Multiple quantum correlated spectroscopy revamped by asymmetric z-gradient echo detection signal intensity as a function of the read pulse flip angle as verified by heteronuclear 1H/31P experiments

Heteronuclear multiple quantum (n=+/-0 and n=+/-2) correlated spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED) experiments were performed on the spins 31P and 1H in a H3PO4 solution in order to determine the optimum flip angle for the read pulse. It has been shown that for the negative quantum signals, the maximum signals appear at beta=0, and for the positive quantum signals, the maximum signals appear at beta=pi. The CRAZED signals were compared to the single quantum signals in two-pulse two-gradient experiments. It is found that the CRAZED signals can also be distinguished into gradient echoes and spin echoes. The gradient-echo-type CRAZED signal requires beta=0 and the spin-echo-type CRAZED signal requires beta=pi for maximum echo intensities, in the same way as in single quantum experiments.     

Theoretical formalism and experimental verification of line shapes of NMR intermolecular multiple-quantum coherence spectra

Although the theories and potential applications of intermolecular multiple-quantum coherences (iMQCs) have been under active investigations for over a decade, discussion of iMQC NMR signal formation was mainly confined in the time domain. In this paper, a full line-shape theory was developed to describe iMQC signals in the frequency domain. Relevant features of the line shape, such as peak height, linewidth, and phase, were investigated in detail. Predictions based on the theory agree well with experimental and simulated results. Since radiation-damping effects always couple with iMQCs in highly polarized liquid-state NMR systems, and strongly radiation-damped signals have many spectral characteristics similar to those of iMQCs, a detailed comparison was also made between them from different spectral aspects. With detailed comparison of peak height, linewidth, and phase, this work demonstrates that the iMQC and radiation-damping phenomena result from two completely different physical mechanisms despite that both present similar signal features and coexist in highly polarized liquid-state NMR systems.     

Observation of satellite signals due to scalar coupling to spin-1/2 isotopes in solid-state nuclear magnetic resonance spectroscopy

A method is introduced to select the signal from a spin-1/2 nucleus I specifically bound to another spin-1/2 nucleus S for solid-state magic angle spinning nuclear magnetic resonance (NMR) spectroscopy via correlation through the heteronuclear J coupling. This experiment is analogous to the bilinear rotation decoupling (BIRD) sequence in liquid-state NMR spectroscopy which selects for signals from 1H directly bound to 13C. The spin dynamics of this modified BIRD experiment is described using the product-operator formalism, where experimental considerations such as rotor synchronization and the effect of large chemical shielding anisotropies on I and S are discussed. Two experiments are proposed that accommodate large chemical shielding anisotropies on S: (1) by stepping the inversion pulse frequency through the entire S spectral range or (2) by adiabatically inverting the S spins. Both these experiments are shown to successfully select the signal of 19F bound to 129Xe in XeF+ salts, removing the contributions from isotopomers containing non-spin-1/2 Xe isotopes. The feasibility in obtaining isotope-selective 19F spectra of inorganic fluoride compounds is discussed, and further modifications are proposed to expand the application to other chemical systems.     

Effectively doubling the magnetic field in spin-1/2-spin-1, HSQC, HDQC, coupled HSQC, and coupled HDQC in solution NMR

Pulse sequences for spin-1/2-spin-1 pair heteronuclear single quantum correlation (HSQC), heteronuclear double quantum correlation (HDQC), and coupled-HSQC, and coupled-HDQC NMR spectroscopies are outlined, and experimental realization for a (13)C-(2)H pair is demonstrated in solution state. In both the coupled versions, conditions for generation of in-phase and antiphase multiplets in either dimension are arrived at. The patterns and the intensity ratios are explained. The double quantum (2Q) experiments confirm doubling of both the shift frequency and the splitting due to coupling (to spin 1/2) of the 2Q coherence emanating from spin 1. The frequency doubling is equivalent to the corresponding single quantum (1Q) coherence at double the magnetic field strength. The coupling doubling, however, is independent of the magnetic field strength and a signature feature of the 2Q coherence. The ramification of the relative relaxation rates of 1Q and 2Q coherences is discussed.     

Dipolar recoupling of I = 1/2, S = 3/2 spin pairs with SEDOR for static and spinning samples

Spin-echo, double-resonance (SEDOR) dipolar recoupling experiments are illustrated on an I = 1/2, S = 3/2 spin system for static and spinning samples. An (15)N-(23)Na spin system is used to show that the simple pulse sequence is very effective in causing (15)N dipolar dephasing using either a (23)Na pi/2 recoupling pulse or a long radio-frequency (r.f.) recoupling pulse.     

Complete solution for two coupled spin-1/2 nuclei evolving under chemical shift and dipolar interaction

The spin dynamics for two coupled spin-1/2 nuclei evolving under isotropic chemical shifts and a dipole-dipole interaction, in the high-field limit, is presented using Fano's unit spherical tensors as a basis set. For simplicity, relaxation effects are ignored. The results are used to discuss the frequently used dipole-dipole approximations for both homonuclear and heteronuclear spin systems. In particular, it is shown that it is possible to analyse NMR spectra of two spin-1/2 systems where the chemical shift differences and the dipolar interactions are of similar magnitude.     

Flat pancake distant dipolar fields for enhancement of intermolecular multiple-quantum coherence signals

Intermolecular multiple-quantum coherences (iMQCs) originated from distant dipolar field (DDF) possess some appealing unique properties for magnetic resonance imaging (MRI). DDF is usually induced with continuous wave (i.e., sine- or square-wave) magnetization modulation in the whole sample. In this article, a spatially localized and enhanced DDF was optimally tailored in a thin slice with an adiabatic inversion pulse. Evidence was provided to show that careful tailoring of the spatially localized DDF can generate highly efficient iMQC signals, with more than two-fold enhancement compared to the conventional sine-wave magnetization modulation method, and 1.5 times of that with the square-wave modulation under the similar condition. Theoretical predictions, simulation results, and experimental verifications agree well with each other. Practical implementation of this approach for efficient iMQC MRI was explored.     

Simultaneous recording of spin-state-selective NMR spectra for different InS spin systems

Heteronuclear magnetization transfer occurring during heteronuclear cross-polarization mixing processes in liquid-state NMR experiments can be easily monitored as a function of the involved in-phase, antiphase, and multiple-quantum magnetization components. The theoretical background on the simultaneous detection of E.COSY-type, TROSY-type, or spin-edited multiplet patterns for different IS and I(2)S spin systems in the same solution-state NMR spectrum is described. The proposed pulse scheme preserves high sensitivity levels and shows good tolerance to the presence of undesired cross-talk artifacts for both NH and NH(2) multiplicities providing an interesting NMR tool for biomolecular applications.     

Multiple Quantum Coherences Hyperpolarized at Ultra-Low Fields

The development of hyperpolarization technologies enabled several yet exotic NMR applications at low and ultra-low fields (ULF), where without hyperpolarization even the detection of a signal from analytes is a challenge. Herein, we present a method for the simultaneous excitation and observation of homo- and heteronuclear multiple quantum coherences (from zero up to the third-order), which give an additional degree of freedom for ULF NMR experiments, where the chemical shift variation is negligible. The approach is based on heteronuclear correlated spectroscopy (COSY); its combination with a phase-cycling scheme allows the selective observation of multiple quantum coherences of different orders. The nonequilibrium spin state and multiple spin orders are generated by signal amplification by reversible exchange (SABRE) and detected at ULF with a superconducting quantum interference device (SQUID)-based NMR system.     

Heteronuclear recoupling in solid-state magic-angle-spinning NMR via overtone irradiation

A heteronuclear dipolar recoupling scheme applicable to I-S spin pairs undergoing magic-angle-spinning (MAS) is introduced, based on the overtone irradiation of one of the coupled nuclei. It is shown that when I is a quadrupole, for instance (14)N, irradiating this spin at a multiple of its Larmor frequency prevents the formation of MAS dipolar echoes. The ensuing S-spin signal dephasing is significant and dependent on a number of parameters, including the I-S dipolar coupling, the magnitude of I's quadrupolar coupling, and the relative orientations between these two coupling tensors. When applied to a spin-1 nucleus, this overtone recoupling method differs from hitherto proposed recoupling strategies in that it involves only the +/-1> I(z) eigenstates. Its dephasing efficiency becomes independent of first-order quadrupolar effects yet shows a high sensitivity to second-order offsets. A constant-time/variable-offset recoupling sequence thus provides a simple route to acquire, in an indirect fashion, (14)N overtone spectra from rotating powders. The principles underlying this kind of S-(14)N experiments and different applications involving S = (13)C, (59)Co sites are presented.     

Quenching echo modulations in NMR spectroscopy.

In NMR spectroscopy, homonuclear scalar couplings normally lead to modulations of spin echoes that tend to interfere with the accurate determination of transverse relaxation rates by Carr-Purcell-Meiboom-Gill (CPMG) multiple refocusing experiments. Surprisingly, the echo modulations are largely cancelled when the refocusing pulses applied to the coupling partner deviate slightly from ideal pi rotations due to tilted effective radio-frequency (RF) fields, even at offsets that are much smaller than the radio-frequency amplitude. Experiments and simulations illustrate these effects for two-spin IS systems containing donor and acceptor (15)N nuclei I=N (D) and S=N(A) in RNA Watson-Crick base pairs with homonuclear scalar couplings J(IS)=(2h)J(N(D), N(A)) across the hydrogen bonds.     

An electron paramagnetic resonance study of Mn2(H2O)(OAc)4(tmeda)2 (tmeda = N,N,N',N'-tetramethylethylenediamine): a model for dinuclear manganese enzyme active sites

The complex Mn2(H2O)(OAc)4(tmeda)2 (tmeda = N,N,N',N'-tetramethylethylenediamine) is a model for the active site of hydrolase enzymes containing acetate-bridged dimanganese cores. The two high-spin Mn(II) ions are antiferromagnetically coupled, as determined by previous magnetic susceptibility studies (Yu, S.-B; Lippard, S. J.; Shweky, I; Bino, A. Inorg. Chem. 1992, 31, 3502-3504) to yield a spin "ladder" with total spin S = 0, 1, 2, ..., 5 in increasing energy. In this study, the complex was characterized by Q-band and X-band EPR spectroscopy in frozen solution. Analysis of the temperature dependence of these EPR spectra indicates that the primary spectral contribution is from the S = 2 manifold. The EPR spectra were simulated using a full spin Hamiltonian for this manifold of a coupled spin system, which provided the fit parameters J = -2.9 cm-1, g = 2.00, and D2 = -0.060 +/- 0.003 cm-1. An additional multiline EPR signal is observed which is proposed to arise from the total spin S = 5/2 ground state of a Mn(II) trimer of the type Mn3(OAc)6(tmeda)2.     

Cross polarization from spins I=12 to spins S=1 in nuclear magnetic resonance with magic angle sample spinning

Spin locking of the nuclear magnetization of a spin with S=1 such as deuterium in the presence of a radio-frequency field under magic angle spinning (MAS) is described in terms of adiabatic modulations of the energy levels. In a brief initial period, part of the initial density operator nutates about the Hamiltonian and is dephased. The remaining spin-locked state undergoes persistent oscillatory transfer processes between various coherences with a periodicity given by the rotation of the sample. While all crystallites in the powder undergo such periodic transfer processes, the phases of the oscillations depend on the angle gamma of the crystallites. The angle gamma is the azimuthal angle defining the orientation of the unique axis of the quadrupolar interaction tensor in a rotor-fixed frame. The theory is extended to describe cross-polarization between spins S=1 and I=12 under MAS. There are four distinct Hartmann-Hahn matching conditions that correspond to four zero-quantum matching conditions, all of which are shifted and broadened compared to their spin S=12 counterparts. These matching conditions are further split into a family of sideband conditions separated by the spinning frequency. The theory allows the calculation of both shifts and broadening factors of the matching conditions, as verified by simulations and experiments.     

Communication: partial polarization transfer for single-scan spectroscopy and imaging

A method is presented to partially transfer nuclear spin polarization from one isotope S to another isotope I by the way of heteronuclear spin couplings, while minimizing the loss of spin order to other degrees of freedom. The desired I spin polarization to be detected is a design parameter, while the sequence of pulses at the two Larmor frequencies is optimized to store the greatest unused S spin longitudinal polarization for subsequent use. The unitary evolution for the case of I(N)S spin systems illustrates the potentially ideal efficiency of this strategy, which is of particular interest when the spin-lattice relaxation time of S greatly exceeds that of I. Explicit timing and pulses are tabulated for the cases for which M ≤ 10 partial transfers each result in equal final polarization of 1/M or more compared to the final I polarization expected in a single transfer for N = 1, 2, or 3 I spins. Advantages for the ratiometric study of reacting molecules and hyperpolarized initial conditions are outlined.     

Multiple Quantum Transitions Induced by Irradiating Single Quantum Coherence in a Heteronuclear Spin System

In a heteronuclear spin system, with initially prepared single quantum (SQ) coherence of X-spin, the irradiation of the proton spins will induce all the possible transitions including those SQs and multiple quanta (MQs) available in the system to be studied. The MQs appear in a rather weak irradiation while a strong irradiation results in a complete decoupling situation. Theoretical analysis is made to explain this phenomenon and is agreed qualitatively with experiments in the CH₂ group. This phenomenon of MQ induction may happen in a decoupling experiment, when the effective irradiation strength is rather weak and it may also provide a convenient approach to create and then to manipulate MQ coherences in heteronuclear spin systems.     

The use of high field/frequency EPR in studies of radical and metal sites in proteins and small inorganic models

Low temperature electron paramagnetic resonance (EPR) spectroscopy with frequencies between 95 and 345 GHz and magnetic fields up to 12 T have been used to study radicals and metal sites in proteins and small inorganic model complexes. We have studied radicals, Fe, Cu and Mn containing proteins. For S = 1/2 systems, the high frequency method can resolve the g-value anisotropy. It was used in mouse ribonucleotide reductase (RNR) to show the presence of a hydrogen bond to the tyrosyl radical oxygen. At 285 GHz the type 2 Cu(II) signal in the complex enzyme laccase is clearly resolved from the Hg(II) containing laccase peroxide adduct. For simple metal sites, the systems over S = 1/2 can be described by the spin Hamiltonian: H(S) = BgS + D[Sz2 - S(S + 1)/3 + E/D (Sx2 - Sy2)]. From the high frequency EPR the D-value can be determined directly by, (I) shifts of g(eff) for half-integer spin systems with large D-values as observed at 345 GHz on an Fe(II)-NO-EDTA complex, which is best described as S = 3/2 system with D = 11.5 cm(-1), E = 0.1 cm(-1) and gx = gy = gz = 2.0; (II) measuring the outermost signal, for systems with small D values, distant of (2S - 1) x absolute value(D) from the center of the spectrum as observed in S= 5/2 Fe(III)-EDTA. In Mn(II) substituted mouse RNR R2 protein the weakly interacting Mn(II) at X-band could be observed as decoupled Mn(II) at 285 GHz.     

NMR spectroscopy of citrate in solids: cross-polarization kinetics in weakly coupled systems

Solid-state NMR spectroscopy is a potentially powerful method for obtaining molecular level structural information crucial for understanding the specific relationship between calcite crystals and occluded organic molecules that are important in biomineralization and biomimetic materials. In this work, a method is developed based on cross-polarization/magic angle spinning (CP/MAS) NMR to measure the heteronuclear distances and obtain structural information for large intracrystalline citrate defects in a synthetic calcite/citrate composite. Using compounds with well-characterized crystal structures, Mg(II) citrate and Sr(II) citrate, a correlation is established between T(IS), the CP time, and M(2) (IS), the van Vleck heteronuclear dipolar second moment, which contains distance and structural information. This correlation is supported by peak assignments obtained from calculations of the (13)C chemical shifts for crystalline Mg(II) citrate. On the basis of T(IS) (-1) versus M(2) (IS) correlation, measurement of T(IS) for carbonate ions associated with citrate defects in a calcite((13)C-enriched)/citrate coprecipitate yields an estimate for the distance between citrate and the nearest carbonate carbon that indicates close spatial proximity and provides useful constraints for future computational study. The applicability of T(IS) (-1) versus M(2) (IS) correlations to other weakly coupled spin-1/2 systems is discussed in terms of the effects of (1)H homonuclear dipolar coupling, using the CP kinetics of Zn(II) dihydroxybenzoate and kaolinite for comparison. The results suggest a limited range of correlation constants and indicate that quantitative information can be obtained from CP/MAS kinetics obtained under similar experimental conditions.     

pi-topology and spin alignment utilizing the excited molecular field: observation of the excited high-spin quartet (S = 3/2) and quintet (S = 2) states on purely organic pi-conjugated spin systems.

As a model system for the photoinduced/photoswitched spin alignment in a purely organic pi-conjugated spin system, 9-[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1a), 9-[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1b), 9,10-bis[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2a), and 9,10-bis[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2b) were designed and synthesized. In these spin systems, 9-phenylanthracene and 9,10-diphenylanthracene were chosen as photo spin couplers and iminonitroxide was chosen as a dangling stable radical. Time-resolved electron spin resonance (TRESR) spectra of the first excited states with resolved fine-structure splittings were observed for 1a and 2a in an EPA or a 2-MTHF rigid glass matrix. Using the spectral simulation based on the eigenfield method, the observed TRESR spectra for 1a and 2a were unambiguously assigned as an excited quartet (S = 3/2) spin state (Q) and an excited quintet (S = 2) spin state (Qu), respectively. The g value and fine-structure splitting for the quartet state of 1a were determined to be g(Q) = 2.0043, D(Q) = 0.0235 cm(-1), and E(Q) = 0.0 cm(-1). The relative populations (polarization) of each M(S)() sublevel in Q were determined to be P(+1/2') = P(-1/2') = 0.5 and P(+3/2') = P(-3/2') = 0.0 with an increasing order of energy in zero magnetic field. The spin Hamiltonian parameters for Qu are g = 2.0043, D = 0.0130 cm(-1), and E = 0.0 cm(-1), and the relative populations in Qu were determined to be P(0') = 0.30, P(-1') = P(+1') = 0.35 and P(-2') = P(+2') = 0.0. These are the first observations of a photoexcited quartet and a quintet high-spin state in pi-conjugated triplet-radical pair systems. In contrast high-spin excited states were not observed for 1b and 2b, the pi-topological isomers of 1a and 2a, showing the role of pi-topology in the spin alignment of the excited states. Since a weak antiferromagnetic exchange interaction was observed in the ground state of 2a, the clear detection of the excited quintet high-spin state shows that the effective exchange coupling between the two dangling radicals through the diphenylanthracene spin coupler has been changed from antiferromagnetic to ferromagnetic upon photoexcitation. Thus, a photoinduced spin alignment utilizing the excited triplet molecular field was realized for the first time in the purely organic pi-conjugated spin system. Furthermore, the mechanism for the generation of dynamic electron spin polarization was investigated for the observed quartet and quintet states, and a plausible mechanism of the enhanced selective intersystem crossing was proposed. Ab initio molecular orbital calculations based on density functional theory were carried out to determine the electronic structures of the excited high-spin states and to understand the mechanism of the spin alignment utilizing the excited molecular field. The role of the spin delocalization and the spin polarization mechanisms were revealed on the photoexcited state.     

Improved methods for 1H-3H heteronuclear shift correlation

A better understanding of the structure of complex 3H-labeled molecules can be obtained by complete assignment of their 1H and 3H solution-state NMR spectra. The assignment process is aided by the detection of heteronuclear chemical shift correlations between 1H and 3H nuclei. Heteronuclear correlation (HETCOR) experiments previously applied to this task exhibit several drawbacks caused by the nature of both the pulse sequences and 1H-3H spin systems. The range of J-couplings involved in 1H-3H coupling networks make it challenging to perform correlation experiments using methods that rely on coherences created during free precession periods and interrupted by transfer pulses. Two alternative HETCOR experiments are demonstrated for 1H-3H systems in the present work and are shown to have advantages over earlier methods. The first experiment is known as hetero-TOCSY and correlates heteronuclear chemical shifts using J-cross polarization. This experiment achieves both homonuclear and heteronuclear mixing and connects the chemical shifts of all 1H and 3H nuclei in a coupling network. A second HETCOR experiment uses the heteronuclear Overhauser effect to obtain through-space correlations between nearby nuclei. The 1H-3H HETCOR experiments are phase sensitive and typically contain more correlations than other methods, which is beneficial for assignment purposes, while being sensitive enough to be applicable to routine analytical samples. The experiments were used to analyze 3H incorporation in sub-milligram quantities of 3H-labeled pharmaceutical derivatives with complex labeling schemes.     

The 2H,2H-INADEQUATE experiment: Detection of One- and Two-Spin Double Quantum Coherences

The INADEQUATE experiment for spin-1 nuclei has been extended to deuteron spin systems of the AX, AA′ XX′, and AA′MM′X type. Measurements with (3,3-D₂)norcampher, 4-chloro(2,3,5,6-D₄)pyridine, and (D₅)pyridine revealed correlation peaks due to two-spin double quantum coherences and additional one-spin double quantum peaks with coordinates F₁ = 2 ω_i, F₂ =(ω_ii = A, X) which were not observed in the ⁶Li, ⁶Li,-INADEQUATE experiment described earlier.