peptide hormone [val(5)]angiotensin. As reported by the NOEs, many cross relaxation terms for interactions of these organic cosolvents are adequately accounted for using a hard spheres interaction model in which encounters of peptide and cosolvent molecules take place by mutual diffusion. However, there are indications of localized solvent-peptide interactions that are not well described by this model. In dimethyl sulfoxide-water at 0 °C, organic solvent near the C-terminal Phe8 residue and the Val3 residue produce strongly enhanced cross-relaxation terms. NOEs for all peptide N-H protons and the protons of the Tyr4 aromatic ring were significantly more positive than expected in 33% acetone-water (v/v) at 0 °C, while those for most side-chain protons were close to predictions of the hard sphere model. All peptide-organic solvent NOEs in 35% acetonitrile water (v/v) at 0 °C are consistent with the hard spheres interaction model.

Interactions of 2,2,2‐trifluoroethanol with melittin

Melittin dissolved in 42% trifluoroethanol-water at pH 2 has been shown to be alpha-helical between residues 6 and 12 and between residues 13 and 25, with the two helical regions separated by a bend at the Leu13 residue. The inter-helix angle was found to be 154 +/- 3 degrees at 0 degrees C and 135 +/- 3 degrees at 25 degrees C. The dominant conformation of the peptide is thus similar to those observed by previous workers for the peptide in a variety of media. At 25 degrees C, intermolecular nuclear Overhauser effects arising from nuclear spin dipole-dipole interactions between melittin hydrogens and fluorines of the solvent are essentially those expected for a system that is homogeneous as regards concentration and translational diffusion of the peptide and fluoroalcohol components. However, at 0 degrees C, peptide-trifluoroethanol cross-relaxation terms are negative, a result consistent with the conclusion that fluoroalcohol molecules associate with the peptide for times (approximately 1 ns) that are long compared to the time of a typical peptide-fluoroalcohol diffusive encounter (approximately 0.2 ns). Such interactions may be responsible for the reduction of the translational diffusion coefficient of trifluoroethanol produced by dissolved peptides.

Spectroscopic Properties of Pr3+ Ions in a PbWO4 Single Crystal

Site-selective fluorescence laser spectroscopy of Pr (3+) ions in lead tungstate single crystal were investigated at temperatures from 10 to 300 K. The site-selective emission spectra and fluorescence decays from the (3)P J ( J = 0, 1, 2) and (1)D 2 states were analyzed. The (3)P J ( J = 0, 1, 2) level shows its predominantly radioactive character with the typical greenish-blue luminescence ascribed to (3)P J transition. The emission from the (1)D 2 level is only observed when this level is directly excited. The decay kinetic of the (1)D 2 level was measured under site-selective excitation and discussed in terms of cross-relaxation. The up-conversion emission from levels (3)P 1 and (3)P 0 following excitation of the (1)D 2 state was observed in the PbWO 4 crystal between 10 and 300 K. The main up-conversion mechanism, together with the understanding the quenching of the (1)D 2 fluorescence in this Pr (3+) heavily doped PbWO 4 were discussed. The presence of the complex structures of the emission spectra and different decay profiles indicate that several processes contribute to the quenching of the (1)D 2 fluorescence of Pr (3+) ions. It was found that the up-conversion fluorescence intensity had a quadratic dependence on the laser input power. The temporal behavior of the up converted emission indicates that an energy-transfer up-conversion is the dominant process.

Solvent Interactions with [Val5]angiotensin II in Ethanol−Water

Intermolecular NOE experiments have been used to explore the interactions of water and ethanol molecules in 35% ethanol/65% water (v/v) with the octapeptide hormone [val (5)]angiotensin II at temperatures from 0 to 25 degrees C. Magnetic dipole-dipole cross relaxation terms sigma(HH)(NOE) and sigma(HH)(ROE) for interaction of both solvent components suggest that ethanol molecules interact with the peptide backbone atoms strongly enough to associate for times comparable to the rotational correlation time of the peptide; comparison of observed ROE and NOE cross relaxation terms indicate that lifetimes of these interactions are of the order 0.4 ns at 5 degrees C. Formation of such peptide-ethanol complexes can also account for larger-than-expected values of the cross relaxation terms at higher temperatures. Alternative explanations of the observations reported are shown to be unlikely, primarily because they require unreasonable and highly localized concentrations of the ethanol near the peptide. Side chains of the peptide appear to experience no unusual interactions with ethanol. Cross relaxation terms for water-peptide backbone interactions indicate long-lived interactions of water with the backbone atoms although the nonpolar side chains of the peptide (Val3, Val5, Pro7, and possibly Phe8) do not interact in any specific way with water molecules. Cross relaxation terms for protons of the polar (Tyr4 and His6) side chains may reflect strong interactions with water, but analysis of these is confounded by solvent proton exchange and possible spin diffusion effects.

Solvent interactions with the Trp-cage peptide in 35% ethanol-water

Intermolecular NOE experiments have been used to explore interactions of water and ethanol molecules in 35% ethanol/65% water (v/v) with the peptide Trp-cage at temperatures from 5 to 25 degrees C. Magnetic dipole-dipole cross-relaxation terms sigma(HH) (NOE) and sigma(HH) (ROE) for interaction of solvent components with spins of the peptide suggest that ethanol molecules associate with backbone atoms for times of the order of nanoseconds at 5 degrees C. Formation of peptide-ethanol complexes can also account for the larger-than-expected values of cross-relaxation terms at higher temperatures. Hydrocarbon side chains of the peptide do not appear to experience such interactions with ethanol. Cross relaxation resulting from water-peptide interactions are consistent with long-lived water interactions with the backbone atoms. Water cross relaxation with nonpolar side chains of the peptide (Leu2, Ile4, Leu7, and proline residues) are only those expected for bulk solvent. However, long-lived association of both water and ethanol with the polar side chains of Tyr3 and Trp6 is indicated by the data.

Efficient photoluminescence of Dy 3+ at low concentrations in nanocrystalline ZrO 2

Nanocrystalline ZrO 2:Dy 3+ were prepared by sol–gel and the structural and photoluminescence properties characterized. The crystallite size ranges from 20 to 50 nm and the crystalline phase is a mixture of tetragonal and monoclinic structure controlled by dopant concentration. Strong white light produced by the host emission band centered at ∼460 nm and two strong Dy 3+ emission bands, blue (488 nm) and yellow (580 nm), under direct excitation at 350 nm were observed. The highest efficiency was obtained for 0.5 mol% of Dy 3+. Emission is explained in terms of high asymmetry of the host suggesting that Dy 3+ are substituted mainly into Zr 4+ lattice sites at the crystallite surface. Luminescence quenching is explained in terms of cross-relaxation of intermediate Dy 3+ levels.

NMR structural model of the interaction of herbicides with the photosynthetic reaction center from Rhodobacter sphaeroides.

The interaction of the herbicides acifluorfen and paraquat with the photosynthetic reaction center from Rhodobacter sphaeroides has been studied by NMR relaxation measurements. Interaction in aqueous solution has been demonstrated by evaluating motional features of the bound form through cross-relaxation terms of protons at fixed distances on the herbicides. Contributions to longitudinal nonselective relaxation rates different from the proton-proton dipolar relaxation were inferred, most probably due to paramagnetic effects originating from the high-spin nonheme Fe(II) ion in the reaction center. Paramagnetic contributions to proton relaxation rates were converted into distance constraints in order to build a model for the interaction. The models place paraquat in the QB site, where most herbicides interact, in agreement with docking calculations, whereas acifluorfen was placed between the metal and the QB site, as also demonstrated by the induced paramagnetic shifts. Acifluorfen could therefore act to break the electron-transfer pathway between the QA and QB sites.

Energy transfer processes of Nd3+ in Y2O3 ceramic

The paper presents the results on the energy transfer processes of Nd3+ doped Y2O3 transparent polycrystalline ceramics. The 4F3/2 decays are highly non-exponential at concentrations larger than 0.1at%. For concentrations up to 3at% the emission kinetics can be described in terms of a static cross-relaxation (calculated by using the discrete structure and two types of interactions—short range and dipole–dipole) plus a migration term. Only C2 sites are considered in the quenching. The transfer parameters that describe coherently the decays up to 3at% are determined. The high concentration effects and the concentration dependence of the quantum efficiency are discussed.

Power dependence of luminescence of Tb 3+-doped KYb(WO4) 2 crystal

The effect of excitation intensity on luminescence properties of Tb 3+-doped KYb(WO 4) 2 crystal is reported. In particular, emissions following the Stokes and anti-Stokes excitations were investigated. It was found that, in addition to the Tb 3+ luminescence, an intense Yb 3+ luminescence at room temperature was observed after a direct excitation of Tb 3+ ions in spite of the fact that both ions are far off-resonance. The intensities of Tb 3+ and Yb 3+ luminescences were dependent on the excitation power and changed by a sublinear function (slope<1). The mechanism of the non-resonant energy transfer from Tb 3+ to Yb 3+ is discussed in terms of the phonon-assisted cross-relaxation and the down-conversion contributing to the process. The up-conversion emission of Tb 3+ (after a direct excitation of Yb 3+ ions) was observed at room temperature and its intensity varied with the slope <2. It was found that the emission decay times were strongly dependent on the excitation power. The mechanism of this behaviour is discussed in terms of many-body energy transfer.

Investigation of the spectroscopic properties of Nd3+-doped phosphate glasses

Optical properties of Nd3+-doped phosphate glasses have been studied on the basis of the Judd-Ofelt theory. With the intermediate cross-section value and the weak 4/6 parameter, we expect a relatively prominent 4F3/24I11/2 laser emission. The quenching effect of the emission intensity is discussed in terms of cross-relaxation between the Nd3+ ions as well as energy transfer processes within the host matrix. From the investigation of the decay rate from the 4F3/2 state with the neodymium concentration, we suggest that self-quenching is insured by dipole-dipole interaction. This result was also confirmed by the simulation of the decay with the Inokuti-Hyrayama model.