Measurement of Small One-Bond Proton–Carbon Residual Dipolar Coupling Constants in Partially Oriented C Natural Abundance Oligosaccharide Samples: Analysis of Heteronuclear JCH-Modulated Spectra with the BIRD Inversion Pulse

Two 2D J-modulated HSQC-based experiments were designed for precise determination of small residual dipolar one-bond carbon–proton coupling constants in ¹³C natural abundance carbohydrates. Crucial to the precision of a few hundredths of Hz achieved by these methods was the use of long modulation intervals and BIRD pulses, which acted as semiselective inversion pulses. The BIRD pulses eliminated effective evolution of all but ¹J_CH couplings, resulting in signal modulation that can be described by simple modulation functions. A thorough analysis of such modulation functions for a typical four-spin carbohydrate spin system was performed for both experiments. The results showed that the evolution of the ¹H–¹H and long-range ¹H–¹³C couplings during the BIRD pulses did not necessitate the introduction of more complicated modulation functions. The effects of pulse imperfections were also inspected. While weakly coupled spin systems can be analyzed by simple fitting of cross peak intensities, in strongly coupled spin systems the evolution of the density matrix needs to be considered in order to analyse data accurately. However, if strong coupling effects are modest the errors in coupling constants determined by the “weak coupling” analysis are of similar magnitudes in oriented and isotropic samples and are partially cancelled during dipolar coupling calculation. Simple criteria have been established as to when the strong coupling treatment needs to be invoked.     

Phase modulation in dipolar-coupled A2 spin systems: effect of maximum state mixing in H NMR in vivo

Coupling constants of nuclear spin systems can be determined from phase modulation of multiplet resonances. Strongly coupled systems such as citrate in prostatic tissue exhibit a more complex modulation than AX connectivities, because of substantial mixing of quantum states. An extreme limit is the coupling of n isochronous spins (A_n system). It is observable only for directly connected spins like the methylene protons of creatine and phosphocreatine which experience residual dipolar coupling in intact muscle tissue in vivo. We will demonstrate that phase modulation of this “pseudo-strong” system is quite simple compared to those of AB systems. Theory predicts that the spin-echo experiment yields conditions as in the case of weak interactions, in particular, the phase modulation depends linearly on the line splitting and the echo time.     

描述自旋体系密度算符演化的新方法

提出一种描述自旋体系密度算符演化的新方法,可统一处理核磁共振波谱学中强、弱偶合自旋体系及弱射频场存在下密度算符的演化。具体分析一些核磁共振实验:自旋轻扰、自旋锁定、偶极耦合作用、强耦合AB和ABX自旋体系的演化。     

Spectral editing of weakly coupled spins using variable flip angles in PRESS constant echo time difference spectroscopy: Application to GABA

A general in vivo magnetic resonance spectroscopy editing technique is presented to detect weakly coupled spin systems through subtraction, while preserving singlets through addition, and is applied to the specific brain metabolite γ-aminobutyric acid (GABA) at 4.7 T. The new method uses double spin echo localization (PRESS) and is based on a constant echo time difference spectroscopy approach employing subtraction of two asymmetric echo timings, which is normally only applicable to strongly coupled spin systems. By utilizing flip angle reduction of one of the two refocusing pulses in the PRESS sequence, we demonstrate that this difference method may be extended to weakly coupled systems, thereby providing a very simple yet effective editing process. The difference method is first illustrated analytically using a simple two spin weakly coupled spin system. The technique was then demonstrated for the 3.01 ppm resonance of GABA, which is obscured by the strong singlet peak of creatine in vivo. Full numerical simulations, as well as phantom and in vivo experiments were performed. The difference method used two asymmetric PRESS timings with a constant total echo time of 131 ms and a reduced 120° final pulse, providing 25% GABA yield upon subtraction compared to two short echo standard PRESS experiments. Phantom and in vivo results from human brain demonstrate efficacy of this method in agreement with numerical simulations.     

Determination of signs of hyperfine coupling constants for an S = ½ system through E.P.R., ENDOR and double ENDOR

A systematic method of obtaining relative signs of hyperfine coupling constants is described. It applies to systems consisting of (a) a set of one or more nuclei coupled fairly strongly to the electron spin, and possessing a two-fold (or higher) axis of symmetry, together with (b) a set of weakly coupled nuclei defining superhyperfine transitions. ENDOR measurements for several E.P.R. hyperfine transitions, with the field oriented along the symmetry axis, give relative signs of hyperfine components for this direction. Signs for the other directions can then be obtained through ENDOR measurements on a single hyperfine transition at various field orientations. Additional double ENDOR measurements may be necessary for very weakly coupled nuclei. This method can complement double ENDOR studies in favourable cases. It is illustrated by the determination of signs of coupling constants of protons and of ⁷⁵As in the AsO₄ ^4- radical in KH₂AsO₄.     

Spin-polarization-dependent transport in a quantum dot array coupled with an Aharonov-Bohm ring

In this paper the quantum transport in a dot-array coupled with an Aharonov–Bohm (AB) ring is investigated via single-band tight-binding Hamiltonian. It is shown that the output spin current is a periodic function of the magnetic flux in the quantum unit Φ₀. The resonance positions of the total transmission probability do not depend on the size of the AB ring but the electronic spectrum. Moreover, the persistent currents in the AB ring is also spin-polarization dependent and different from the isolated AB ring where the persistent current is independent of spin polarization.     

J Coupling between C and H across Hydrogen Bonds in Proteins

Two new two- or three-dimensional NMR methods for measuring ^3hJ_C′N and ^2hJ_C′H coupling constants across hydrogen bonds in proteins are presented. They are tailored to suit the size of the TROSY effect, i.e., the degree of interference between dipolar and chemical shift anisotropy relaxation mechanisms. The methods edit 2D or 3D spectra into two separate subspectra corresponding to the two possible spin states of the ¹H^N spin during evolution of ¹³CO coherences. This allows ^2hJ_C′H to be measured in an E.COSY-type way while ^3hJ_C′N can be measured in the so-called quantitative way provided a reference spectrum is also recorded. A demonstration of the new methods is shown for the ¹⁵N,¹³C-labeled protein chymotrypsin inhibitor 2.     

On the spectroscopy in ee annihilation

The properties of the radial excitations of the , ω and mesons are discussed. In particular it is proposed to identify the recently observed states at √s 1.5, 1.82 and 2.13 GeV in e⁺e⁻ annihilation with the _D ≡ ³D₁(λλ), ″ and ′″ mesons respectively. The ′ meson is suggested to lie in the vicinity of 1.5 GeV and strongly coupled to the _D. The ″(1.6) width is also suggested to be smaller than previously reported.     

Accurate Measurement of Small Spin–Spin Couplings in Partially Aligned Molecules Using a Novel J-Mismatch Compensated Spin-State-Selection Filter

The accurate measurement of small spin–spin coupling constants in macromolecules dissolved in a liquid crystalline phase is important in the context of molecular structure investigation by modern liquid state NMR. A new spin-state-selection filter, DIPSAP, is presented with significantly reduced sensitivity to J-mismatch of the filter delays compared to previously proposed pulse sequences. DIPSAP presents an attractive new approach for the accurate measurement of small spin–spin coupling constants in molecules dissolved in anisotropic solution. Application to the measurement of ¹⁵N–¹³C′ and ¹H^N–¹³C′ coupling constants in the peptide planes of ¹³C, ¹⁵N labeled proteins demonstrates the high accuracy obtained by a DIPSAP-based experiment.     

Development of injection-seeded, pulsed optical parametric generator/oscillator systems for high-resolution spectroscopy

We report the development and application of pulsed optical parametric generator (OPG) and optical parametric oscillator (OPO) systems that are injection seeded with near-infrared distributed feedback diode lasers. The OPG is injection seeded at the idler wavelength without the use of a resonant cavity. Two counter-rotating, beta-barium-borate (β-BBO) crystals are used in the OPG. These crystals are pumped by the third harmonic, 355-nm output of an injection-seeded Nd:YAG laser. An OPO version of the system has also been developed by placing two flat mirrors around the two β-BBO crystals to form a feedback cavity at the signal wavelength. The OPO cavity length is not actively controlled. The output signal beam from the OPG or OPO is amplified using an optical parametric amplifier (OPA) stage with four β-BBO crystals. The frequency bandwidths of the signal and idler laser radiation from OPG/OPA and OPO/OPA systems have been determined to be slightly greater than 200 MHz. The temporal pulses from each system are smooth and near-Gaussian. High-resolution optical absorption measurements of acetylene (C₂H₂) were performed as another check of the frequency spectrum of the idler beam. The frequency-doubled signal output of the OPO/OPA system was used to perform high-resolution, single-photon, laser-induced fluorescence (LIF) spectroscopic studies of the (0,0) vibrational band of the A ²Σ⁺−X ²Π electronic transition of nitric oxide (NO) at low pressure. Excellent agreement was obtained between the theory and the experiment. The signal output of the OPG/OPA system was also used for sub-Doppler, two-photon LIF spectroscopic studies of the same vibration–rotation manifold of NO.This revised version was published online in August 2005 with a corrected cover date.     

Measurement of Cross Correlation between Dipolar Coupling and Chemical Shift Anisotropy in the Spin Relaxation ofC,N-Labeled Proteins

We present a simple method for extracting interference effects between chemical shift anisotropy (CSA) and dipolar coupling from spin relaxation measurements in macromolecules, and we apply this method to extracting cross-correlation rates involving interference of amide¹⁵N CSA and¹⁵N–¹H dipolar coupling and interference of carbonyl¹³C′ CSA and¹⁵N–¹³C′ dipolar coupling, in a small protein. A theoretical basis for the interpretation of these rates is presented. While it proves difficult to quantitatively separate the structural and dynamic contributions to these cross-correlation rates in the presence of anisotropic overall tumbling and a nonaxially symmetric chemical shift tensor, some useful qualitative correlations of data with protein structure can be seen when simplifying assumptions are made.     

Measurement of one and two bond N–C couplings in large proteins by TROSY-based J-modulation experiments

Residual dipolar couplings (RDCs) between NC′ and NC^α atoms in polypeptide backbones of proteins contain information on the orientation of bond vectors that is complementary to that contained in NH RDCs. The ¹J_NC^α and ²J_NC^α scalar couplings between these atoms also display a Karplus relation with the backbone torsion angles and report on secondary structure. However, these N–C couplings tend to be small and they are frequently unresolvable in frequency domain spectra having the broad lines characteristic of large proteins. Here a TROSY-based J-modulated approach for the measurement of small ¹⁵N–¹³C couplings in large proteins is described. The cross-correlation interference effects inherent in TROSY methods improve resolution and signal to noise ratios for large proteins, and the use of J-modulation to encode couplings eliminates the need to remove frequency distortions from overlapping peaks during data analysis. The utility of the method is demonstrated by measurement of ¹J_NC′, ¹J_NC^α, and ²J_NC^α scalar couplings and ¹D_NC′ and ¹D_NC^α residual dipolar couplings for the myristoylated yeast ARF1·GTPγs protein bound to small lipid bicelles, a system with an effective molecule weight of 70 kDa.     

Spin-spin relaxation in heteropolynuclear clusters containing iron(III) and copper(II) ions

The magnetic coupling between Fe(III) and Cu(II) and the relaxation behavior of the spin centered at the iron have been studied in di- and trinuclear clusters. The experimental spectra have been analyzed in terms of a simple cross-relaxation model. In the strongly coupled trimer the relaxation rates are determined by a Boltzmann factor and a single field- and temperature-independent scaling parameter¹ w ₀, while the weakly coupled dimer exhibits obviously a more complex behavior.     

Analysis of longitudinal multispin orders in strongly coupled spin systems

Longitudinal multispin orders provide an effective way for measurement of scalar couplings and also to probe molecular interactions and dynamics. Analysis of longitudinal orders has been made in strongly coupled AB and ABX spin systems to determine the dependence of strong coupling parameter on these orders. Experimental and simulated spectra of various longitudinal orders are illustrated for these spin systems. This general procedure can be extended to broad range of spin systems to understand the influence of strong coupling on longitudinal orders.     

High resolution spectroscopy of and

Extensions in frequency coverage coupled with sensitive spectroscopic techniques have enabled high resolution measurements of pure rotational spectra of deuteromethane and its 13-C substituted counterpart up to J^′=7. The current work reveals a small inconsistency in previously reported frequency measurements of ¹²CH₃D at J^′=5.     

EPR spectroscopy of weak exchange interactions between Co2+ ions in ZnO

Electron paramagnetic resonance (EPR) is used to identify the next nearest neighbour Co²⁺ pairs coupled by spin–spin interaction in Co‐doped ZnO single crystals grown by the hydrothermal technique. These dimer centers are described by a spin Hamiltonian with exchange coupling terms written as interaction between identical effective spins S^eff = 1/2 of the lowest ground state Kramers doublets of the two Co²⁺ ions. The exchange parameters of weakly ferromagnetically coupled next nearest neighbor Co²⁺ pairs are estimated. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A turn-on fluorescent chemosensor for Hg based on phenanthroline fluorophore

Dipyrido[3,2-a:2′,3′-c]-phenazine (L) was employed as a selectively fluorescent chemosensor for Hg²⁺ in DMF solution under buffered conditions with its fluorescence being strongly increased. The fluorescence increasing was attributed to the formation of L–Hg²⁺ by 1:1 complex ratio (K=3.7×10⁵ M⁻¹), which constitutes the basis for the determination of Hg²⁺ with the prepared chemosensor. The experiment results also show that the response behavior of L to Hg²⁺ is pH independent in the range of pH 6.0–9.0 and show excellent sensitivity and selectivity for Hg²⁺ over other examined metal ions.     

Simulation of CPMAS Signals at High Spinning Speeds

The spin dynamics of anS( )I_Nsystem during the CP mixing time of continuous wave and variable amplitude cross-polarization magic angle spinning (CWCPMAS and VACPMAS) experiments is discussed. The signal enhancement of a low abundantSspin, coupled to a set ofN= 6 coupled spins withI= , is evaluated as a function of the length of the mixing time. For CWCPMAS this signal is first evaluated in the frequency domain and then transformed to the time domain. These calculations provide some additional insight into the CP spin dynamics and enable a practical approach toward the evaluation of CP signals of large spin systems. In addition the adiabatic character of the ramped VACPMAS experiments is discussed andS-spin signals of a spin system withN= 6 are simulated. Estimates of the upper bounds of the CP signals as a function of the number ofIspins in anS( )I_Nsystem are given and compared with the calculated values.     

A precise measurement of the spin-dependent neutron scattering length of 3He

The poor knowledge of the spin-dependent neutron scattering length of ³He has until now handicapped nuclear four body theory and the interpretation of excitations in the quantum liquid. We have measured, for the first time directly, the real part of the bound incoherent neutron scattering length, b_i′ of ³He. A neutron spin echo spectrometer was used to detect pseudomagnetic precession of polarised neutrons passing through polarised ³He gas. Any absolute calibrations of sample and beam parameters were avoided using simple transmission measurements with non-polarised neutrons. The only a priory information required was the spin-dependent neutron absorption cross section of ³He. The result is b_i′ = -2.365(20) fm, which reduces the prior uncertainty by a factor 30. The corresponding new value of the bound incoherent scattering cross section is σ_i = 1.532(12) barn. Including the known value of the coherent neutron scattering length, we obtain new values for the real parts of the free triplet and singlet neutron scattering lengths, a_-′ = 7.370(58) fm and a₊′ = 3.278(53) fm.