Cross-relaxation Terms Research Articles

Conformational analyses of quinolines substituted with a vinyloxy or vinylthio group at the ortho position by means of the NMR selective relaxation method. Experimental verification of the planar conformation in solution

Conformations of 2-vinyloxy-4-methylquinoline (1) and 2-vinylthioquinoline (2) have been investigated by means of the NMR selective relaxation method where selective non-inversion T1, T1SNI, has been used to obtain the cross relaxation terms, σij. The interproton distances, rij, have been estimated using these σij values. For the rigid part of the molecule, the values of rij obtained experimentally give good agreement with the values calculated by the ab initio method. The conformations defined by the rotations about C(2)–X(O or S) and X(O or S)–C(1′) have been analyzed using the equilibrium model, in which the values of σij obtained experimentally have been compared with the total average of σij, σijtotal, and the fractional populations of the pertinent conformers have been determined. The predominance in solution of conformer 1 (see Scheme 2), in which 1′-H is close to the aromatic nitrogen, supporting the planar conformation, has been confirmed for both compounds. Although, for 2, a small proportion of conformer 3′, with a non-planar conformation, is observed as the second-minimum energy one, the conformational equilibria of 1 lie on the side of the conformer 1.

Anti-Stokes emission in LaCl 3 doped with U 3+ and Pr 3+ ions

An efficient blue emission of Pr 3+ ions was observed in a LaCl 3:U 3+, Pr 3+ crystal after direct excitation of the U 3+ ions by a red laser line. Evidence of up-conversion in U 3+ ions is shown. The phenomena observed are discussed in terms of cross-relaxation, successive absorption and up-conversion processes.

Comparison of ROESY and EXSY Methods Using Bloch Equations

A comparison of longitudinal and transverse frame magnetization transfer methods is provided using modified Bloch equations for a two-spin system representative of a methylene group exchanging between two chemically distinct sites. This simple formalism incorporates chemical shift, auto- and cross-relaxation terms, chemical exchange first-order rate constants and r.f. field terms for ROESY experiments. Overhauser and exchange magnetization transfer is emulated at three limits of molecular reorientation time (τc). The calculated build up curves suggest a parasitic effect of ROE on chemical exchange magnetization transfer. In addition, for molecules harboring spin systems exchanging between sites such that the exchange rate approximates the transverse cross-relaxation rate, and reorienting in solution at rates slower than that defined by the CAMELSPIN limit (ω0τc≈1.118), the calculations suggest observation of exchange or Overhauser cross peaks may be compromised. Finally, the effect of off-resonance spin locking r.f. power level is explored upon exchange or Overhauser build-up curve profiles. The results suggest off-resonance approaches that at low r.f. spin locking power may adversely affect ROE or exchange build-up rates and may compromise either exchange rate determination or internuclear distance estimation.

Monte Carlo estimation of errors in 13C-NMR relaxation studies of a DNA oligomer duplex.

An analysis of errors has been done with the Monte Carlo method for natural abundance 13C-NMR relaxation studies of a DNA duplex. Repeated measurements of the longitudinal relaxation time, T1, and the heteronuclear NOE were made at 90.6 MHz on the duplexed DNA pentanucleotide, [d(TCGCG)]2. The deviations averaged over all carbons were 13% for T1 and 9% for NOE. These relative deviations were applied to generate 100 values of T1 and NOE with normal distributions about the measured mean values for each carbon. A new version of MOLDYN, called McMOLDYN, has been written, which was used to generate 100 values of T1 and NOE with normal distributions corresponding to the measured errors; the same error distributions were also applied to measurements at 125.8 MHz. The order parameter, S2, and the effective internal correlation time, tau e, in the Model-Free Approach have been optimized from the distributions simulated by McMOLDYN. McMOLDYN also permits the automated entry of multiple sets of initial guesses for the output parameters S2, tau e, and tau m. In addition, McMOLDYN adds cross-relaxation terms from chemical shift anisotropy, increasingly important as spectrometer magnetic fields get higher. Between the two parameters optimized, S2 has the smallest relative error, estimated at 15% on average, which means that S2 is a well-defined parameter. However, tau e is very poorly defined with the average relative error estimated 85%; it is typically found in the range of 30-300 ps.

Equation of motion for two coupled spin-1 nuclei under a coherent and random Hamiltonian.

The time dependence of the density operator for two coupled spin-1 nuclei, such as dideuterium D[sub 2] and dinitrogen [sup 14]N[sub 2], under the second rank intramolecular dipolar and the first rank spin-rotation Hamiltonians has been derived. It is shown that the relaxation rates for the [ital ortho] and [ital para] species are not identical. We report the five independent rates for the [ital para] and the 21 independent rates for the [ital ortho] species. Due to cross-relaxation between spin orders which precess at a common frequency, the relaxation for the [ital ortho] configuration is not always single exponential. There are 15 cross-relaxation terms between certain spin orders for the [ital ortho] species in the slow-motion regime. The complexity of our coupled spin system is discussed for the solid and the Jeener-Broekaert echo sequences. The Zeeman spin-lattice relaxation rates [ital R][sub 10] for [ital ortho]- and [ital para]-dideuterium have been measured experimentally in patially oriented nematic solvents at 304 K using [sup 2]H-NMR. The solvents used are PCH-7, 1132 and a 55 wt.% 1132/EBBA mixture. These oriented solvents make it possible to separate the resonances of the [ital ortho] and [ital para] configurations thereby facilitating the [ital R][sub 10] determinationmore » directly without multiexponential analysis. The [ital R][sub 10] of [ital para]-dideuterium is about twice that of [ital ortho]-dideuterium in all experiments. Using our calculated expressions for [ital R][sub 10] based on quadrupolar, dipolar, and spin-rotation interactions, and performing the proper averaging over the rotational states, we obtain excellent agreement with our experimental results. Good agreement has been obtained with published low-temperature studies as well.« less

Conformational Features of Erythrocyte-Bound Chloroquine from Double-Selective 1H NMR Relaxation

The interaction between the antimalarial drug chloroquine and red blood cells was investigated by measuring 1H NMR spin-lattice relaxation rates upon double-selective excitation of dipolarly connected spin pairs within the ligand molecule. Geometric and dynamic features were inferred, for the free as well as for the bound ligand, by the cross-relaxation terms between protons at fixed and time-dependent distances, respectively. A change in orientation of the side chain in respect of the aromatic moiety was shown to result from the binding interaction.

Nuclear overhauser effects and the flexibility of saccharides: methyl β-xylobioside

The behavior of methyl β-xylobioside ( 1) has been analysed by 1H-n.m.r. spectroscopy for solutions in water and methanol in the range —15° to 85°. Experimental n.O.e. values did not change with temperature and solvent in contrast to the inter-glycosidic 3 J C,H values. Experimental n.O.e. data accorded with the values calculated from time-averaged cross-relaxation terms. The tumbling times were determined from relaxation data and were in the sub-ns range.

A relaxation trend in the electron spin resonance phenomena in copper tellurite glasses containing NiO, CoO and Lu2O3

Copper tellurite glasses containing NiO, CoO and Lu2O3 were prepared by the melt-quenching technique. The composition used was 65TeO2-(35-x)CuO-xTMO (mol%), where TMO indicates NiO, CoO, Lu2O3, and for NiO- and CoO-doped glasses,x has the values 0, 0.5, 1 to 4, and for Lu2O3 doped glasses x=0 to 4. Electron spin resonance (ESR) spectra of all glasses were recorded at room temperature. The results on glasses doped with NiO, CoO and Lu2O3 are discussed in terms of oxidation-reduction, cross-relaxation and interelectronic repulsion processes, respectively. Cobalt oxide is found to be more effective in relaxing the ESR spectrum than nickel and lutetium oxides when substituted in copper tellurite glasses.

1H and13C NMR relaxation investigation of the calcium complex of β-alanyl-L-histidine (carnosine) in aqueous solution

13 C and 1H NMR chemical shifts, J couplings, and spin–lattice relaxation rates were measured for carnosine in aqueous solution at pH 7.0. A Dreiding model of the ‘most-probable’ conformation was built up, showing predominance of the g– rotamer and folding of the β-alanyl moiety towards the imidazole ring. The structural and dynamic picture was shown not to be severely changed by stepwise titration with CaII ions. Exchange of carnosine among the bulk solution, a monomer complex (Ca2+ liganded by carboxy and carbonyl oxygens) and a dimer complex (Ca2+ liganded also by imidazole nitrogen) was suggested. The formation constant was evaluated at ca. 25 dm3mol–1. A Dreiding model of the dimeric complex was also built up on the basis of measured cross-relaxation terms of proton pairs, showing occurrence of intermolecular interactions between peptide molecules mediated by CaII ions.

Accurate determination of frequency dependent cross-relaxation terms in homonuclear and heteronuclear two spin-1/2 systems

One-dimensional N.M.R. methods are proposed for accurately determining homo- and heteronuclear longitudinal cross-relaxation terms. These sequences employ exclusively hard pulses and do not require critical adjustments of the instrument even in the case of selective perturbation (inversion) of one magnetization in a homonuclear two-spin system. Emphasis is put on the initial conditions which prevail at the beginning of the relaxation interval; they are determined by means of adequate experimental or numerical procedures. An algorithm, based on analytical solutions of Solomon equations, is described; it allows to analyse as many sets of data as desired for arbitrary initial conditions. These methods have been applied to the 1H-1H spin system of ortho protons in an aromatic ring, which bears in para positions the polar head and the aliphatic chain of benzene-sulphonate surfactants. Proton homonuclear cross-relaxation parameters are complemented by heteronuclear carbon-proton parameters and classical carbon-...

Relaxation of the 35Cl response in K 20SCl 6 to the Jeener-Broekaert pulse sequence

This paper presents the first attempt to apply the Jeener-Broekaert pulse sequence to an inhomogeneous pure quadrupolar resonance as a means to study motional processes. The decay of the 35Cl Jeener-Broekaert echo is measured in polycrystalline and single-crystal samples of K 2OsCl 6 at 295 and 77 K for various separations τ 1 of the preparation pulse pair as a function of the delay time τ 2 of the observe pulse following the pulse pair. The observed decays are represented by double exponential functions. For long τ 1 the response is that of a stimulated echo and is characteristic of a system in which no spin temperature can be defined. The present experiments have characterized the decay but provide no real understanding of the underlying mechanism. For short τ 1 the dominant response is that characteristic of dipolar order. The long time constant of the decay is as predicted for the motion narrowing regime. The short time constant is interpreted in terms of cross relaxation by spin-spin diffusion between single-spin and spin-spin energy reservoirs. Sample-dependent aspects are attributed to the different defect structures in the polycrystalline and single-crystal samples.

Energy transfer, up-conversion and nonequivalent crystal field effects in the KYP4O12:Pr3+ system

Under selective pulsed dye laser excitation in the 1D2 state fluorescence of Pr3+ ions in KYP4O12 has been studied. The presence of complex structures in the excitation and emission spectra associated with ion pairs and perturbed single ions is revealed. It is shown that in low concentration crystals most of the 1D2 fluorescence originates from the perturbed Pr3+ ions. The kinetics of the 1D2 decay is measured and discussed in terms of cross relaxation, energy transfer to traps and up-conversion processes. These results are in satisfactory agreement with the developed rate equation model. The contribution of short-range interaction, probably superexchange, in the quenching of 1D2 fluorescence in strongly concentrated crystals is examined. The first observations of 1D2 and up-converted 3P0 emissions in stoichiometric Pr3+ phosphate are reported.

Conformational dynamics of cytochalasin B in solution as detected by 13C and 1H NMR relaxation rates.

13C and 1H NMR spin-lattice relaxation rates have been measured for cytochalasin B in [2H]6DMSO. Motional features have been interpreted in terms of nearly isotropic reorientation of the whole molecule with few additional internal motions. The 'isotropic' reorientational correlation time was evaluated at 0.21 ns at room temperature. By using selective and double-selective excitation techniques, relevant cross-relaxation terms have been obtained wherefrom proton-proton distances have been calculated. A Dreiding model of the 'preferred' conformation in solution has been built, yielding evidence of a strong similarity between solution and solid state structures of cytochalasin B.

Relaxation time study of diammonium hydrazinium (+2) tetraperchlorate

The relaxation times T1, and T1ρ, and T1D of protons in (NH4)2N2H6(ClO4)4 were measured in the temperature range from 77 to 430 K. T1 measurements were made at 36 and 18 MHz, and T1ρ measurements were made as a function of H1. The temperature and frequency dependence of T1 is explained in terms of dipole–dipole interactions within anisotropically reorienting NH3 groups and spin-rotational interactions within reorienting NH+4 ions. The value of the T1ρ minimum due to reorienting NH3 groups is unusually large with weak H1 dependence, possibly due to tunneling motions. The T1ρ measurements exhibit a strong H1 dependence on the high-temperature side of the T1ρ minimum. An analysis of the H1 dispersion in terms of cross relaxation between protons and nitrogen nuclei within the N2H++6 ions has yielded values for the nitrogen T1.

Proton spin relaxation in solutions of the complex tris(acetylacetonato)chromium(III)

Field-dependent proton spin-lattice and spin-spin relaxation times have been measured for CH 3 in dilute chloroform solutions of tris(acetylacetonato)chromium(III) (Cr(acac) 3) complex over a temperature range of 253–323 K and a proton resonance frequency range of 10–90 MHz. The results have been interpreted in terms of scalar, dipolar, and scalar-dipolar cross relaxation. The effects of methyl group internal rotation are included. In particular, coupled modulation of ESR relaxation and electron-nuclear dipolar relaxation by reorientational motion of the complex is treated. The reorientational correlation times of the complex obtained are consistent with those measured from 13C T 1 in the analogous diamagnetic Co(III) complex, but significant deviations from the Stokes-Einstein relation with the consideration of microviscosity are observed. Solvation of the complex with chloroform is further identified from chemical-shift measurements and is utilized to interpret the results. The proton T, data are discussed in terms of their relevance to the zero-field-splitting interaction and to the contribution of scalar-dipolar cross relaxation.

Deformation of ESR line of O - radicals caused by cross relaxation with solvated electrons in alkaline ice

In degassed alkaline ices, the O - signal became less saturated at 4K. For aerated samples, a striking asymmetry was seen at 4K in the difference spectrum obtained by subtracting the O - line from the mixed spectrum of O - and e - sol. The asymmetry was ascribed to deformation of the O - line caused by cross relaxation. A revised saturation treatment with inclusion of cross-relaxation terms is presented and successfully accounts for the deformation of the O - line.

Nuclear magnetic relaxation studies of water in frozen biological tissues. Cross-relaxation effects between protein and bound water protons

Water proton longitudinal relaxation has been investigated in frozen mouse tissues including tumors. The nonfreezable water which gives rise to a relatively sharp NMR signal at this temperature (263 K) is identified as water bound to macromolecules. Measurements have been carried out by the nonselective inversion-recovery method at 90 and 6 MHz. Partially selective inversion has been achieved at 90 MHz by the DANTE sequence. The experimental data are analyzed by means of Solomon-type equations. This analysis provides the cross-relaxation term from which the dipolar contribution to water relaxation rate, arising from interactions with macromolecular protons, is calculated. This contribution seems to be dominant. The number of water protons interacting with a given macromolecular proton is found to be of the order of 10. The data at both frequencies can be consistently interpreted in terms of water diffusion, with a characteristic time of about 10 -9 sec. These conclusions are valid for all the tissues investigated here, their relaxation parameters exhibiting only slight differences.

Spectroscopic properties of Cs2NaLa1−xNdxCl6 crystal. Concentration quenching of fluorescence

Abstract The results of the absorption, infrared, Raman and fluorescence spectra of the title compound are reported. An assignment of the vibronic peaks of the fluorescence spectra is proposed. It was found that the concentration dependence of the fluorescence decay times was temperature dependent. The mechanism of concentration quenching of Nd 3+ fluorescence is discussed in terms of resonant and non-resonant cross relaxation.

Molecular motion in anisotropic medium. II. The proton relaxation study of CH3CN: The general A3 spin system

The coupled relaxation of an A3 spin system (CH3CN) dissolved in a perdeuterated nematic phase (EBBA-d23) is studied for the first time. The partially averaged dipolar interaction allows one to observe a triplet pattern on which four different spin perturbations have been performed: a selective soft π pulse on either an external or the central transition, and a nonselective π pulse affecting all three transitions with or without spin tickling on one of the external transitions. The last perturbation provides a measure of the time evolution of the doublet components in the central line of the triplet. The relaxation processes are modeled as intramolecular dipole–dipole interactions plus other mechanisms which are treated collectively as external random magnetic fields (RF). A restricted diffusion model is used which takes account of the molecular orientation of the solute molecule. Cross relaxation terms are very important, and RF mechanisms are not negligible even in a perdeuterated nematic phase. The anisotropy of the rotational diffusion is obtained, and R, the rotational diffusion constant of an axis perpendicular to the axis of symmetry of the rotor, is found to be 25 times smaller than in neat liquid. The diffusion constant about the principal axis R∥ is not affected significantly by the liquid crystal. The values of R⊥ and R∥ give correlation times which allow extreme narrowing approximations, but in accordance with theoretical predictions it is demonstrated experimentally that the zeroth, first, and second order projections of the dipolar spectral densities are not equal due to preferential ordering of CH3CN in the liquid crystal. Optimal values of the effective ordering parameter SZZ obtained from dipolar splittings in the spectrum and from the dipolar relaxation parameters differ from each other. This difference may be rationalized on the basis of models which account for the two different physical phenomena.

Relaxation in an A2 coupled spin system dissolved in a perdeuterated liquid crystal: The intermolecular dipolar random field cross relaxation term

A perdeuterated nematic liquid crystal N- (p-ethoxybenzylidene) -p-n-butylaniline (EBBA-d23, 98% deuterium) was synthesized to study relaxation of small molecules dissolved in anisotropic solvents. Proton spin–lattice relaxation in the coupled A2 spin system of methylene chloride (CH2Cl2) was studied using both perhydro and perdeutero EBBA as the solvent. The relaxation was modeled as intramolecular dipole–dipole interactions plus other mechanisms that were treated collectively as fluctuating external random magnetic fields. Comparison of relaxation in EBBA and EBBA-d23 allowed the first experimental measurement of the correlation coefficient between external random fields at the two protons due to dipolar interactions of the methylene protons with the protons or deuterons of the solvent (78% correlated). The selective nuclear Overhauser effect was also studied.