W-Band pulse EPR distance measurements in peptides using Gd(3+)-dipicolinic acid derivatives as spin labels

We present high field DEER (double electron-electron resonance) distance measurements using Gd(3+) (S = 7/2) spin labels for probing peptides' conformations in solution. The motivation for using Gd(3+) spin labels as an alternative for the standard nitroxide spin labels is the sensitivity improvement they offer because of their very intense EPR signal at high magnetic fields. Gd(3+) was coordinated by dipicolinic acid derivative (4MMDPA) tags that were covalently attached to two cysteine thiol groups. Cysteines were introduced in positions 15 and 27 of the peptide melittin and then two types of spin labeled melittins were prepared, one labeled with two nitroxide spin labels and the other with two 4MMDPA-Gd(3+) labels. Both types were subjected to W-band (95 GHz, 3.5 T) DEER measurements. For the Gd(3+) labeled peptide we explored the effect of the solution molar ratio of Gd(3+) and the labeled peptide, the temperature, and the maximum dipolar evolution time T on the DEER modulation depth. We found that the optimization of the [Gd(3+)]/[Tag] ratio is crucial because excess Gd(3+) masked the DEER effect and too little Gd(3+) resulted in the formation of Gd(3+)-tag(2) complexes, generating peptide dimers. In addition, we observed that the DEER modulation depth is sensitive to spectral diffusion processes even at Gd(3+) concentrations as low as 0.2 mM and therefore experimental conditions should be chosen to minimize it as it decreases the DEER effect. Finally, the distance between the two Gd(3+) ions, 3.4 nm, was found to be longer by 1.2 nm than the distance between the two nitroxides. The origin and implications of this difference are discussed.     

Simulating electron spin resonance spectra of macromolecules labeled with two dipolar-coupled nitroxide spin labels from trajectories

An efficient method for simulating continuous-wave electron spin resonance spectra (ESR) of molecules labeled with two dipolar-coupled nitroxides from trajectories of the molecular motion is presented. Two approximate treatments of the dipolar spin evolution, resulting in significantly shorter simulation times, are examined in order to determine their range of applicability. The approach is illustrated in the context of a double-helical B-DNA. ESR spectra for DNA undergoing anisotropic global diffusion and internal stretching dynamics are calculated for three different labeling geometries with the spin labels bracketing, respectively, three, two and one base pairs. While multifrequency spectra of all three labeling schemes are very sensitive to DNA tumbling, the last one is found to be most informative about the local DNA dynamics.     

Simulating electron spin resonance spectra of nitroxide spin labels from molecular dynamics and stochastic trajectories

Simulating electron spin resonance spectra of nitroxide spin labels from motional models is necessary for the quantitative analysis of experimental spectra. We present a framework for modeling the spin label dynamics by using trajectories such as those from molecular dynamics (MD) simulations combined with stochastic treatment of the global protein tumbling. This is achieved in the time domain after two efficient numerical integrators are developed: One for the quantal dynamics of the spins and the other for the classical rotational diffusion. For the quantal dynamics, we propagate the relevant part of the spin density matrix in Hilbert space. For the diffusional tumbling, we work with quaternions, which enables the treatment of anisotropic diffusion in a potential expanded as a sum of spherical harmonics. Time-averaging arguments are invoked to bridge the gap between the smaller time step of the MD trajectories and the larger time steps appropriate for the rotational diffusion and/or quantal spin dynamics.     

Distance measurements in Au nanoparticles functionalized with nitroxide radicals and Gd(3+)-DTPA chelate complexes

Nanosized gold particles were functionalised with two types of paramagnetic surface tags, one having a nitroxide radical and the other one carrying a DTPA complex loaded with Gd(3+). Selective measurements of nitroxide-nitroxide, Gd(3+)-nitroxide and Gd(3+)-Gd(3+) distances were performed on this system and information on the distance distribution in the three types of spin pairs was obtained. A numerical analysis of the dipolar frequency distributions is presented for Gd(3+) centres with moderate magnitudes of zero-field splitting, in the range of detection frequencies and resonance fields where the high-field approximation is only roughly valid. The dipolar frequency analysis confirms the applicability of DEER for distance measurements in such complexes and gives an estimate for the magnitudes of possible systematic errors due to the non-ideality of the measurement of the dipole-dipole interaction.     

Explicit treatment of spin labels in modeling of distance constraints from dipolar EPR and DEER

Current SDSL-EPR methods allow measurement of dipolar distances in the 8-70 A range; however, the use of extrinsic probes complicates the interpretation of these distances in modeling macromolecular structure and conformational changes. The data presented here show that interprobe distances correlate only weakly with Cbeta-Cbeta distances, especially for distances that are on the order of the spin label tether lengths. Explicitly incorporating the spin label into the modeling process increases the experiment/model correlation 4-fold and reduces the distance error from 6 A to 3 A.     

Growth and Spectroscopic Properties of Nd3＋：Sr3Gd2（BO3）4 Crystal

<正>This paper reports the growth,X-ray diffraction and spectroscopy of Nd~(3+):Sr_3Gd_2(BO_3)_4 crystal.A Nd~(3+):Sr_3Gd_2(BO_3)_4 crystal with dimensions ofφ20×45 mm~3 has been grown by the Czochralski method.Nd~(3+):Sr_3Gd_2(BO_3)_4 crystal belongs to the orthorhombic system,space group Pnma(D_(2h))with a=0.7401,b=1.604 and c=0.8755 nm.The absorption and emission spectra of Nd~(3+):Sr_3Gd_2(BO_3)_4 were investigated.The absorption cross sectionσ_a is 3.11×10~(-20)cm~2 at 808 nm. The absorption transition at 808 nm has an FWHM of 14 nm.The luminescence lifetimeτ_f is 51.7 us.The emission cross sectionσ_e at 1064 nm wavelength is 1.09×10~(-19)cm~2.     

Energy transfer between Gd3+ and Sm3+ the effect of Gd3+ on quenching of Sm3+ and intensity parameters of Sm3+ in borate glasses

Intensity parameters of Sm³⁺ in borate glasses were obtained by fitting the oscillator strengths to the Judd-Ofelt formula and a study of energy transfer from gadolinium to samarium was performed. An increase of samarium fluorescence originating from the ${}^4\text{G}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ level was observed in the presence of gadolinium, in the concentration range of 0.1–3 wt% samarium with gadolinium constant at 3 wt%. The intensity of samarium fluorescence on excitation at 273 nm increased by one order of magnitude in the presence of gadolinium. From the excitation spectrum of the double-doped glasses (Gd + Sm), it was deduced that energy absorbed by gadolinium is transferred from ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ gadolinium levels to the ${}^4\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and ${}^4\text{P}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ samarium levels.The mechanism of this energy transfer was obtained by plotting the energy transfer probabilities as a function of samarium concentration. A linear dependence of $\text{η}{}_0\text{η}$ (η intensity of gadolinium in the presence of samarium) versus square of concentration of Sm + Gd is obtained. From this it is concluded that the transfer is of electric-multipolar type, mainly dipole-dipole. A small increase (about 10%) of fluorescence of samarium in the presence of gadolinium excited at levels where no energy transfer can take place is attributed to the fact that the quenching of samarium occurring by the cross relaxation $\text{(}{}^4\text{G}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{→}{}^6\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{) (}{}^6\text{H}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{→}{}^6\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{)}$ is suppressed by the presence of gadolinium as seen from concentration dependence of samarium doped glasses compared to double-doped glasses.     

Interaction of Cs+, Sr2+ and Gd3+ with humin

Humin is separated from a soil sample and characterized by elemental analysis, IR and potentiometric titration. These investigations indicated that humin displayed similar characteristics as humic acid. The sorption behavior of¹³⁷Cs,⁹⁰Sr and¹⁵³Gd with humin was investigated from aqueous solution. The results indicated that the sorption process follows the order: Gd³⁺≫Sr²⁺>Cs⁺. It was time dependent and increased with pH. The sorption in the presence of humic acid or EDTA showed that both form a metal-ligand complex and consequently decrease the % uptake.     

High-field EPR and ESEEM investigation of the nitrogen quadrupole interaction of nitroxide spin labels in disordered solids: toward differentiation between polarity and proticity matrix effects on protein function

The combination of high-field electron paramagnetic resonance (EPR) with site-directed spin labeling (SDSL) techniques employing nitroxide radicals has turned out to be particularly powerful in revealing subtle changes of the polarity and proticity profiles in proteins enbedded in membranes. This information can be obtained by orientation-selective high-field EPR resolving principal components of the nitroxide Zeeman (g) and hyperfine ( A) tensors of the spin labels attached to specific molecular sites. In contrast to the g- and A-tensors, the (14)N ( I = 1) quadrupole interaction tensor of the nitroxide spin label has not been exploited in EPR for probing effects of the microenvironment of functional protein sites. In this work it is shown that the W-band (95 GHz) high-field electron spin echo envelope modulation (ESEEM) method is well suited for determining with high accuracy the (14)N quadrupole tensor principal components of a nitroxide spin label in disordered frozen solution. By W-band ESEEM the quadrupole components of a five-ring pyrroline-type nitroxide radical in glassy ortho-terphenyl and glycerol solutions have been determined. This radical is the headgroup of the MTS spin label widely used in SDSL protein studies. By DFT calulations and W-band ESEEM experiments it is demonstrated that the Q(yy) value is especially sensitive to the proticity and polarity of the nitroxide environment in H-bonding and nonbonding situations. The quadrupole tensor is shown to be rather insensitive to structural variations of the nitroxide label itself. When using Q(yy) as a testing probe of the environment, its ruggedness toward temperature changes represents an important advantage over the g xx and A(zz) parameters which are usually employed for probing matrix effects on the spin labeled molecular site. Thus, beyond measurenments of g xx and A(zz) of spin labeled protein sites in disordered solids, W-band high-field ESEEM studies of (14)N quadrupole interactions open a new avenue to reliably probe subtle environmental effects on the electronic structure. This is a significant step forward on the way to differentiate between effects from matrix polarity and hydrogen-bond formation.     

Synthesis and structure of a nucleoside with pi-conjugated nitroxide spin label forming a one-dimensional ferromagnetic chain

Spin labeled 2'-deoxyuridine, in which a significant fraction of the spin density is delocalized from a nitroxide radical to the DNA base residue, was prepared as a crystalline solid, stable at ambient conditions. The crystal packing, which includes multiple hydrogen bonds, leads to one-dimensional chains of molecules with predominant intrachain ferromagnetic coupling and weaker interchain antiferromagnetic coupling.     

Identification of protein surfaces by NMR measurements with a pramagnetic Gd(III) chelate.

Gd-diethylenetriamine pentaacetic acid-bismethylamide, Gd(DTPA-BMA), is shown to be a reagent suitable for the identification of protein surfaces. Compared to the conventionally used spin-label TEMPOL, Gd(DTPA-BMA) is a stronger relaxation agent, requiring lesser concentrations to achieve the same paramagnetic relaxation enhancement of solvent-exposed protein protons. It is also less hydrophobic and therefore less prone to specific binding to proteins. Relaxation enhancements predicted by a second-sphere interaction model correlated with experimental data recorded with ubiquitin, while the correlation with corresponding data recorded with TEMPOL was poor.     

Nitrate ion association with La3+, Gd3+, and Dy3+

Using the solvent extraction method it was found that the stability constantsK ₁ ⁰ of the La, Gd, DyNO₃ ²⁺ complexes are 41, 39, 21, respectively. The change of stability constants ofLnNO₃ ²⁺ complexes in the lanthanide series was related to the change of the amount of inner sphere complexation.

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Zur Assoziation von Nitrationen mit La³⁺, Gd³⁺ und Dy³⁺

Zusammenfassung Mittels der Lösungsmittelextraktionsmethode wurden die Stabilitäts-konstantenK ₁ ⁰ von La, Gd und DyNO ₃ ²⁺ zu 41, 39 und 21 bestimmt. Der Unterschied der Stabilitätskonstanten derLnNO ₃ ²⁺ -Komplexe innerhalb der Lanthanidenserie wurde mit dem Wechsel der Inneren-Sphären-Komplexierung verknüpft.

     

Characterization of the motion of spin probes and spin labels in amorphous polymers with two-dimensional field-step ELDOR

The motion of nitroxide spin probes and spin labels in amorphous polymers is studied below the glass transition temperature with a two-dimensional pulsed electron double-resonance experiment. Polystyrene and a liquid crystalline side group polymer are studied using both spin probes and spin labels covalently bound to specific sites along the polymer chain. Two methyl acrylic polymers differing only in their side group structure and polyvinylacetate are compared and large differences in the molecular dynamics deduced from both the nuclear and the electron spin relaxation rates are observed as the glass transition is approached. The results demonstrate the complexity of small amplitude motion in simple polymers below the glass transition temperature and show that it is very sensitive to the packing in the polymer. © 1996 John Wiley & Sons, Inc.     

多孔二氧化硅中Gd3+ → Eu3+的能量传递

通过水热反应法，获得了单掺和双掺Eu³⁺，Gd³⁺的多孔二氧化硅组装体，研究了掺杂体系的光谱特性，观察到Gd³⁺ → Eu³⁺的能量传递。分析了能量传递过程，探讨了在多孔二氧化硅中Gd³⁺→Eu³⁺的能量传递的机理，其机理主要为电偶极-电偶极相互作用。     

Synthesis and Luminescent Properties of Eu 3+ , Gd 3+ , and Tb 3+ Complexes with Quinoline-4-carboxylic Acids

New complex compounds LnL3·nH2O (n = 5–10) have been synthesized on the basis of Eu3+, Gd3+, and Tb3+ salts and quinoline-4-carboxylic acid derivatives obtained via the Pfitzinger reaction. Composition and structure of the ligands and the resulting complex compounds have been confirmed by NMR and IR spectroscopy, thermogravimetry, and complexometric titration. Europium complex with 1,2,3,4-tetrahydroacridine-9-carboxylic acid has exhibited efficient luminescence.     

Luminescence Centers in BaFCl:Ce3+ and BaFCl:Gd3+ Crystals

Introduction In BaFCl crystal which is of PbFCl type structure in tetragonal system, Ba~(2+) ions situate at sites of C_(4?) point symmetry, while F~- and Cl~- ions occupy sites of D_(2d) and C_(4?) symmetry, respectively. The rare earth ions RE~(3+) doped in BaFCl should randomly replace the sites of Ba~(2+) ions, and their extra positive charges may be compensated by