Two-dimensional Exchange Spectroscopy Research Articles

Nuclear spin relaxation in chloroform dissolved in a nematic phase: spectral densities at high frequencies

Longitudinal relaxation data are reported for 13C labelled CHCl3 in a nematic liquid crystalline solvent at several magnetic field strengths. Two-dimensional exchange spectroscopy has been applied together with transfer-modes analysis to obtain the relaxation matrix including the cross-correlation terms. The spectral densities were found to be frequency dependent, which is interpreted in terms of a fast local motion and a slow process, both contributing to relaxation. When taking this into account, the cross-correlation rates were found to be linear in the magnetic field, and they can be attributed to DD/CSA interference. In addition, a small field-independent contribution is found for 1H. This can be related to interference effects between intramolecular DD and intermolecular DD relaxation.

Determination of Monomer Conformations in Noncrystalline Solid Polymers by Two-Dimensional NMR Exchange Spectroscopy

We propose and demonstrate a new approach to the determination of the conformations of molecules or monomers in noncrystalline solids. The method makes use of two-dimensional NMR exchange spectroscopy in combination with specific isotopic labeling. We present experimental results on specifically labeled poly(methyl methacrylate), poly(methyl acrylate), and poly(ethyl methacrylate). Through comparisons between the experimental and simulated two-dimensional spectra, we determine that the ester side groups of poly(methyl methacrylate) and poly(methyl acrylate) have planar, trans conformations and that the side groups of poly(ethyl methacrylate) are also predominantly in the planar, trans conformation in the noncrystalline solid state

2D EPR Study of a Photoinduced Proton Abstraction in the System Anthraquinone and 4-Methyl-2,6-di-tert-butylphenol in 2-Propanol

Abstract The present study demonstrates the potential of two-dimensional exchange spectroscopy for the investigation of transient radicals. The appearance of cross peaks in a 2D plot unambiguously demonstrates the process of photoinduced interconversion of the neutral hydroxyanthroxyl radical (AQH·) into the anthrasemiquinone anion radical (AQ−·) by proton abstraction in the system anthraquinone/4-methyl-2.6-di-tert-butylphenol in 2-propanol. An analysis of the cross-peak pattern reveals the correlation between hyperfine lines of the parent and product radical and yields information about the assignment of coupling constants to molecular positions including their sign. The rate constant of interconversion k could also be determined by a variation of the mixing time Tm.

Application Of 2D Exchange Spectroscopy to the Studies of Imidazole Binding to CytochromeC

Abstract Two-dimensional 1H exchange spectroscopy has been applied to the studies of imidazole binding to cytochrome c. The magnetization exchange rate constants, the substitution reaction rate constants and the equilibrium constants have been evaluated. Some resonances of imidazole substituted forms (Im-cyt c) have been assigned by 2D exchange NMR. The observed hyperfine shifts of heme methyl groups of Im-cyt c and cyt c have been corrected for contact and dipolar contributions. The detailed analysis of these results indicates that the axial ligands and their orientations, and the substituents of heme are important determinants of the electronic structure of the heme in cyt c and its derivatives.

1,2-Metallotropic shifts in trimethylplatinum(IV) and tricarbonylrhenium(I) halide complexes of pyridazine and 4-methylpyridazine

Pyridazine (pydz) and 4-methylpyridazine (4Me-pydz) form stable bis(monodentate) complexes of general formulae fac-[PtXMe3L2](X = Cl, Br or I) and fac-[ReX(CO)3L2](X = Cl, Br or I). These complexes in above-ambient temperature solutions exhibit 1,2-fluxional shifts between the nitrogen donor pairs of each ring. All linkage isomers of the pydz complexes are equivalent whereas three NMR-distinct linkage isomers, namely (1,1), (1,2)/(2,1) and (2,2) species, occur in solutions of the 4Me-pydz complexes. Fluxional exchange kinetics have been measured by variable-temperature 1H NMR bandshape analysis and two-dimensional exchange spectroscopy (EXSY). Platinum-195 NMR data have been obtained for the complexes [PtXMe3(4Me-pydz)2](X = Br or I) and their fluxional dynamics investigated by 195Pt two-dimensional EXSY and total bandshape analysis. Energy barriers, ΔG‡(298.15 K) for the 1,2-M–N fluxion are in the ranges 69–74 kJ mol–1 for the PtIV complexes and 84–90 kJ mol–1 for the ReI complexes. The crystal structure of [PtClMe3(pydz)2] shows the cis pydz rings oriented in a propeller arrangement with their unco-ordinated nitrogens oriented away from the cis positioned chlorine.

Rotational energy barrier in PtII–N7(purine) bonds by one- and two-dimensional nuclear magnetic resonance spectroscopy

Rotational energy barriers about PtII-N7 purine bonds for cis-[Pt(NH3)2L2][L = 1-methyladenosine, adenosine 5′-mono-, -di- or -tri-phosphate, or guanosine 5′-monophosphate) were evaluated from one-and two-dimensional phosphorus-31 exchange spectroscopy. Two diastereomers in a head-to-tail conformation exist for all the aminopurine complexes in the temperature range 25–60 °C due to slow rotation about Pt–N7 bonds. The rotational activation energies for all the 6-aminopurine complexes lie in the range 46–95 kJ mol–1 while the 6-oxopurine complex exhibits the lowest energy barrier, 25 kJ mol–1. The two exocyclic amines of the aminopurine complexes in a head-to-tail configuration are positioned above and below the platinum square plane. The increased rotational energy barrier in the AMP compared to that of GMP complexes is attributed to the absence of hydrogen bonding with amines, a direct interaction of the lone-pair electrons of the 6-NH2 group with the σ- bonding orbitals of the platinum atom, and a decrease in the crystal-field stabilization energy due to the interaction with the filled dz2 orbital. The rotational energy barrier can be taken as the ground-state energy difference between the head-to-tail and head-to-head configurations since the latter is encountered near to the highest potential-energy surface during the conversion of one head-to-tail rotamer into the other. An energy difference >45 kJ mol–1 may be estimated between the bis(GMP) complex and the AMP analogue in a head-to-head configuration.

3] Two-dimensional exchange spectroscopy of proteins

Publisher Summary This chapter discusses Two-dimensional (2D) implementations of the separation procedures. The techniques described can be generalized to higher dimensions. 2D exchange spectroscopy is a very convenient tool for studying dynamic processes in liquids. It is designed specifically for the elucidation of incoherent magnetization transfer processes: chemical exchange and cross-relaxation. However, the 2D exchange experiment also detects coherent magnetization transfer caused by scalar coupling. In laboratory-frame, 2D exchange spectra of macromolecules, cross-relaxation and chemical exchange are indistinguishable because both give rise to positive cross-peaks. In rotating-frame exchange spectra of macromolecules, cross-relaxation and chemical exchange are distinguishable because cross-peaks of the former are negative with respect to the diagonal whereas those of the latter are positive. However, for 2D exchange spectroscopy, only slow processes in which the observed spins change their resonance frequencies are suitable. In NMR spectroscopy, slow refers to an exchange rate, k ij , between sites i and j that is smaller than the difference in the resonance frequencies of the two exchange sites—that is, exchange does not influence the shapes and positions of the individual resonance lines.

Does phosphoglucoisomerase display anomeric specificity or selectivity towards alpha-D-glucose 6-phosphate? An assessment by two-dimensional phase-sensitive 31P exchange spectroscopy (EXSY) n.m.r.

The study by two-dimensional phase-sensitive 31P exchange spectroscopy (EXSY) n.m.r. of hexose 6-phosphates interconversion in the reaction catalysed by yeast phosphoglucoisomerase reveals that the enzyme displays anomeric selectivity, rather than specificity, towards alpha-D-glucose 6-phosphate. Indeed, beta-D-glucose 6-phosphate participates for about 20% to the total and direct conversion of the aldohexose into oxohexose ester.

Mathematical properties of the amplitude matrix of NMR exchange spectroscopy

By mathematical study of NMR two-dimensional chemical exchange spectroscopy (2D-EXSY), it has been found that the eigenvalues of the amplitude matrix A are always real, positive and do not exceed 1. These properties form a theoretical basis for the methods of evaluating kinetic constants from A by directly diagonalizing the matrix A and by directly expanding its logarithm In A. The conclusion is also valid for 2D nuclear Overhauser effect spectroscopy (NOESY).

Conformational properties of monosubstituted cyclohexanes in their thiourea inclusion compounds and in solution: variable-temperature one-dimensional and two-dimensional carbon-13 NMR investigations

Temperature-dependent changes in high-resolution solid state C-13 NMR spectra of thiourea inclusion compounds containing monosubstituted cyclohexane guest moleculeS (C6H11X: X = CH3, NH2, OH, Cl, Br, I) have demonstrated that a ''chair-chair'' ring inversion process occurs for these guest molecules inside the thiourea tunnel, and the energy barriers for this process have been determined. The relative populations of the axial and equatorial conformers of the guest molecules have been determined directly from the NMR spectra recorded (at low temperature) under slow exchange conditions. On this basis, the guest molecules studied can be divided into two classes: those with X = Cl, Br, and I have a predominance of the axial conformer (relative populations of equatorial conformer almost-equal-to 0.05-0.15), whereas those with X = CH3, NH2, and OH have a predominance of the equatorial conformer (relative populations of equatorial conformer almost-equal-to 0.82-0.97). In addition, conformational properties of these monosubstituted cyclohexanes in solution have been reinvestigated via variable-temperature C-13 NMR measurements. It is shown, from the reported results for methylcyclohexane, that two-dimensional exchange spectroscopy can successfully identify minor conformers with relative populations as low as 0.01 in solution and 0.03 in the solid state.

Deuterium two-dimensional exchange nuclear magnetic resonance by rotor-synchronized magic angle spinning

The method of two-dimensional exchange spectroscopy under condition of magic angle sample spinning (MAS) and synchronization of the mixing time with the rotor period is extended to spin I=1 nuclei. Theoretical equations are derived for the cross peak intensities as a function of the magnetic and kinetic parameters of the system and the method is demonstrated on a number of deuterated compounds. Dimethylmalonic acid–d6 is first used to illustrate the effect of rotor synchronization by the complete absence of cross peaks when no exchange takes place. The method is then applied to two dynamic systems, viz. dimethylsulfone–d6 and thiourea–C6D12 inclusion compound. The experimental results are compared with simulations as well as with analogous experiments on nonspinning samples. Since chemical shift effects are often negligible in deuterium NMR the time domain sampling can in principle be reduced to the number of the desired spinning sidebands, resulting in considerable time savings. The main advantage of the method is the gain in sensitivity at the expense of the characteristic ridge pattern of static two-dimensional exchange experiments. The gain in sensitivity may be sufficient for performing such experiments on deuterium in isotopically normal compounds. The feasibility of such experiments is demonstrated by the recording of a one-dimensional deuterium MAS spectrum of a nonenriched sample.

Halogenotrimethylplatinum(IV) complexes of 2,6-bis(p-tolylthiomethyl)pyridine (L1): nuclear magnetic resonance studies of their solution state stereodynamics and the crystal structure of fac-[PtBrMe3L1

Under mild conditions 2,6-bis(p-tolylthiomethyl) pyridine (L1) reacted with halogenotrimethylplatinum(IV) to afford complexes of the type fac-[PtXMe3L1](X = Cl or Br). This potentially terdentate ligand acts here in a five-membered S/N chelate bidentate fashion. Solution dynamic NMR studies revealed the presence of three fluxional processes: pyramidal inversion of the co-ordinated sulfur atom, S-S switching with correlated pyramidal inversion, and S-S switching. The mechanism for the S-S exchange process was elucidated by two-dimensional NMR exchange spectroscopy. The crystal structure of fac-[PtBrMe3L1] confirms the bidentate chelate nature of the ligand in the solid state, with a N-Pt-S(1) angle of 81.5°. IR data for the Pt-C, Pt-N and Pt-X stretching modes in these complexes are also presented.

Molecular orientational dynamics in K3C60 probed by two-dimensional nuclear magnetic resonance.

We characterize the orientational dynamics of C 60 -3 anions in K 3 C 60 , using two-dimensional NMR exchange spectroscopy. We find an activated temperature dependence for the reorientation rate between 195 and 220 K, with E a =460±60 meV. Molecular reorientation proceeds via small (<44 o ) rotational jumps, implying that K 3 C 60 is more highly orientationally disordered than previously proposed

The orientations and dynamics of ethanol guest molecules in the clathrate of Dianin's compound

In the clathrates of Dianin's compound the guests are trapped in cages formed by six host molecules. Two ethanol molecules can be enclosed by each cage. Deuteron nuclear magnetic resonance spectra of single crystals of Dianin's compound with ethanol-OD, ethanol-1,1-D2 and ethanol-2,2,2-D3 as a guest were recorded in the temperature range 10 K < T < 310 K. Only at T < 25 K the guests are found to be immobile enough to allow the determination of the bond directions of the different deuterons from the unique eigenvectors of the quadrupole coupling (qc) tensors. The measured qc tensors of the hydroxyl and of the methylene deuterons are used to determine the orientations of the ethanol molecules as a whole. These can occupy six distinct sites. The two ethanol molecules in a given cage are linked by a weak hydrogen bond. The dynamics of the ethanol guests is elucidated by line shape analyses and by deuteron two-dimensional exchange spectroscopy. It is found to be rather complex, consisting of three different ty...

Exchange processes in diselenium and selenium-sulfur dihalides, Se2X2 and SeSX2 (X = Br, Cl). A selenium-77 2D-EXSY study

The application of two-dimensional NMR exchange spectroscopy (2D-EXSY) to the study of 77 Se exchange kinetics in mixtures of S 2 Cl 2 and Se 2 Br 2 , shows that 77 Se magnetization transfer takes place predominantly by an exchange process, in which only SeX bonds are broken. Direct exchange via mechanisms, involving ChX radicals, four-chalcogen center transition states and mono- and trichalcogen dihalide species are not consistent with the cross peaks observed in the 2D spectra.

Sequential nuclear magnetic resonance assignment of beta-1,2-linked mannooligosaccharides isolated from the phosphomannan of the pathogenic yeast Candida albicans NIH B-792 strain.

The H-1 and H-2 signals of beta-1,2-linked mannooligosaccharides isolated from the phosphomannan of Candida albicans NIH B-792 strain by mild acid hydrolysis were assigned by a sequential NMR assignment method that combines two-dimensional 1H-1H correlated spectroscopy (COSY) and two-dimensional nuclear Overhauser enhancement and exchange spectroscopy (NOESY). The results indicated that the H-1 and H-2 of each beta-1,2-linked mannopyranose unit show largely different signals compared with those of the alpha-linked ones and that the correlation between linkages and signals could not be explained by a conventional additivity rule. Furthermore, a regular proportional downfield shift of the H-1 signal was observed in the order of the mannose unit from the reducing terminal except those of the reducing and nonreducing terminal positions. Although the 1H NMR spectra of these oligosaccharides were complicated due to the presence of a large portion of the beta-anomer from the reducing terminal mannose unit, reduction of the oligosaccharides with NaBH4 to the corresponding alcohols gave simple and more readily interpretable 1H NMR spectra. Unexpectedly, however, a shift of H-1 signals by this reduction occurred not only on the second mannose unit but also on the third and fourth mannose units from the modified reducing terminal group of each oligosaccharide alcohol. This result indicates that the reducing terminal mannose unit is able to affect up to the fourth mannose unit from the reducing terminal. The presence of a long-distance interresidue NOE also suggests that the beta-1,2-linked mannooligosaccharides have a compactly folded conformation in solution.

Models of the cytochromes b. 8. Two-dimensional nuclear Overhauser and exchange spectroscopy studies of paramagnetic "cavity" type (tetraphenylporphinato)iron(III) complexes of planar ligands. [Erratum to document cited in CA115(22):246370d

Models of the cytochromes b. 8. Two-dimensional nuclear Overhauser and exchange spectroscopy studies of paramagnetic "cavity" type (tetraphenylporphinato)iron(III) complexes of planar ligands. [Erratum to document cited in CA115(22):246370d

Models of the cytochromes b. 8. Two-dimensional nuclear Overhauser and exchange spectroscopy studies of paramagnetic "cavity" type (tetraphenylporphinato)iron(III) complexes of planar ligands

The paramagnetic bis-2-methylimidazole complex of (tetramesitylporphinato)iron(III) chloride, with short TI and T, relaxation times, was investigated in CD2C12 by NOESY (nuclear Overhauser and exchange spectroscopy) experiments as a function of temperature (-29, -54, -74 C). The two-dimensional (2-D) maps obtained at -29 and -54 C revealed cross peaks due to both chemical exchange and the nuclear Overhauser effect (NOE), while the map obtained at -74 C showed only NOE cross peaks. The NOE cross correlations are consistent with the 2-MeImH ligands being positioned over or near the meso carbon atoms, rather than over the porphyrin nitrogens, as previously suggested by Nakamura and Groves (Tetrahedron 1988, 44, 3225). The nonbonded interactions between the methyl groups of the 2-methylimidazole (2-MeImH) ligands and the mesityl groups of the porphyrin phenyl rings for this orientation of the axial ligands produce a strongly ruffled porphyrin core having two oblong cavities at right angles to each other, one above and one below the plane of the porphyrin ring, which hold the hindered axial ligands in perpendicular planes over the meso positions. The chemical exchange cross peaks are indicative of interconversion of all resonances of a particular type, due to rotation of the axial ligands rather than axial ligand exchange. This interpretation is confirmed by the fact that there are no cross peaks observed between free and coordinated 2-MelmH resonances at any of the temperatures investigated. Rotation of the 2-MeImH ligands requires that alternate phenyl rings flap up and down as the ligands rotate. The coupling pattern between the pyrrole-H protons is consistent with that expected on the basis of the extrapolated intercepts of the Curie plot.

Subunit interactions change the heme active-site geometry in p-cresol methylhydroxylase.

The enzyme p-cresol methylhydroxylase [4-cresol: (acceptor) oxidoreductase (methyl-hydroxylating), EC 1.17.99.1] contains two subunits: a cytochrome c (electron transfer) subunit (cytochrome cpc) and a flavin (catalytic) subunit. When these subunits are separated by isoelectric focusing, a stable cytochrome subunit is obtained. Significant differences are observed between the one-dimensional NMR spectra of oxidized cytochrome cpc and of oxidized p-cresol methylhydroxylase. Analysis of the two-dimensional nuclear Overhauser enhancement and exchange spectroscopy (NOESY) spectrum of reduced cytochrome cpc suggests that the axial ligand, Met-50, of the stable subunit reorients by a rotation about the C gamma-S delta bond when cytochrome cpc binds to the flavin subunit. This reorientation must result in a change in bonding at the heme, which is reflected both in the para-magnetically shifted resonances and in the redox potential. p-Cresol methylhydroxylase thereby provides an interesting example of the coupling of subunit interactions to active-site structure and reactivity.

Two-dimensional magnetization exchange spectroscopy of Anabaena 7120 ferredoxin. Nuclear Overhauser effect and electron self-exchange cross peaks from amino acid residues surrounding the iron-sulfur (2Fe-2S) cluster

Hyperfine 1H NMR signals of the 2Fe-2S* vegetative ferredoxin from Anabaena 7120 have been studied by two-dimensional (2D) magnetization exchange spectroscopy. The rapid longitudinal relaxation rates of these signals required the use of very short nuclear Overhauser effect (NOE) mixing times (0.5-20 ms). The resulting pattern of NOE cross-relaxation peaks when combined with previous 1D NOE results [Dugad, L. B., La Mar, G. N., Banci, L., & Bertini, I. (1990) Biochemistry 29, 2263-2271] led to elucidation of the carbon-bound proton spin systems from each of the four cysteines ligated to the 2Fe-2S* cluster in the reduced ferredoxin. Additional NOE cross peaks were observed that provide information about other amino acid residues that interact with the iron-sulfur cluster. NOE cross peaks were assigned tentatively to Leu27, Arg42, and Ala43 on the basis of the X-ray coordinates of oxidized Anabaena 7120 ferredoxin [Rypniewski, W.R., Breiter, D.R., Benning, M.M., Wesenberg, G., Oh, B.-H., Markley, J.L., Rayment, I., & Holden, H. M. (1991) Biochemistry 30, 4126-4131]. Three chemical exchange cross peaks were detected in magnetization exchange spectra of half-reduced ferredoxin and assigned to the 1H alpha protons of Cys49 and Cys79 [both of whose sulfur atoms are ligated to Fe(III)] and Arg42 (whose amide nitrogen is hydrogen-bonded to one of the inorganic sulfurs of the 2Fe-2S* cluster). The chemical exchange cross peaks provide a means of extending assignments in the spectrum of reduced ferredoxin to assignments in the spectrum of the oxidized protein.(ABSTRACT TRUNCATED AT 250 WORDS)