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.     

Doubly compensated multiplicity-edited HSQC experiments utilizing broadband inversion pulses

We propose a family of doubly compensated multiplicity-edited heteronuclear single quantum coherence (HSQC) pulse sequences. The key difference between our proposed sequences and the compensation of refocusing inefficiency with synchronized inversion sweeps (CRISIS)-HSQC experiments they are based on is that the conventional rectangular 180 degrees pulses on the proton channel in the latter have been replaced by the computer-optimized broadband inversion pulses (BIPs) with superior inversion performance as well as much improved tolerance to B(1) field inhomogeneity. Moreover, all adiabatic carbon 180 degrees pulses during the INEPT and reverse-INEPT periods in the CRISIS-HSQC sequences have also been replaced with the much shorter BIPs, while the adiabatic sweeps during the heteronuclear spin echo for multiplicity editing are kept in place in order to maintain the advantage of the CRISIS feature of the original sequences, namely J-independent refocusing of the one-bond (1)H--(13)C coupling constants. These modifications have also been implemented to the preservation of equivalent pathways (PEP)-HSQC experiments. We demonstrate through a detailed comparison that replacing the proton 180 degrees pulses with the BIPs provide additional sensitivity gain that can be mainly attributed to the improved tolerance to B(1) field inhomogeneity of the BIPs. The proposed sequences can be easily adapted for (19)F--(13)C correlations.     

Broadband and band-selective IMPRESS-gHMBC: compensation of refocusing inefficiency with synchronized inversion sweep

Compensation of refocusing inefficiency in a gHMBC experiment by replacing the rectangular pi pulse with a pair of adiabatic pulses with synchronized inversion sweep (CRISIS) significantly improves the performance of the gHMBC experiment. The CRISIS-gHMBC experiment retains the pure absorptive shapes in F1 and hence results in better lineshape and higher resolution than the current versions of magnitude mode gHMBC spectra. When used as a broadband experiment, CRISIS-gHMBC, owing to better refocusing efficiency of the adiabatic pulse pairs, gives improved performance across the 13C spectral width. Moreover, it is shown that CRISIS-gHMBC is a robust and improved alternative and when used along with the IMPRESS (Improved Resolution using Symmetrically Shifted pulses) technique further increases the sensitivity and resolution without additional experimental time. The IMPRESS-CRISIS combination is demonstrated for broadband gHMBC and band-selective gHMBC experiments. The ICbs-gHMBC [IMPRESS-CRISIS-band-selective gHMBC] experiment is an attractive and better alternative to individual band-selective gHMBC.     

J-modulated ADEQUATE experiments using different kinds of refocusing pulses

Owing to the recent developments concerning residual dipolar couplings (RDCs), the interest in methods for the accurate determination of coupling constants is renascenting. We intended to use the J-modulated ADEQUATE experiment by K?vér et al. for the measurement of (13)C - (13)C coupling constants at natural abundance. The use of adiabatic composite chirp pulses instead of the conventional 180 degrees pulses, which compensate for the offset dependence of (13)C 180 degrees pulses, led to irregularities of the line shapes in the indirect dimension causing deviations of the extracted coupling constants. This behaviour was attributed to coupling evolution, during the time of the adiabatic pulse (2 ms), in the J-modulation spin echo. The replacement of this pulse by different kinds of refocusing pulses indicated that a pair of BIPs (broadband inversion pulses), which behave only partially adiabatic, leads to correct line shapes and coupling constants conserving the good sensitivity obtained with adiabatic pulses.     

CRAPT: an improved version of APT with compensation for variations in JCH

A modified version of the attached proton test (APT) sequence for ¹³C spectral editing, which we call CRisis‐APT (CRAPT), is developed and tested on representative organic compounds. CRAPT incorporates ¹³C compensation for refocusing inefficiency with synchronized inversion sweeps (CRISIS) pulses in combination with ¹H broadband inversion pulses to give improved compensation for variations in ¹J_CH along with improved refocusing efficiency. It is shown that CRAPT gives edited ¹³C spectra with only small losses in sensitivity (between 8% and 15% for strychnine, 1 , menthol, 2 , cholecalciferol, 3 , and isotachysterol, 4 ), compared with basic ¹³C spectra obtained on the same compounds. CRAPT also gives significantly better signal/noise than DEPTQ for nonprotonated carbons. Therefore, we conclude that CRAPT is an improvement over APT or DEPTQ or a combination of DEPT135 with a full ¹³C spectrum for routine ¹³C spectral editing of organic compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

Theory of pulsed reaction yield detected magnetic resonance

We propose pulse sequences for Reaction Yield Detected Magnetic Resonance (RYDMR), which are based on refocusing the zero-quantum coherences in radical pairs by non-selective microwave pulses and using the population of a radical pair singlet spin state as an observable. The new experiments are analogues of existing EPR experiments such as the primary echo, Carr-Purcell, ESEEM, stimulated echo and Mims ENDOR. All pulse sequences are supported by analytical results and numerical calculations. The pulse sequences can be used for more efficient and highly detailed characterization of intermediates of chemical reactions and charge carriers in organic semiconductors.     

Rotation and diffusion of H2 in hydrogen-ice clathrate by 1H NMR

A recently reported hydrogen-ice clathrate carries up to four H(2) in each large cage and one H(2) in each small cage. We report pulsed proton NMR line shape measurements on H(2)-D(2)O clathrate formed at 1500 bar and 250 K. The behavior of the two-pulse spin-echo amplitude with respect to the nutation angle of the refocusing pulse shows that intramolecular dipolar broadening, modulated by H(2) molecular reorientations, dominates the line width of the ortho-H(2). Dipolar interaction between H(2) guests and host D atoms explains the echo variation with the relative phases of the pulses. From 12 to 120 K, the line width varies as 1/T, demonstrating that the three sublevels of J = 1 are split by a constant energy, epsilon. The splitting arises from distortion in the otherwise high-symmetry cages from frozen-out D(2)O orientational disorder. Above 120 K, further line-narrowing signals the onset of H(2) diffusion from cage to cage. At the lowest temperature, 1.9 K, the spectrum has Pake powder doublet-like features; the doublet is not fully developed, indicating a broad distribution of order parameters and energies epsilon.     

Multiple refocusing in NMR spectroscopy: compensation of pulse imperfections by scalar couplings.

When applying multiple refocusing pulses to characterize the cross-correlated relaxation of heteronuclear multiple quantum coherence 2NxHx in biomolecules, the unavoidable effects of pulse imperfections are compensated by the scalar couplings between nitrogen atoms and protons. The experiment, which is useful as a tool for studying slow internal dynamics of biomolecules, greatly benefits from this compensation. The underlying effect is a manifestation of an interchange between three noncommuting components of the density operator. One perturbing Hamiltonian is counteracted by another, which leads to a nearly complete suppression of the perturbation. The effect proves to be an example of a hitherto unknown phenomenon in NMR spectroscopy.     

Multi-frequency improved constant amplitude pulses for broadband inversion

Optimization of constant amplitude broadband inversion pulses for maximum inversion over a range of multiple fields yields more regular pulses with better tolerance to B(1) errors than those optimized for a single field. These multi-frequency improved constant amplitude (MICA) pulses as constructed for (13)C broadband inversion give best results for HMQC and especially HSQC type experiments. Most of the advantages of MICA pulses versus other inversion pulses in these experiments can be attributed to their relatively short durations. Linearly truncated versions of these pulses do not offer any advantage over MICA pulses for these applications. MICA inversion pulses can also be used for decoupling and a (13)C decoupling example gives twice the decoupling bandwidth as the GARP1 sequence at the same power level with no appreciable increase in decoupling sidebands.     

The influence of laser field noise on controlled quantum dynamics

The influence of laser noise on the dynamics of simple quantum systems is analyzed. An anharmonic ladder is chosen for illustration and several pulses are obtained that optimize the yield of a quantum transition by constraining the laser parameters. The following models of laser noise are introduced: Amplitude white noise, phase white noise, frequency white noise and shot-to-shot static noise in the different pulse parameters. It is shown that the optimal pulses are robust to white amplitude noise, since the system acts as a dynamical filter. White phase noise affects the optimal pulses in a similar way by reducing the pulse area. This effect can be easily compensated for by pulse amplitude rescaling, up to a high level of noise. White frequency noise reduces the pulse area and induces spectral broadening, more strongly affecting the high frequency components. It can be partially compensated for by amplitude rescaling. The effects of static noise in the parameters cannot be easily corrected. It is shown that optimal pulses that drive n-photon transitions become more sensitive to noise in the amplitude and less sensitive to noise in the frequency as n increases. The effects of noise in the relative phase rapidly become constant for a large number of interfering pathways.     

Broadband RF‐amplitude‐dependent flip angle pulses with linear phase slope

Pulse sequences in NMR spectroscopy sometimes require the application of pulses with effective flip angles different from 90° and 180°. Previously (Magn. Reson. Chem. 2015, 53, 886‐893), offset‐compensated broadband excitation pulses with RF‐amplitude‐dependent effective flip angles (RADFA) were introduced that are applicable in such cases. However, especially RF‐amplitude‐restricted RADFA pulses turned out to perform not as good as desired in terms of achievable bandwidths. Here, a class of RF‐amplitude‐restricted RADFA pulses with linear phase slope is introduced that allows excitation over much larger bandwidths with better performance. In this theoretical work, the basic principle of the pulse class is explained, their physical limits explored, and their properties, also compared with other pulse classes, discussed in detail. Copyright © 2017 John Wiley & Sons, Ltd.     

High-voltage electroconduction in molten chlorides of alkaline-earth metals

Effects of high-voltage pulses on electroconduction of molten chlorides of alkaline-earth metals and their mixtures with potassium chloride are studied experimentally. It is established that the electroconductivity of the melts subjected to the pulses increases proportionally to the voltage amplitude and the number of activating pulses in a pulse series. Oscillatory relaxation of nonequilibrium melts is discovered.     

Discrimination of PHIP Signals Through their Evolution in Multipulse Sequences

The antiphase character of the PHIP associated signals after a hydrogenation reaction is particularly sensitive to line broadening introduced by magnetic field inhomogeneities and interferences by the presence of resonance lines steaming from a large amount of thermally polarized spins. These obstacles impose a limitation in the detection of reaction products as well as in the experimental setups. A simple way to overcome these impediments consists of acquiring the signal with a train of refocusing pulses instead of a single r.f. pulse. We present here a number of examples where this multipulse acquisition, denominated PhD-PHIP, displays its potentiality in improving the information related to hyperpolarized spins performed in a sample, where the former parahydrogen nuclei are part of a complex J-coupling network.     

Qualitative analysis by online nuclear magnetic resonance using Carr-Purcell-Meiboom-Gill sequence with low refocusing flip angles

The Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence has been used in many applications of magnetic resonance imaging (MRI) and low-resolution NMR (LRNMR) spectroscopy. Recently, CPMG was used in online LRNMR measurements that use long RF pulse trains, causing an increase in probe temperature and, therefore, tuning and matching maladjustments. To minimize this problem, the use of a low-power CPMG sequence based on low refocusing pulse flip angles (LRFA) was studied experimentally and theoretically. This approach has been used in several MRI protocols to reduce incident RF power and meet the specific absorption rate. The results for CPMG with LRFA of 3π/4 (CPMG₁₃₅), π/2 (CPMG₉₀) and π/4 (CPMG₄₅) were compared with conventional CPMG with refocusing π pulses. For a homogeneous field, with linewidth equal to Δυ = 15 Hz, the refocusing flip angles can be as low as π/4 to obtain the transverse relaxation time (T₂) value with errors below 5%. For a less homogeneous magnetic field, Δυ = 100 Hz, the choice of the LRFA has to take into account the reduction in the intensity of the CPMG signal and the increase in the time constant of the CPMG decay that also becomes dependent on longitudinal relaxation time (T₁). We have compared the T₂ values measured by conventional CPMG and CPMG₉₀ for 30 oilseed species, and a good correlation coefficient, r = 0.98, was obtained. Therefore, for oilseeds, the T₂ measurements performed with π/2 refocusing pulses (CPMG₉₀), with the same pulse width of conventional CPMG, use only 25% of the RF power. This reduces the heating problem in the probe and reduces the power deposition in the samples.     

Low-power composite CPMG HSQMBC experiment for accurate measurement of long-range heteronuclear coupling constants

A modified version of CPMG-HSQMBC pulse scheme is presented for the measurement of long-range heteronuclear coupling constants. The method implements adiabatic inversion and refocusing pulses on the heteronucleus. Low-power composite 180° XY-16 CPMG pulse train is applied on both proton and X nuclei during the evolution of long-range couplings to eliminate phase distortions due to co-evolution of homonuclear proton-proton couplings. The pulse sequence yields pure absorption antiphase multiplets allowing precise and direct measurement of the (n)J(XH) coupling constants regardless from the size of the proton-proton couplings. The applicability of the method is demonstrated using strychnine as a model compound. The selective 1D version of the method is also presented.     

Extending the Scope of Singlet‐State Spectroscopy

Different decoupling sequences are tested—using various shaped radio‐frequency (RF) pulses—to achieve the longest possible lifetimes of singlet‐state populations over the widest possible bandwidths, that is, ranges of offsets and relative chemical shifts of the nuclei involved in the singlet states. The use of sinc or refocusing broadband universal rotation pulses (RE‐BURP) for decoupling during the intervals where singlet‐state populations are preserved allows one to extend the useful bandwidth with respect to prior state‐of‐the‐art methods based on composite‐pulse WALTZ decoupling. The improved sinc decoupling sequences afford a more reliable and sensitive measure of the lifetimes of singlet states in pairs of spins that have widely different chemical shifts, such as the two aromatic protons H⁵ and H⁶ in uracil. Similar advantages are expected for nucleotides in RNA and DNA. Alternative approaches, in particular frequency‐modulated decoupling sequences, also appear to be effective in preserving singlet‐state populations, even though the profiles of the apparent relaxation rate constants as a function of the offset are somewhat perturbed. The best decoupling sequences prove their utility in sustaining longer lifetimes of singlet states than previously achieved for the side‐chain tyrosine protons in bovine pancreatic trypsin inhibitor (BPTI) at 600 MHz (14.1 T), where the differences of chemical shifts between coupled protons are a challenge.     

Measurement of slow (micros-ms) time scale dynamics in protein side chains by (15)N relaxation dispersion NMR spectroscopy: application to Asn and Gln residues in a cavity mutant of T4 lysozyme.

A new NMR experiment is presented for the measurement of micros-ms time scale dynamics of Asn and Gln side chains in proteins. Exchange contributions to the (15)N line widths of side chain residues are determined via a relaxation dispersion experiment in which the effective nitrogen transverse relaxation rate is measured as a function of the number of refocusing pulses in constant-time, variable spacing CPMG intervals. The evolution of magnetization from scalar couplings and dipole-dipole cross-correlations, which has limited studies of exchange in multi-spin systems in the past, does not affect the extraction of accurate exchange parameters from relaxation profiles of NH(2) groups obtained in the present experiment. The utility of the method is demonstrated with an application to a Leu --> Ala cavity mutant of T4 lysozyme, L99A. It is shown that many of the side chain amide groups of Asn and Gln residues in the C-terminal domain of the protein are affected by a chemical exchange process which may be important in facilitating the rapid binding of hydrophobic ligands to the cavity.     

Modulation-aided signal enhancement in the magic angle spinning NMR of spin-5/2 nuclei

We report signal enhancement schemes using fast amplitude modulated pulses for the one-dimensional (1D) nuclear magnetic resonance (NMR) of spin-5/2 nuclei under magic-angle spinning. Signal enhancement by a factor of around 2.5 is observed when amplitude modulated pulses precede selective excitation of the central transition. This enhancement is a result of the redistribution of energy level populations through partial saturation of the satellite transitions. Results are shown for ²⁷Al and ¹⁷O. The gain in signal intensity is very useful for the observation of weak signals from low abundance quadrupolar nuclei. The scheme works for wide ranges of quadrupole interactions and rf powers.     

Effects of changes in pattern of LHRH pulse on LH secretion by perfused anterior pituitary cells of male rats

In the present study, a perfusion system of dispersed cells was used to investigate the effects of LHRH pulse amplitude and frequency, and LHRH continuous stimulation on LH secretion by anterior pituitary cells of adult male rats. The results have shown that, in the range of LHRH concentrations from 1 X 10(-10) to 1 X 10(-6) mol/L, the dose-response curve of LH secretion was linear. LHRH pulse frequency generated a biphasic LH response: increasing LHRH pulse frequency increased the basal LH secretion and decreased LH/pulse. When 1 X 10(-9) mol/L or greater LHRH was given at frequencies of 3 pulses/h or higher, it was observed that a maximal LH peak was induced and then the LH release declined progressively to its LH basal level, i.e. LHRH self-priming effect and LH desensitization occurred. Enhancement of amplitude of LHRH pulses could reduce pulse frequency required for priming. Increases in frequency of LHRH pulses with high amplitude would provoke the priming effect more quickly. In addition, continuous perfusion of LHRH with different concentrations could also elicit the LHRH self-priming effect and lH desensitization. LHRH with low concentration (1 X 10(-10) mol/L) would take much longer to evoke a self-priming effect. These results indicate that the LH secretion pattern is dependent on LHRH pulsatile amplitude and frequency, and will help to clarify the kinetics mechanisms by which LH pulses fluctuate in vivo.     

Optimization of bulk cell electrofusion in vitro for production of human-mouse heterohybridoma cells

Cell electrofusion is a phenomenon that occurs, when cells are in close contact and exposed to short high-voltage electric pulses. The consequence of exposure to pulses is transient and nonselective permeabilization of cell membranes. Cell electrofusion and permeabilization depend on the values of electric field parameters including amplitude, duration and number of electric pulses and direction of the electric field. In our study, we first investigated the influence of the direction of the electric field on cell fusion in two cell lines. In both cell lines, applications of pulses in two directions perpendicular to each other were the most successful. Cell electrofusion was finally used for production of human-mouse heterohybridoma cells with modified Koehler and Milstein hybridoma technology, which was not done previously. The results, obtained by cell electrofusion, are comparable to usually used polyethylene glycol mediated fusion on the same type of cells.