Modified z-gradient filtering as a mean to obtain phased deuterium autocorrelation 2D NMR spectra in oriented solvents

We describe a modified z-gradient filter scheme specifically designed to obtain pure absorption mode deuterium 2D NMR spectra recorded in oriented solvents. The proposed technique is investigated by analysing the evolution of the density operator for a spin I=1. The method is applied to the recently designed Q-COSY and Q-resolved 2D experiments to simplify the analysis of chiral molecules dissolved in weakly orienting chiral liquid crystals. The efficiency of this z-gradient filtering technique is illustrated using the perdeuterated 1-butanol, a prochiral molecule of average Cs symmetry, dissolved in an organic solution of poly-gamma-benzyl-L-glutamate (PBLG). The experimental results as well as the advantages of the new experiments compared with the previous ones are described and discussed.     

Homo- and heteronuclear 2D NMR approaches to analyse a mixture of deuterated unlike/like stereoisomers using weakly ordering chiral liquid crystals

We describe several homo- and heteronuclear 2D NMR strategies dedicated to the analysis of anisotropic (2)H spectra of a mixture of dideuterated unlike/like stereoisomers with two remote stereogenic centers, using weakly orienting chiral liquid crystals. To this end, we propose various 2D correlation experiments, denoted "D(H)(n)D" or "D(H)(n)C" (with n=1, 2), that involve two heteronuclear polarization transfers of INEPT-type with one or two proton relays. The analytical expressions of correlation signals for four pulse sequences reported here were calculated using the product-operators formalism for spin I=1 and S=1/2. The features and advantages of each scheme are presented and discussed. The efficiency of these 2D sequences is illustrated using various deuterated model molecules, dissolved in organic solutions of polypeptides made of poly-gamma-benzyl-L-glutamate (PBLG) or poly-epsilon-carbobenzyloxy-L-lysine (PCBLL) and NMR numerical simulations.     

S(3)E-E.COSY methods for the measurement of (19)F associated scalar and dipolar coupling constants

A (1)H-(19)F spin state selective excitation (S(3)E) pulse sequence element has been applied in combination with (1)H homonuclear mixing to create E.COSY-type experiments designed to measure scalar J(HF2') and J(HH2') and residual dipolar D(HF2') and D(HH2') couplings in 2'-deoxy-2'-fluoro-sugars. The (1)H-(19)F S(3)E pulse sequence element, which resembles a simple INEPT sequence, achieves spin-state-selective correlation between geminal (1)H-(19)F spin pairs by linear combination of in-phase (19)F magnetization and anti-phase magnetization evolved from (1)H. Since the S(3)E sequence converts both (19)F and (1)H steady-state polarization into observable coherences, an approximately twofold signal increase is observed for fully relaxed (1)H-(19)F spin pairs with respect to a standard (1)H coupled (19)F 1D experiment. The improved sensitivity and resolution afforded by the use of (1)H-(19)F S(3)E E.COSY-type experiments for measuring couplings is demonstrated on the nucleoside 9-(2',3'-dideoxy-2'-fluoro-beta-D-threo-pentofuranosyl)adenine (beta-FddA) and on a selectively 2'-fluorine labeled 21mer RNA oligonucleotide.     

NMR Studies of Hindered Rotation. The Diels-Alder Adduct of Phencyclone with Maleic Anhydride: Restricted Motion of Bridgehead Phenyls

¹H NMR studies at 300 MHz have been performed for the Diels-Alder adduct of phencyclone and maleic anhydride in CDCl₃ at ambient temperatures. The 1D spectrum shows four equal (2H) intensity doublets in the aryl region (in addition to other absorptions) which is fully consistent with a slow exchange limit (SEL) spectrum of a system in which the unsubstituted bridgehead phenyls exhibit hindered rotation around the C(sp²)-C (sp³) bond on the NMR timescale. These protons are assigned to H-1,8 and H-4,5 of the phenanthrene moiety and to H-2′ and H-6′ of the phenvls based on the two-dimensional (2D) homonuclear chemical shift correlation spectrum (COSY) together with arguments regarding carbonyl and aromatic ring anisotropy. Full proton assignments are given.     

57Fe Mössbauer spectroscopy on the cyclic spin-cluster Fe6(tea)6(CH3OH)6

We present ⁵⁷Fe Mössbauer spectroscopy experiments on the cyclic spin-cluster Fe₆(tea)₆(CH₃OH)₆ (tea = triethanolaminato(-3)). In former studies, the spin cluster has been treated as a homogenous, quasi-one-dimensional spin S=5/2 Heisenberg antiferromagnet. Our experiments reveal spectra, which consists of two different quadrupolar doublets. In consequence, there are two different Fe sites among the hexanuclear iron spin-cluster.     

Characterization of microporous aluminophosphate IST-1 using (1)H Lee-Goldburg techniques

The presence of two independent methylamine species in microporous aluminophosphate IST-1 (|(CH(3)NH(2))(4)(CH(3)NH(+)(3))(4)(OH(-))(4)|[Al(12)P(12)O(48)]) has been shown previously by synchrotron powder X-ray diffraction. One of these species, [N(1)-C(1)], links to a six-coordinated framework Al-atom [Al(1)], while the other methylamine [N(2)-C(2)] is protonated and hydrogen-bonded to three O-atoms [O(1), O(2) and O(12)]. We revisit the structure of IST-1 and report the complete assignment of the (1)H NMR spectra by combining X-ray data and high-resolution heteronuclear/homonuclear solid-state NMR techniques based on frequency-switched Lee-Goldburg homonuclear decoupling and (31)P-(31)P homonuclear recoupling. Careful analysis of the 2D (1)H-X homonuclear correlation (X=(1)H) and 2D heteronuclear correlation (X=(13)C, (31)P and (27)Al) spectra allowed the distinction of both methylamine species and the assignment of all (31)P and (13)C resonances. For the first time at a relatively high (9.4 T) magnetic field, symmetric doublet patterns have been observed in the (13)C spectra, caused by the influence of the (14)N second-order quadrupolar interaction.     

［Co(tren)(amino acidato-N,O)］X2·nH2O在溶液中的NMR研究

报道了配合物［Co(tren)(DL-phenylalaninato)］(ClO₄)₂ (Ⅰ)、Co(tren)(L-prolinato)］I₂·H₂O (Ⅱ)、［Co(tren) (L-valinato)］I₂·1/3H₂O (Ⅲ)、［Co(tren)(L-isoleucinato)］I₂ (Ⅳ)、［Co(tren) ( L-leucinato)］I₂ (Ⅴ) 在溶液中的NMR研究. 通过同核和异核二 维核磁共振实验归属了配合物的¹H和¹³C NMR ，发现在配合物的空间结构 中，四齿配体(tren)的3个螯合臂中有两个构象稳定，而另一个构象不稳定，这与晶体学数据吻合. 通过变温实验和溶剂效应实验，发现在配合物Ⅲ、Ⅳ、Ⅴ中存在分子内氢键，而在晶体结构中，所有配合物都存在分子间氢键. 配合物Ⅰ、Ⅱ在溶液中的行为需进一步的研究 .     

1H and 13C NMR studies of glycine in anisotropic media: double-quantum transitions and the effects of chiral interactions

The (1)H NMR spectrum of glycine in stretched gelatin gel and in cromolyn liquid crystal displays a well-resolved doublet due to (1)H-(1)H dipolar interaction. Multiple spectra were obtained within a wide range of offset frequencies of partially saturating radio-frequency (RF) radiation to generate steady-state irradiation envelopes or z-spectra of glycine. Maximal suppression of the doublet occurred when the irradiation was applied exactly at the centre frequency, between the two glycine peaks. This phenomenon is due to double-quantum transitions and is similar to our previous work on quadrupolar nuclei (2)H (HDO) and (23)Na(+). When the (13)C isotopomer glycine-2-(13)C was used, the same effect was found in twice, split by (1)J(CH)+2D(CH). Additional signals in (1)H and (13)C NMR due to prochiral-chiral interactions were found when glycine-2-(13)C was dissolved in chiral anisotropic gelatin and κ-carrageenan gels. The NMR spectra were successfully simulated assuming a (2)J(HH) coupling constant of -16.5Hz and two distinct dipolar coupling constants for the -(13)CH(2)- group (D(C,HA), and D(C,HB)).     

Determination of membrane Peptide orientation by 1H-detected 2H NMR spectroscopy

We demonstrate the application of the proton inverse detected deuteron (PRIDE) NMR technique to the measurement of the orientation of membrane-bound peptides with enhanced sensitivity. Gramicidin D, a transmembrane peptide, and ovispirin, a surface-bound peptide, were used as model systems. The peptides were 2H-labeled by 1H/2H exchange and oriented uniaxially on glass plates. The directly detected 2H spectra of both peptides showed only a strong D(2)O signal and no large quadrupolar splittings. In contrast, the PRIDE spectrum of gramicidin exhibited quadrupolar splittings as large as 281 kHz, consistent with its transmembrane orientation. Moreover, the large D(2)O signal in the directly detected 2H spectra was cleanly suppressed in the PRIDE spectrum. For ovispirin, the 1H indirectly detected 2H spectrum revealed a 104 kHz splitting and a zero-frequency peak. The former reflects the in-plane orientation of most of the helix axis, while the latter results from residues with a magic-angle orientation of the N-D bonds. These are consistent with previous 15N NMR results on ovispirin. The combination of PRIDE and exchange labeling provides an economical and sensitive method of studying membrane peptide orientations in lipid bilayers without the influence of D(2)O and with the ability to detect N-D bonds at the magic angle from the bilayer normal.     

Lightweight hydrogen-storage material Mg(0.65)Sc(0.35)D2 studied with 2H and 2H-{45Sc} MAS NMR exchange spectroscopy

Using double-quantum (2)H MAS NMR with (45)Sc recoupling and Bloch-Siegert compensated (2)H-{(45)Sc} TRAPDOR we have identified the overlapping NMR signals of deuterium with and without scandium neighbors in Mg(0.65)Sc(0.35)D(2), a candidate lightweight material for hydrogen storage. At room temperature we also observe a third type of mobile deuterium. Deuterium mobility among the three NMR-distinct sites has been investigated by means of one-and two-dimensional exchange spectroscopy (Exsy). Complete deuterium exchange within 0.1s is observed, which indicates that the three NMR-distinct sites are close together in the crystal lattice. The weak temperature- and MAS-rate dependences observed in Exsy are indicative for a combination of chemical exchange and spin diffusion.     

Q-shear transformation for MQMAS and STMAS NMR spectra

The multiple-quantum magic-angle spinning (MQMAS) and satellite-transition magic-angle spinning (STMAS) experiments refocus second-order quadrupolar broadening of half-integer quadrupolar spins in the form of two-dimensional experiments. Isotropic shearing is usually applied along the indirect dimension of the 2D spectra such that an isotropic projection free of anisotropic quadrupolar broadening can be obtained. An alternative shear transformation by a factor equal to the coherence level (quantum number) selected during the evolution period is proposed. Such a transformation eliminates chemical shift along the indirect dimension leaving only the second-order quadrupolar-induced shift and anisotropic broadening, and is expected to be particularly useful for disordered systems. This transformation, dubbed Q-shearing, can help avoid aliasing problems due to large chemical shift ranges and spinning sidebands. It can also be used as an intermediate step to the isotropic representation for expanding the spectral window of rotor-synchronized experiments.     

(1)H-(13)C INEPT MAS NMR correlation experiments with (1)H-(1)H mediated magnetization exchange to probe organization in lipid biomembranes

Two-dimensional (1)H-(13)C INEPT MAS NMR experiments utilizing a (1)H-(1)H magnetization exchange mixing period are presented for characterization of lipid systems. The introduction of the exchange period allows for structural information to be obtained via (1)H-(1)H dipolar couplings but with (13)C chemical shift resolution. It is shown that utilizing a RFDR recoupling sequence with short mixing times in place of the more standard NOE cross-relaxation for magnetization exchange during the mixing period allowed for the identification and separation of close (1)H-(1)H dipolar contacts versus longer-range inter-molecular (1)H-(1)H dipolar cross-relaxation. These 2D INEPT experiments were used to address both intra- and inter-molecular contacts in lipid and lipid/cholesterol mixtures.     

Residual 2H quadrupolar couplings in weakly aligned carbohydrates.

We report a novel two-dimensional NMR pulse scheme for the 1H-detected observation of 2H in isotopically 13C, 2H-enriched carbohydrates. This scheme is used for the indirect observation of residual quadrupolar couplings in 13C, 2H-enriched methyl-beta-D-glucopyranoside weakly aligned in a dilute lyotropic liquid-crystalline medium comprising 20% (w/v) dihexanoyl-phosphatidylcholine/dimyristoyl-phosphatidylcholine (1:3 mol/mol) in D2O. The observed residual quadrupolar couplings are substantially larger than residual dipolar one-bond 13C-1H couplings under the same experimental conditions. These quadrupolar couplings are thus a useful alternative to dipolar couplings for the structural analysis of small molecules that align very weakly in dilute liquid-crystalline media. Moreover, since the quadrupolar coupling constant is very uniform throughout endocyclic deuterons of the carbohydrate, these data suggest that adoption of a single average value of this parameter in 2H relaxation studies on the glycan moieties of glycoproteins and glycopeptides is a valid assumption.     

3'(,3")-(二)甲酰基(二)苯并冠醚的2D NMR研究

测定了3'(,3")-(二)甲酰基(二)苯并冠醚(1~7)的¹H、¹³C谱,首次结合¹H-¹H COSY、¹³C-¹H COSY、COLOC二维谱对这些化合物的¹H、¹³C化学位移进行了归属。用NOESY谱确定了合成产物冠4、冠5的结构,并对溶液中的构象异构体及其溶剂影响进行了初探。作者以冠1~8的¹H、¹³C谱的归属为参照,提出了正确归属冠醚醚环上次甲基¹H、¹³C化学位移的一般规则。     

Influence of local molecular motions on the determination of 1H-1H internuclear distances measured by 2D 1H spin-exchange experiments

Analysis of spin-exchange build-up curves obtained by measurement of 2D 1H CRAMPS spectra of alpha-glycine was performed to evaluate the rate of 1H-1H spin-exchange process with respect to the influence of variation in internal molecular motion. Differences in local motions significantly affect spin-exchange constants even in highly rigid organic solids with virtually uniform motion behavior. The polarization transfer between nonequivalent alpha-protons is described by the spin-exchange constant D=0.77 nm(2)ms(-1), while the polarization transfer involving spin exchange between alphaH and NH(3)(+) protons is characterized by D=0.24-0.21 nm(2)ms(-1). This significant decrease corresponds to rotation of hydrogen-bonded amino groups. Neglecting this variation in local spin-exchange constants the resulting calculated 1H-1H distance can be overestimated by up to 100%. Complications following from relayed and back polarization transfer involving the nearest spins within one functional group (e.g., CH(2) and/or NH(3)(+)) and intermolecular spin exchange are discussed. It was shown that 2H quadrupolar splitting determined for selected sites directly correlates with the experimentally observed differences in spin-exchange coefficients. It is also demonstrated that a medium level quantum chemical calculation of molecular dynamics provides relevant data that can be used to estimate differences in molecular motions.     

Side-chain H and C resonance assignment in protonated/partially deuterated proteins using an improved 3D(13)C-detected HCC-TOCSY

We propose the use of (13)C-detected 3D HCC-TOCSY experiments for assignment of (1)H and (13)C resonances in protonated and partially deuterated proteins. The experiments extend 2D C-13-start and C-13-observe TOCSY type experiments proposed earlier. Introduction of the third (1)H dimension to 2D TOCSY: (i) reduces the peak overlap and (ii) increases the sensitivity per unit time, even for highly deuterated (>85%) protein samples, which makes this improved method an attractive tool for the side-chain H and C assignment of average sized proteins with natural isotope abundance as well as large partially deuterated proteins. The experiments are demonstrated with a 16 kDa (15)N, (13)C-labeled non-deuterated apo-CcmE and a 48 kDa uniformly (15)N, (13)C-labeled and fractionally ( approximately 90%) deuterated dimeric sFkpA. It is predicted that this method should be suitable for the assignment of methyl (13)C and (1)H chemical shifts of methyl protonated, highly deuterated and (13)C-labeled proteins with even higher molecular weight.     

Observing in-phase single-quantum 15N multiplets for NH2/NH3+ groups with two-dimensional heteronuclear correlation spectroscopy

Two-dimensional (2D) F1-(1)H-coupled HSQC experiments provide 3:1:1:3 and 1:0:1 multiplets for AX(3) and AX(2) spin systems, respectively. These multiplets occur because, in addition to the 2S(y)H(z)(a)-->2S(y)H(z)(a) process, the coherence transfers such as 2S(y)H(z)(a)-->2S(y)H(z)(b) occurring in t(1) period provide detectable magnetization during the t(2) period. Here, we present a 2D F1-(1)H-coupled (1)H-(15)N heteronuclear correlation experiment that provides a 1:3:3:1 quartet for AX(3) spin system and a 1:2:1 triplet for AX(2). The experiment is a derivative of 2D HISQC experiment [J. Iwahara, Y.S. Jung, G.M. Clore, Heteronuclear NMR spectroscopy for lysine NH(3) groups in proteins: unique effect of water exchange on (15)N transverse relaxation. J. Am. Chem. Soc. 129 (2007) 2971-2980] and contains a scheme that kills anti-phase single-quantum terms generated in the t(1) period. The purge scheme is essential to observe in-phase single-quantum multiplets. Applications to the NH(2) and NH(3)(+) groups in proteins are demonstrated.     

Enantiomeric excess measurements in weakly oriented chiral liquid crystal solvents through 2D H selective refocusing experiments

In this article, a simple and robust method is proposed for simplifying the analysis of proton spectra of molecules dissolved in weakly oriented chiral media. The NMR approach investigated is based on the use of proton selective refocusing 2D experiments (SERF) to measure proton–proton dipolar couplings from unresolved lines. This technique is applied to the case of enantiomers dissolved in chiral polypeptide liquid crystals. It is shown that an accurate determination of enantiomeric excess is possible within a short experimental time.     

Nuclear spin resonance of (129)Xe doped with O(2)

Spin-lattice relaxation of (129)Xe nuclei in solid natural xenon has been investigated in detail over a large range of paramagnetic O(2) impurity concentrations. Direct measurements of the ground state magnetic properties of the O(2) are difficult because the ESR (electron spin resonance) lines of O(2) are rather unstructured, but NMR measurements in the liquid helium temperature region (1.4-4 K) are very sensitive to the effective magnetic moments associated with the spin 1 Zeeman levels of the O(2) molecules and to the O(2) magnetic relaxation. From these measurements, the value of the D[Sz(2)-(1/3)S(2)] spin-Hamiltonian term of the triplet spin ground state of O(2) can be determined. The temperature and magnetic field dependence of the measured paramagnetic O(2)-induced excess line width of the (129)Xe NMR signal agree well with the theoretical model with the spin-Hamiltonian D=0.19 meV (2.3 K), and with the reasonable assumption that the E[S(x)(2)-S(y)(2)] spin-Hamiltonian term is close to 0 meV. An anomalous temperature dependence between 1.4 K and 4.2K of the (129)Xe spin-lattice relaxation rate, T(1n)(-1)(T), is also accounted for by our model. Using an independent determination of the true O(2) concentration in the Xe-O(2) solid, the effective spin lattice relaxation time (which will be seen to be transition dependent) of the O(2) at 2.3 K and 0.96 T is determined to be approximately 1.4 x 10(-8)s. The experimental results, taken together with the relaxation model, suggest routes for bringing highly spin-polarized (129)Xe from the low temperature condensed phase to higher temperatures without excessive depolarization.     

Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR

We report bulk superconductivity (SC) in Eu(0.2)Sr(0.8)(Fe(0.86)Co(0.14))(2)As(2) single crystals by means of electrical resistivity, magnetic susceptibility and specific heat measurements with T(c) is approximately equal to 20 K and an antiferromagnetic (AFM) ordering of Eu(2+) moments at T(N) is approximately equal to 2.0 K in zero field. (75)As NMR experiments have been performed in the two external field directions (H is parallel to ab) and (H is parallel to c). (75)As-NMR spectra are analysed in terms of first-order quadrupolar interaction. Spin-lattice relaxation rates (1/T(1)) follow a T(3) law in the temperature range 4.2-15 K. There is no signature of a Hebel-Slichter coherence peak just below the SC transition, indicating a non-s-wave or s(±) type of superconductivity. In the temperature range 160-18 K 1/T(1)T follows the C/(T+θ) law reflecting 2D AFM spin fluctuations.