Two-photon Lee-Goldburg nuclear magnetic resonance: Simultaneous homonuclear decoupling and signal acquisition

We present NMR signals from a strongly coupled homonuclear spin system, (1)H nuclei in adamantane, acquired with simultaneous two-photon excitation under conditions of the Lee-Goldburg experiment. Small coils, having inside diameters of 0.36 mm, are used to achieve two-photon nutation frequencies of approximately 20 kHz. The very large rf field strengths required give rise to large Bloch-Siegert shifts that cannot be neglected. These experiments are found to be extremely sensitive to inhomogeneity of the applied rf field, and due to the Bloch-Siegert shift, exhibit a large asymmetry in response between the upper and lower Lee-Goldburg offsets. Two-photon excitation has the potential to enhance both the sensitivity and performance of homonuclear dipolar decoupling, but is made challenging by the high rf power required and the difficulties introduced by the inhomogeneous Bloch-Siegert shift. We briefly discuss a variation of the frequency-switched Lee-Goldburg technique, called four-quadrant Lee-Goldburg (4QLG) that produces net precession in the x-y plane, with a reduced chemical shift scaling factor of 13.     

PITANSEMA-MAS, a solid-state NMR method to measure heteronuclear dipolar couplings under MAS

A 2D NMR method is presented for the measurement of the dipole-dipole interaction between a proton and a low-frequency nuclear spin species in the solid state under the magic angle spinning. It employs the time averaged nutation concept to dramatically reduce the required radio frequency (rf) power on the low γ nuclear channel and spin exchange at the magic angle is used to suppress (1)H-(1)H dipolar interactions and chemical shifts. The flexibility in choosing the spinning speed, rf power and the scaling factor of the pulse sequence are of considerable importance for the structural studies of biological solids. The performance of the pulse sequence has been numerically and experimentally demonstrated on several solids.     

PITANSEMA, a low-power PISEMA solid-state NMR experiment

A low radio frequency power polarization inversion spin exchange at the magic angle (PISEMA) pulse sequence is described for the measurement of heteronuclear dipolar couplings from solids. The method employs a time averaged nutation concept to significantly reduce the rf power required to spin-lock low gamma nuclear spins in PISEMA experiments. The efficacy of the 2D method is demonstrated on a single crystal of n-acetyl-L-(15)N-valyl-L-(15)N-leucine dipeptide to measure (1)H-(15)N dipolar couplings and a liquid crystal sample to measure (1)H-(13)C dipolar couplings.     

Theory of stochastic dipolar recoupling in solid-state nuclear magnetic resonance

Dipolar recoupling techniques in solid-state nuclear magnetic resonance (NMR) consist of radio frequency (rf) pulse sequences applied in synchrony with magic-angle spinning (MAS) that create nonzero average magnetic dipole-dipole couplings under MAS. Stochastic dipolar recoupling (SDR) is a variant in which randomly chosen rf carrier frequency offsets are introduced to cause random phase modulations of individual pairwise couplings in the dipolar spin Hamiltonian. Several aspects of SDR are investigated through analytical theory and numerical simulations: (1) An analytical expression for the evolution of nuclear spin polarization under SDR in a two-spin system is derived and verified through simulations, which show a continuous evolution from coherent, oscillatory polarization exchange to incoherent, exponential approach to equilibrium as the range of random carrier offsets (controlled by a parameter f(max)) increases; (2) in a many-spin system, polarization transfers under SDR are shown to be described accurately by a rate matrix in the limit of large f(max), with pairwise transfer rates that are proportional to the inverse sixth power of pairwise internuclear distances; (3) quantum mechanical interferences among noncommuting pairwise dipole-dipole couplings, which are a complicating factor in solid-state NMR studies of molecular structures by traditional dipolar recoupling methods, are shown to be absent from SDR data in the limit of large f(max), provided that coupled nuclei have distinct NMR chemical shifts.     

1H to 2H uniform cross-polarization nuclear magnetic resonance using 2H Lee-Goldburg irradiation in static powders

Application of conventional cross polarization (CP) to (2)H results in only a narrowband enhancement of the powder line shape due to the quadrupole interaction. We propose a CP scheme to uniformly enhance (2)H spectra in static powders. In this method, a Hartmann-Hahn matched (2)H rf field is applied on the Lee-Goldburg (LG) condition to remove the zeroth-order quadrupole interaction. In order to achieve a uniformly enhanced (2)H powder line shape with a limited (2)H rf intensity, the (1)H rf amplitude in CP is stepwise altered during the contact time. We develop a spin-thermodynamic theory to describe polarization transfer due to CP with LG irradiation (LG-CP) under the influence of the quadrupole interaction, which can successfully reproduce the LG-CP line shapes observed under various experimental conditions. Experimental and simulated (2)H powder spectra are reported for some compounds.     

Resonant transfer of the nuclear spin dipolar order in the rotating frame

A method for transferring the nuclear spin dipolar energy in a solid lattice is presented. It is shown that order may be transferred between the Zeeman and nuclear spin dipolar systems in either direction by rf pulses of suitable amplitude. This method may also be used to measure the dipolar relaxation time. Experimental data on ammonium chloride are shown to be in close agreement with the theory.     

Effective Hamiltonians by optimal control: solid-state NMR double-quantum planar and isotropic dipolar recoupling

We report the use of optimal control algorithms for tailoring the effective Hamiltonians in nuclear magnetic resonance (NMR) spectroscopy through sophisticated radio-frequency (rf) pulse irradiation. Specifically, we address dipolar recoupling in solid-state NMR of powder samples for which case pulse sequences offering evolution under planar double-quantum and isotropic mixing dipolar coupling Hamiltonians are designed. The pulse sequences are constructed numerically to cope with a range of experimental conditions such as inhomogeneous rf fields, spread of chemical shifts, the intrinsic orientation dependencies of powder samples, and sample spinning. While the vast majority of previous dipolar recoupling sequences are operating through planar double-or zero-quantum effective Hamiltonians, we present here not only improved variants of such experiments but also for the first time homonuclear isotropic mixing sequences which transfers all I(x), I(y), and I(z) polarizations from one spin to the same operators on another spin simultaneously and with equal efficiency. This property may be exploited to increase the signal-to-noise ratio of two-dimensional experiments by a factor of square root 2 compared to conventional solid-state methods otherwise showing the same efficiency. The sequences are tested numerically and experimentally for a powder of (13)C(alpha),(13)C(beta)-L-alanine and demonstrate substantial sensitivity gains over previous dipolar recoupling experiments.     

Selective excitation in spin systems with homogeneous broadening

We have found that the application of a weak radio frequency (rf) pulse to a spin system with indirect spin-spin (J) couplings can produce a narrow inverted peak in spectral regions where there are many overlapping peaks. Examples of three compounds with (1)H-(1)H J couplings and one with (1)H-(13)C J couplings are given. The dependences of the signals on the frequency, duration, and amplitude of the weak rf irradiation have been studied. For an rf power of gammaB(1)/2pi=1.0 Hz, pulse widths longer than 0.25 s consistently produce inverted narrow peaks in spectral regions with unresolved peaks. An interpretation of the origin of this unusual result of selective excitation in spin systems with homogeneous broadening is given: the inverted signal observed can be considered as a cumulative effect of the weak rf irradiation acting on many uncorrelated transitions that are slightly off resonance on both sides of the irradiating frequency; the only role of the J couplings (or dipolar couplings in liquid crystals) is to produce a large number of closely spaced peaks that overlap with each other. Computer simulations of both coupled and noninteracting spin systems have been carried out, and the results support this interpretation.     

Spin dynamics in the modulation frame: application to homonuclear recoupling in magic angle spinning solid-state NMR

We introduce a family of solid-state NMR pulse sequences that generalizes the concept of second averaging in the modulation frame and therefore provides a new approach to perform magic angle spinning dipolar recoupling experiments. Here, we focus on two particular recoupling mechanisms-cosine modulated rotary resonance (CMpRR) and cosine modulated recoupling with isotropic chemical shift reintroduction (COMICS). The first technique, CMpRR, is based on a cosine modulation of the rf phase and yields broadband double-quantum (DQ) (13)C recoupling using >70 kHz omega(1,C)/2pi rf field for the spinning frequency omega(r)/2=10-30 kHz and (1)H Larmor frequency omega(0,H)/2pi up to 900 MHz. Importantly, for p>or=5, CMpRR recouples efficiently in the absence of (1)H decoupling. Extension to lower p values (3.5相似文献   

Measurement of carbon-proton dipolar couplings in liquid crystals using DAPT

Dipolar couplings provide valuable information on order and dynamics in liquid crystals. For measuring heteronuclear dipolar couplings in oriented systems, a new separated local field experiment is presented here. The method is based on the dipolar assisted polarization transfer (DAPT) pulse sequence proposed recently (Chem. Phys. Lett. 2007, 439, 407) for transfer of polarization between two spins I and S. DAPT utilizes the evolution of magnetization of the I and S spins under two blocks of phase shifted BLEW-12 pulses on the I spin separated by a 90 degree pulse on the S spin. Compared to the rotating frame techniques based on Hartmann-Hahn match, this approach is easy to implement and is independent of any matching conditions. DAPT can be utilized either as a proton encoded local field (PELF) technique or as a separated local field (SLF) technique, which means that the heteronuclear dipolar coupling can be obtained by following either the evolution of the abundant spin like proton (PELF) or that of the rare spin such as carbon (SLF). We have demonstrated the use of DAPT both as a PELF and as a SLF technique on an oriented liquid crystalline sample at room temperature and also have compared its performance with PISEMA. We have also incorporated modifications to the original DAPT pulse sequence for (i) improving its sensitivity and (ii) removing carrier offset dependence.     

Determination of order parameters of liquid crystals: use of dipolar oscillations enhanced by Lee-Goldburg decoupling

Lee-Goldburg decoupling has been applied to enhance dipolar oscillations during the t1 period of the two-dimensional experiment used for obtaining order parameters of the liquid crystal N-(4-ethoxybenzylidene)-4-n-butylaniline (EBBA) at different temperatures. The order parameters of the aromatic core obtained by this method are in agreement with values obtained from the 2H NMR spectra of a selectively labelled compound and are observed to follow the empirical Haller equation.     

Development of modulated rf sequences for decoupling and recoupling of nuclear-spin interactions in sample-spinning solid-state NMR: application to chemical-shift anisotropy determination

An approach to design modulated rf sequences under sample spinning which decouple/recouple a specific nuclear-spin interaction in solid-state NMR is presented. The Euler angles of the spin rotation caused by a general rf field are forced to fulfill the symmetry principle theory for selecting an interaction of interest. Then, modulated rf sequences are directly obtained from the Euler angles with a large degree of freedom. rf sequences with high performance can be selected from them by numerically optimizing rf sequence parameters. As an example of this approach, an amplitude- and phase-modulated rf sequence to recouple chemical-shift anisotropy (CSA) is developed, which is robust with respect to rf inhomogeneity. Two-dimensional (2D) experiments with this rf sequence under on and off magic-angle spinning (MAS) provide one-dimensional and 2D powder patterns, respectively. The latter enables us to determine the CSA principal values more accurately even for overlapped signals in MAS spectra. The effectiveness of this modulated rf sequence is experimentally demonstrated on [(15)N]-N-acetyl-D,L-alanine for determination of the (15)N and (13)CO CSA principal values.     

Heteronuclear proton assisted recoupling

We describe a theoretical framework for understanding the heteronuclear version of the third spin assisted recoupling polarization transfer mechanism and demonstrate its potential for detecting long-distance intramolecular and intermolecular (15)N-(13)C contacts in biomolecular systems. The pulse sequence, proton assisted insensitive nuclei cross polarization (PAIN-CP) relies on a cross term between (1)H-(15)N and (1)H-(13)C dipolar couplings to mediate zero- and∕or double-quantum (15)N-(13)C recoupling. In particular, using average Hamiltonian theory we derive effective Hamiltonians for PAIN-CP and show that the transfer is mediated by trilinear terms of the form N(±)C(?)H(z) (ZQ) or N(±)C(±)H(z) (DQ) depending on the rf field strengths employed. We use analytical and numerical simulations to explain the structure of the PAIN-CP optimization maps and to delineate the appropriate matching conditions. We also detail the dependence of the PAIN-CP polarization transfer with respect to local molecular geometry and explain the observed reduction in dipolar truncation. In addition, we demonstrate the utility of PAIN-CP in structural studies with (15)N-(13)C spectra of two uniformly (13)C,(15)N labeled model microcrystalline proteins-GB1, a 56 amino acid peptide, and Crh, a 85 amino acid domain swapped dimer (MW=2×10.4 kDa). The spectra acquired at high magic angle spinning frequencies (ω(r)∕2π>20 kHz) and magnetic fields (ω(0H)∕2π=700-900 MHz) using moderate rf fields, yield multiple long-distance intramonomer and intermonomer (15)N-(13)C contacts. We use these distance restraints, in combination with the available x-ray structure as a homology model, to perform a calculation of the monomer subunit of the Crh protein.     

Dipolar DC Collisional Activation in a “Stretched” 3-D Ion Trap: The Effect of Higher Order Fields on rf-Heating

Applying dipolar DC (DDC) to the end-cap electrodes of a 3-D ion trap operated with a bath gas at roughly 1 mTorr gives rise to ‘rf-heating’ and can result in collision-induced dissociation (CID). This approach to ion trap CID differs from the conventional single-frequency resonance excitation approach in that it does not rely on tuning a supplementary frequency to coincide with the fundamental secular frequeny of the precursor ion of interest. Simulations using the program ITSIM 5.0 indicate that application of DDC physically displaces ions solely in the axial (inter end-cap) dimension whereupon ion acceleration occurs via power absorption from the drive rf. Experimental data shows that the degree of rf-heating in a stretched 3-D ion trap is not dependent solely on the ratio of the dipolar DC voltage/radio frequency (rf) amplitude, as a model based on a pure quadrupole field suggests. Rather, ion temperatures are shown to increase as the absolute values of the dipolar DC and rf amplitude both decrease. Simulations indicate that the presence of higher order multi-pole fields underlies this unexpected behavior. These findings have important implications for the use of DDC as a broad-band activation approach in multi-pole traps.     

SLIM: A Short-Linked,Highly Redox-Stable Trityl Label for High-Sensitivity In-Cell EPR Distance Measurements

The understanding of biomolecular function is coupled to knowledge about the structure and dynamics of these biomolecules, preferably acquired under native conditions. In this regard, pulsed dipolar EPR spectroscopy (PDS) in conjunction with site-directed spin labeling (SDSL) is an important method in the toolbox of biophysical chemistry. However, the currently available spin labels have diverse deficiencies for in-cell applications, for example, low radical stability or long bioconjugation linkers. In this work, a synthesis strategy is introduced for the derivatization of trityl radicals with a maleimide-functionalized methylene group. The resulting trityl spin label, called SLIM, yields narrow distance distributions, enables highly sensitive distance measurements down to concentrations of 90 nm , and shows high stability against reduction. Using this label, the guanine-nucleotide dissociation inhibitor (GDI) domain of Yersinia outer protein O (YopO) is shown to change its conformation within eukaryotic cells.     

NMR polarization echoes in a nematic liquid crystal

We have modified the polarization echo (PE) sequence through the incorporation of Lee-Goldburg cross polarization steps to quench the 1H-1H dipolar dynamics. In this way, the 13C becomes an ideal local probe to inject and detect polarization in the proton system. This improvement made possible the observation of the local polarization P(00)(t) and polarization echoes in the interphenyl proton of the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline. The decay of P(00)(t) was well fitted to an exponential law with a characteristic time tau(C) approximately 310 micros. The hierarchy of the intramolecular dipolar couplings determines a dynamical bottleneck that justifies the use of the Fermi Golden Rule to obtain a spectral density consistent with the structural parameters. The time evolution of P(00)(t) was reversed by the PE sequence generating echoes at the time expected by the scaling of the dipolar Hamiltonian. This indicates that the reversible 1H-1H dipolar interaction is the main contribution to the local polarization decrease and that the exponential decay for P(00)(t) does not imply irreversibility. The attenuation of the echoes follows a Gaussian law with a characteristic time tau(phi) approximately 527 micros. The shape and magnitude of the characteristic time of the PE decay suggest that it is dominated by the unperturbed homonuclear dipolar Hamiltonian. This means that tau(phi) is an intrinsic property of the dipolar coupled network and not of other degrees of freedom. In this case, one cannot unambiguously identify the mechanism that produces the decoherence of the dipolar order. This is because even weak interactions are able to break the fragile multiple coherences originated on the dipolar evolution, hindering its reversal. Other schemes to investigate these underlying mechanisms are proposed.     

Accurate heteronuclear distance measurements at all magic-angle spinning frequencies in solid-state NMR spectroscopy

Heteronuclear dipolar coupling is indispensable in revealing vital information related to the molecular structure and dynamics, as well as intermolecular interactions in various solid materials. Although numerous approaches have been developed to selectively reintroduce heteronuclear dipolar coupling under MAS, most of them lack universality and can only be applied to limited spin systems. Herein, we introduce a new and robust technique dubbed phase modulated rotary resonance (PMRR) for reintroducing heteronuclear dipolar couplings while suppressing all other interactions under a broad range of MAS conditions. The standard PMRR requires the radiofrequency (RF) field strength of only twice the MAS frequency, can efficiently recouple the dipolar couplings with a large scaling factor of 0.50, and is robust to experimental imperfections. Moreover, the adjustable window modification of PMRR, dubbed wPMRR, can improve its performance remarkably, making it well suited for the accurate determination of dipolar couplings in various spin systems. The robust performance of such pulse sequences has been verified theoretically and experimentally via model compounds, at different MAS frequencies. The application of the PMRR technique was demonstrated on the H-ZSM-5 zeolite, where the interaction between the Brønsted acidic hydroxyl groups of H-ZSM-5 and the absorbed trimethylphosphine oxide (TMPO) were probed, revealing the detailed configuration of super acid sites.

A new and robust technique dubbed phase modulated rotary resonance (PMRR) was proposed for the accurate determination of heteronuclear dipolar coupling under a broad range of MAS conditions in solid-state NMR spectroscopy.     

Proton line narrowing in solid-state nuclear magnetic resonance: new insights from windowed phase-modulated Lee-Goldburg sequence

We present here a bimodal Floquet analysis of the windowed phase-modulated Lee-Goldburg (wPMLG) sequence for homonuclear dipolar decoupling. One of the main criteria for an efficient homonuclear dipolar decoupling scheme is an effective z-rotation condition. This is brought about by the presence of radio-frequency imperfections in the pulse sequence together with a systematic manipulation of the wPMLG pulses. Additional improvement in the (1)H spectral resolution was obtained by a proper understanding of the off-resonance dependence of the wPMLG irradiation scheme based on bimodal Floquet theory. Numerical investigations further corroborate both theoretical and experimental findings. Theoretical analysis points to accidental degeneracies between the cycle time of the wPMLG sequence and the rotor period leading to the experimentally observed off-resonance dependence of the resolution. Two-dimensional (1)H-(1)H homonuclear single-quantum correlation spectra of model amino acids are also presented, highlighting the improved spectral resolution of wPMLG sequences.