Transfer of CIDNP among coupled spins at low magnetic field

Coherent polarization transfer among groups of dynamically polarized spins is explored and applied to field cycling experiments where spin evolution proceeds at low magnetic field while observation is performed at high field. The case of two nonequivalent spins-1/2 with scalar spin coupling is considered theoretically in detail for the cases of sudden and adiabatic field change. The criterion for efficient polarization transfer is derived theoretically and consistently confirmed experimentally for three photochemical reactions, involving spin systems of increasing complexity that exhibit chemically induced dynamic nuclear polarization: (1) the two polarized protons of the purine base of adenosine monophosphate; (2) four coupled indole protons of tryptophan; and (3) long-range polarization transfer among the aliphatic protons of cycloundecanone. The importance of polarization transfer in other cases with non-equilibrium population of the nuclear spin levels and the possibility of its utilization in field cycling NMR studies are discussed.     

Solid-state NMR spectroscopy using the lost I spin magnetization in polarization transfer experiments

A variation of the cross polarization (CP) experiment is discussed. The method requires two scans where the difference signal is equivalent to the I spin magnetization that is transferred to the S spins. The acquired signal is equivalent to F1 sum projection of a two-dimensional (2D) heteronuclear correlation experiment and is obtained by just two scans without the need to increment the indirect time domain t(1). Any polarization transfer method and any kind of spin manipulations during the t(1) incrementation period of a 2D NMR experiment can be applied. The method allows fast measurements of the CP transfer, particularly if various S spins signal overlap and is good for spectral editing of I spin signals with contact to S spins. Various examples for biomaterials are presented. Most importantly, this novel approach is ideal for detailed investigations of organic-mineral interfaces in bone, here demonstrated for O-phospho-l-serine as simple model compound.     

Incorporating homonuclear polarization transfer into PRESS for proton spectral editing: illustration with lactate and glutathione

A proton spectral editing pulse sequence for the detection of metabolites with spin systems that involve weak coupling is presented. The sequence is based on homonuclear polarization transfer incorporated into the standard PRESS (Point RESolved Spectroscopy) sequence, which is a volume-selective double spin echo method, to enable spatial localization. All peaks in the region of interest are initially suppressed whether they are peaks from the target metabolite or from contaminating background. The target signal is then restored by polarization transfer from a proton that has a resonance outside the suppressed region and to which the target spins are weakly coupled. This is achieved by the application of a 90 degrees hard pulse with phase orthogonal to that of the PRESS excitation pulse at the location of the first echo in PRESS and by optimizing the two PRESS timings, TE(1) and TE(2), for most efficient yield. Background signal not coupled to any protons outside the initially saturated region remains suppressed. The advantage of this sequence compared to multiple quantum filters is that signal from singlet peaks outside the suppressed area are preserved and can thus be used as a reference. The efficacy of the sequence was verified experimentally on phantom solutions of lactate and glutathione at 3.0 T. For the AX(3) spin system of lactate, the sequence timings were optimized by product operator calculations whereas for the ABX spin system of the cysteinyl group of glutathione numerical calculations were performed for sequence timing optimization.     

ENDOR evidence for the quantum exchange of the methylene protons

ENDOR spectroscopy was used for studying methylene proton couplings in the 6-yl (or H-addition) radical in a single crystal of 1-methyluracil at 4.2K. The two methylene protons were magnetically equivalent in all crystal (radical) orientations in the magnetic field, indicating proton quantum exchange. The two protons compose magnetically single entity with spin I=1. The observed small ENDOR line splittings of about 0.60MHz agree well with the theoretical expectations for such a system.     

2D 1H-31P solid-state NMR studies of the dependence of inter-bilayer water dynamics on lipid headgroup structure and membrane peptides

The dynamics of hydration-water in several phospholipid membranes of different compositions is studied by 2D (1)H-(31)P heteronuclear correlation NMR under magic-angle spinning. By using a (1)H T(2) filter before and a (1)H mixing-time after the evolution period and (31)P detection, inter-bilayer water is selectively detected without resonance overlap from bulk water outside the multilamellar vesicles. Moreover the (1)H T(2) relaxation time of the inter-bilayer water is measured. Lipid membranes with labile protons either in the lipid headgroup or in sterols exhibit water-(31)P correlation peaks while membranes free of exchangeable protons do not, indicating that the mechanism for water-lipid correlation is chemical exchange followed by relayed magnetization transfer to (31)P. In the absence of membrane proteins, the inter-bilayer water (1)H T(2)'s are several tens of milliseconds. Incorporation of charged membrane peptides shortened this inter-bilayer water T(2) significantly. This T(2) reduction is attributed to the peptides' exchangeable protons, molecular motion and intermolecular hydrogen bonding, which affect the water dynamics and the chemically relayed magnetization transfer process.     

ESR Spin Trapping of the Reaction Between Urate and Peroxynitrite: the Hydrogen Adduct

Under basic conditions, the hydrogen adduct of the spin traps N-t-butyl-phenylnitrone (PBN) and α-(4-pyridyl-1-oxide)-N-t-butylnitrone (POBN) is formed as a major product in the reaction between the urate anion or dianion and peroxynitrite. It shows a characteristic nine-line spectrum with an intensity pattern of (1:2:1:1:2:1:1:2:1) and hyperfine splitting constants of a(H) = 10.68 G (two magnetically equivalent protons) and a(N) = 16.64 G. The hydrogen adduct of PBN also forms in the absence of urate. In this case, its formation is proposed to follow the non-traditional “inverted spin trapping” mechanism, followed by electron transfer between spin traps and the PBN–OH (or POBN–OH) adduct or one of its decomposition products. The H adduct formation is amplified when uric acid and peroxynitrite are present. Different yields of H adducts obtained from various spin traps supported the inverted spin trapping mechanism and can be rationalized by the relative redox potential of the spin traps.     

Spectral Artifacts Resulting from Multiple-Quantum Coherence Transfers by Spin Decoupling during Acquisition

Off-resonance coherent decoupling of spin X in a heteronuclear SX_N spin system can result in a multiplet with overlapping lines of spin Sand an overall shape similar to that of a doublet. Such lineshape distortions occur if certain multiple-spin coherences are present when the decoupler is switched on. The distortion could cause an apparent change in the multiplicity of an M + 1 multiplet of an SX_NY_M spin system when a selective decoupling of spins X is applied after a polarization transfer from spins X to spin S by INEPT or DEPT. Possibilities for suppressing the distortions are discussed.     

Application of the maximum likelihood principle to separate exponential terms in T2 relaxation of nuclear magnetic resonance

The method of maximum likelihood has been implemented for the estimation of multiple exponential components of T2 decay curves in spin echo NMR measurements on biologic tissues. Each Each component contributes an exponential term described by two parameters (initial amplitude and T2) to the T2 decay curve. The maximum likelihood method estimates the parameters and their standard errors for all terms simultaneously, avoiding the subjectivity inherent in methods such as graphical peeling. In the model used, it was assumed that water protons are compartmentalized and that the measured spin echo signals from the protons undergoing relaxation obey the Poisson distribution. A system of non-linear equations was derived and solved iteratively for the values of the exponential parameters which maximize the likelihood of obtaining the observed data under these assumptions. The approach was implemented for bi- and tri-exponential models on a MicroVAX II computer (Digital Equipment Corporation, Maynard, MA). Simulations of bi- and tri-exponential data, with and without system noise, were analyzed to assess the accuracy and reproducibility of the method. A subset of the simulations was repeated with non-linear least squares techniques and was compared to the results obtained with maximum likelihood. Rabbit muscle and gerbil brain samples were measured and analyzed with the maximum likelihood method. The simulations showed that within specific limits on relative sizes and relaxation rates of components, these parameters can be estimated with errors less than 5%. The comparison to non-linear least squares analysis showed that the maximum likelihood method is generally superior in estimating the parameters in difficult cases. The results from tissue measurements demonstrate that the method is effective even in cases where graphical peeling would clearly not yield reliable results.     

A 1D sensitivity-enhanced 1H spin diffusion experiment for determining membrane protein topology

A sensitivity-enhanced 1D (1)H spin diffusion experiment, CHH, for determining membrane protein topology is introduced. By transferring the magnetization of the labeled protein (13)C to lipid and water protons for detection, the CHH experiment reduces the time of the original 2D (13)C-detected experiment by two orders of magnitude. The sensitivity enhancement results from (1)H detection and the elimination of the (13)C dimension. Consideration of the spin statistics of the membrane sample indicates that the CHH sensitivity depends on the (13)C labeling level and the number of protein protons relative to the mobile protons. 5-35% of the theoretical sensitivity was achieved on two extensively (13)C labeled proteins. The experimental uncertainties arise from incomplete suppression of the equilibrium (1)H magnetization and the magnetization of lipid protons directly bonded to natural-abundance carbons. The technique, demonstrated on colicin Ia channel domain, confirms the presence of a transmembrane domain and the predominance of surface-bound helices.     

Realization of a decoherence-free subspace using multiple quantum coherences

This Letter presents a two-dimensional nuclear magnetic resonance (NMR) approach for constructing a two-logical-qubit decoherence-free subspace (DFS) by using four multiple-quantum coherences of a CH3 spin system as logical qubits. The three protons in this spin system are magnetically equivalent and can only be used as a single qubit in one-dimensional NMR. We have experimentally demonstrated that our DFS can protect against more types of decoherences than those of the one composed of four noisy physical qubits all with different chemical shifts. This idea may provide new insights into extending qubit systems in the sense that it effectively utilizes the magnetically equivalent nuclei.     

A note on the intensities of N.M.R. absorption lines

A proof is given of the proposition that the intensity of the N.M.R. absorption line from a group of n equivalent protons is proportional to n. To do this, it has been necessary to satisfy the Pauli exclusion principle by classifying states according to the irreducible representations of the symmetric group. As far as spin is concerned, this classification is equivalent to one according to the value of I, the total spin. Degeneracy with respect to I, which would otherwise occur, is prohibited by the exclusion principle, and a correlation (not one-to-one) is established between I and the symmetries of the rotational states.     

Two examples of in-flight spin flippers

In precise experiments with polarized beam it’s very often appear a necessity to change beam polarization on opposite. If such operation does not change other beam parameters, it helps to avoid or minimize some systematic errors. It is especially important in experiments, where spin dependent effect is small enough. This paper describes two set of equipments, that make spin flip for extracted beams. In both cases, these devices are absolutely distinct, because they are appropriate for different particles and at different energy range. The first of them is intended for future muon (g-2) experiment, which is under preparation now at JPARC. Here, the muon spin flip will be done by chain of electrostatic and magnetic bends at the kinetic energy 340 keV. A beam matching is provided by a number of short solenoids. The other flipper (or Siberian snake) will rotate spin of protons or antiprotons, which come from Λ-meson decay with the energy up to 40 GeV. This experiment (no. 24) is planed at IHEP, Protvino. In this case, two superconducting helical magnets with opposite helicities and magnetic field 4.5 T will be used. To correct beam trajectory, additional dipole correctors are required.     

IFSERF, an isotope-filtered SERF experiment for the precise measurement of proton-proton coupling constants between chemically equivalent protons

An isotope-filtered selective refocusing (IFSERF) experiment is presented for the sensitive and precise measurement of the proton-proton coupling constant between chemically equivalent protons. The 2D NMR method combines an initial doubly selective isotope filter based on heteronuclear cross-polarization followed by a selective J-resolved block. The coupling topologies obtained from several 2D variants of the IFSERF experiment are described for the simultaneous measurement of both proton-proton and proton-carbon coupling constants in the involved AA'XX' spin system. Application on the determination of the relative configuration of double bonds in symmetrical molecules is illustrated.     

Theoretical Study of Dipolar Relaxation of Coupled Nuclear Spins at Variable Magnetic Field

A theoretical study was made of magnetic field-dependent dipolar relaxation in two- and three-spin systems. The results for the nuclear magnetic relaxation dispersion (NMRD) curves were compared with those for the simpler model of fluctuating local fields. For both models it was found that at low fields spins tend to relax with a common T ₁-relaxation time. Sharp features in the NMRD curves coming from nuclear spin level anti-crossings are also predicted by both models. However, the simple model fails to describe the behavior of so-called long-lived spin states (LLS). We have studied the LLS as function of magnetic field and molecular geometry and simulated experimental results for the LLS in histidine amino acid obtained at the laboratory of Prof. H.-M. Vieth (Free University Berlin, Germany). In addition, we described polarization transfer in a three-spin system where two spins are protons, which are initially hyperpolarized by para-hydrogen induced polarization (PHIP), while the third spin is a spin ½ hetero-nucleus, which acquires polarization in the course of cross-relaxation.     

双磁性中心内嵌富勒烯Y_2C_2@C_(82)-C_2(1)中的超快自旋动力学行为

自旋翻转和自旋转移是实现基于内嵌富勒体系自旋逻辑功能器件设计的先决条件.本文以双磁性中心内嵌富勒烯Y_2C_2@C8_2-C_2(1)体系为例,采用第一性原理计算方法,结合Λ进程理论模型和自编的遗传算法程序,在该内嵌富勒烯体系中分别实现了亚皮秒时间尺度内的自旋翻转和自旋转移过程.计算结果表明,优化后的内嵌Y_2C_2团簇结构和实验得到的各项数据基本吻合,并且会对外部的C8_2-C_2(1)笼结构产生一定的排斥力,但由于富勒烯笼状结构具有很强的稳定性,所以整个体系仍然保持碳笼结构的完整性.通过对自旋密度分布与激光脉冲作用下自旋期望值演化的具体分析,经由Λ进程的自旋翻转是基于两个Y元素的整体自旋翻转;自旋转移则源自两个磁性中心以及碳笼之间在激光脉冲作用下的自旋密度重新分布.本文结果揭示了Y_2C_2@C8_2-C_2(1)体系中的超快自旋动力学机理,可望为基于实际内嵌富勒烯分子的自旋逻辑功能器件设计提供理论依据.     

Evidence for a dipolar-coupled AM system in carnosine in human calf muscle from in vivo 1H NMR spectroscopy

Spin systems with residual dipolar couplings such as creatine, taurine, and lactate in skeletal muscle tissue exhibit first-order spectra in in vivo 1H NMR spectroscopy at 1.5 T because the coupled protons are represented by (nearly) symmetrized eigenfunctions. The imidazole ring protons (H2, H4) of carnosine are suspected to form also a coupled system. The ring's stiffness could enable a connectivity between these anisochronous protons with the consequence of second-order spectra at low field strength. Our purpose was to study whether this deviation from the Paschen-Back condition can be used to detect the H2-H4 coupling in localized 1D 1H NMR spectra obtained at 1.5 T (64 MHz) from the human calf in a conventional whole-body scanner. As for the hydrogen hyperfine interaction, a Breit-Rabi equation was derived to describe the transition from Zeeman to Paschen-Back regime for two dipolar-coupled protons. The ratio of the measurable coupling strength (Sk) and the difference in resonance frequencies of the coupled spins (Deltaomega) induces quantum-state mixing of various degree upon definition of an appropriate eigenbase of the coupled spin system. The corresponding Clebsch-Gordan coefficients manifest in characteristic energy corrections in the Breit-Rabi formula. These additional terms were used to define an asymmetry parameter of the line positions as a function of Sk and Deltaomega. The observed frequency shifts of the resonances were found to be consistent with this parameter within the accuracy achievable in in vivo NMR spectroscopy. Thus it was possible to identify the origin of satellite peaks of H2, H4 and to describe this so far not investigated type of residual dipolar coupling in vivo.     

Clean HMBC: suppression of strong-coupling induced artifacts in HMBC spectra

A new experiment, clean HMBC, is introduced for suppression of strong-coupling induced artifacts in HMBC spectra. The culprits of these artifacts are an inherent shortcoming of low-pass J filters in the presence of strong coupling and the (1)H pi pulse in the middle of the evolution period aimed at suppressing evolution under heteronuclear J couplings and (1)H chemical shifts. A pi pulse causes coherence transfer in strongly coupled spin systems and, as is well known in e.g., homonuclear J spectra, this leads to peaks that would not be there in the absence of strong coupling. Similar artifacts occur in HMBC spectra, but they have apparently been overlooked, presumably because they have been assigned to inefficiency of low-pass J filters or not noticed because of a coarse digital resolution in the spectra. Clean HMBC is the HMBC technique of choice for molecules notorious for strong coupling among protons, such as carbohydrates, and the new technique is demonstrated on D-mannose. Finally, a fundamental difference between HMBC and H2BC explains why strong-coupling artifacts are much less of a problem in the latter type of spectra.     

三自旋体系核自旋单重态的制备与单重态二维谱的实现

核自旋单重态是一种特殊的自旋状态,其寿命远长于相应自旋的横向和纵向弛豫时间,能够被用于研究分子的慢扩散、慢运动、特征信号选择等过程.目前单重态的研究主要集中于孤立的两自旋体系.而本文以N-乙酰基天冬氨酸（NAA）分子中由亚甲基和次甲基的三个氢原子核构成的三自旋体系为研究对象,将亚甲基中的两个氢核制备成单重态.利用优化控制和数值计算方法,分别设计了包含和不包含次甲基氢核耦合的单重态制备脉冲,结果发现,不考虑次甲基氢耦合设计的优化脉冲,其在实际三自旋体系中的单重态制备效率会显著下降.另外,我们以单重态为起点,实现了针对次甲基和亚甲基的信号选择COSY谱和NOESY谱,结果表明基于单重态的二维谱能够有效避免谱峰重叠现象,提高谱图分辨率,并有助于提高分子结构解析的准确性.     

Coherent Polarization Transfer Effects Are Crucial for Interpreting Low-Field CIDNP Data

In this work we demonstrate that low-field chemically induced dynamic nuclear polarization (CIDNP) is strongly affected by re-distribution of polarization, which is formed in the course of spin evolution in transient radical pairs, in diamagnetic reaction products. This phenomenon is of importance when the spins of the reaction product are coupled strongly meaning that spin–spin interactions between them are comparable to the differences in their Zeeman interactions with the external magnetic field. In this case, polarization transfer relies on a coherent mechanism; as a consequence, spins can acquire significant polarization even when they have no hyperfine coupling to the electron spins in the radical pairs, i.e., cannot be polarized directly by CIDNP. This is demonstrated by taking CIDNP of n-butylamine as an example: in this case only the α-CH₂ protons are polarized directly, which is confirmed by high-field CIDNP, whereas the β-CH₂, γ-CH₂ and δ-CH₃ protons get polarized only indirectly due to the transfer of polarization from the α-CH₂ protons. These results show that low-field CIDNP data should be interpreted with care to discriminate between the effects of spin evolution in transient radical pairs and in diamagnetic reaction products.     

Cross-correlation effects involving curie spin relaxation in methyl groups

Cross-correlation effects arising in methyl protons due to the simultaneous presence of dipole-dipole, chemical shift anisotropy, and Curie spin relaxation mechanisms in paramagnetic systems are analyzed. We assess the potential of obtaining structural constraints from the cross-correlation of Curie spin relaxation with dipolar relaxation mechanisms among methyl proton spins. By theoretical analysis and numerical simulations we characterize the transfer functions describing the interconversion processes of different ranks of multispin order. The time dependence of these processes contains a new type of structural information, the orientation of the methyl C(3)-axis with respect to the electron center. Experimental confirmation is found for selected methyl groups in low spin Fe(3+) sperm whale myoglobin.