Measuring oxygen-phosphorus distances and the orientation of electric field gradient tensors using 17O-[31P] REDOR in phosphate glasses

Rotational echo double resonance (REDOR) of spin-12 nuclei is an extremely useful tool for the determination of distances in solids as well as of relative orientations of chemical shift and dipole tensors. We present the corresponding version for measuring the relative orientation of electric quadrupole and dipole tensors and demonstrate its applicability for non-bridging oxygens in phosphate glasses using 17O-[31P] REDOR NMR. The orientational information is found in the changes of the second-order quadrupole patterns as a function of the echo delay. Results and numeric simulations are presented for 17O-[31P] REDOR NMR of 17O-enriched sodium phosphate glasses. For non-bridging oxygens, the symmetric quadrupole tensor is found to be aligned along the phosphorus-oxygen bond. The distance between P and the non-bridging oxygen is calculated for two glasses of different compositions.     

Relative CSA-dipolar orientation from REDOR sidebands

Algebraic expressions are given for the sideband intensities of REDOR dephasing experiments as a function of the relative orientation of the CSA and dipolar tensors. The expressions are straightforward to derive and implement and can be easily modified for variations in the spin systems, including distributions of distances and multiple dephasers. These expressions, along with the high sensitivity, resolution, and general robust nature of REDOR, make determining CSA-dipolar orientations from REDOR experiments reliable and, compared to full simulations, efficient and routine. Additionally, it is shown that even the +/-1 sidebands of fast-spinning samples may contain significant information about orientation. Finally, numerical integration of the expressions supports the intuitive notion that any difference in the sideband dephasing rates is evidence of preferred CSA-dipolar orientations. This fact can be used to gauge the extent of local molecular order in intermolecular dephasing experiments.     

Analysis of the REDOR Signal and Inversion

An inversion of the REDOR signal to recover the dipolar couplings has been recently proposed [K. T. Muelleret al., Chem. Phys. Lett.242, 535 (1995)]: The corresponding integral transform was performed by tabulation of the kernel followed by numerical integration. After explicit determination of the inverse REDOR kernel by the Mellin transform method, we propose an alternative inversion method based on Fourier transforms. Representation of the inverse REDOR kernel by its asymptotic expansion reveals that the inverse REDOR operator is essentially a weighted sum of a cosine transform and of its derivative. Consequently, known properties of Fourier transforms can easily be transposed to the REDOR inversion, allowing for a precise discussion of the value of the method. Moreover, the first term of the asymptotic expansion leading to a derivative of a cosine transform, the REDOR inversion is found to be extremely sensitive to noise, thus considerably reducing the useful part of the theoretical dipolar window.     

聚乙烯结晶区的交叉极化动力学研究

研究了单轴拉伸的聚乙烯（PE）纤维结晶区的¹H-¹³C交叉极化（CP）动力学,发现在魔角旋转(MAS)条件下,质子的同核偶极相互作用变弱,其CP动力学可以用I-I*-S模型描述. 沿MAS转轴整齐排列的纤维样品CP动力学曲线随接触时间的增加振荡上升,与斜方晶相比,单斜晶曲线的振荡更为明显, 而非取向样品的CP动力学曲线则单调上升. 说明经过单轴拉伸的PE样品中斜方晶和单斜晶都在一定程度上沿着拉伸方向排列,而单斜晶的取向度高于斜方晶,上述结果表明测量在MAS条件下的CP动力学可以成为表征分子取向度的一种新方法.     

Characterization of 15N chemical shift tensors via 15N-13C REDOR and 1N-1H dipolar-shift CPMAS NMR spectroscopy

As part of our studies on the characterization of 15N chemical shift anisotropy (CSA) via magic angle spinning (MAS) NMR spectroscopy, we have investigated via numerical simulations the sensitivity of two different REDOR experimental protocols to the angles defining the orientation of the 15N-13C' bond vector in the principal axis system of the 15N CSA tensor of the amide nitrogen in a peptide bond. Additionally, employing polycrystalline samples of 15N and 13C', 15N-labeled acetanilide, we have obtained, in a first study of this type, the orientation of the 15N CSA tensor in the molecular frame by orienting the tensor with respect to the 15N-3C' and 15N-1H dipolar vectors via 15N-13C' REDOR and 15N-1H dipolar-shift MAS experiments, respectively.     

Locating hydrogen atoms in single crystal and uniaxially aligned amino acids by solid-state NMR

We demonstrate a novel method to locate hydrogen atoms in amino acids, which involves measuring the C(alpha)H(alpha) bond vector geometry through orientationally dependent dipolar coupling frequencies measured by Lee-Goldburg cross polarization (LGCP). A 2D LGCP experiment is used to measure the polar angle of the C(alpha)H(alpha) bond vector in a single crystal of the model compound L-alanine. It is also demonstrated that by coupling the 13C(alpha)1H(alpha) LGCP experiment to a 13C(alpha)15N REDOR experiment, one can determine the complete three-dimensional geometry of the C(alpha)H(alpha) and C(alpha)N vectors in a single crystal. These measurements allow for location of hydrogen atoms in crystalline biological macromolecules.     

Spin density description of rotational-echo double-resonance, transferred-echo double-resonance and two-dimensional transferred-echo double-resonance solid state nuclear magnetic resonance

The spin density matrix formalism has been applied to rotational-echo double-resonance (REDOR), transferred-echo double-resonance (TEDOR) and two-dimensional (2D) TEDOR experiments in order to obtain an expression for the signal intensities. TEDOR spectra of ¹⁵N-labeled glycine were measured with different dipolar evolution times. 2D-TEDOR spectra were measured of doubly labeled glycine-2-¹³C, ¹⁵N and of ¹⁵N-labeled glycine. Both the TEDOR and the 2D-TEDOR spectra were readily obtained although the 2D-TEDOR experiment on ¹⁵N-labeled glycine used a lot of machine time. Even though the ¹⁵N-1-¹³C dipolar coupling is relatively small (200 Hz), the 1-C resonance can still be observed.     

Double-quantum filtered rotational-echo double resonance

The homonuclear scalar coupling of a directly bonded 13C-13C pair has been used to create a double-quantum filter (DQF) to remove the natural-abundance 13C background in 13C{15N} rotational-echo double-resonance (REDOR) experiments. The DQF scalar and REDOR dipolar evolution periods are coincident which is important for sensitivity in the event of weak 13C-15N dipolar coupling. Calculated and observed 13C{15N} DQF-REDOR dephasings were in agreement for a test sample of mixed recrystallized labeled alanines. Glycine metabolism in a single uniform-15N soybean leaf labeled for 6 min by 13CO2 was measured quantitatively by 13C{15N} DQF-REDOR with no background interferences.     

The Effect of RF Inhomogeneity on Heteronuclear Dipolar Recoupling in Solid State NMR: Practical Performance of SFAM and REDOR

Practical heteronuclear dipolar recoupling performances under magic angle spinning for SFAM and REDOR have been investigated under well-defined rf inhomogeneity environments with variation of resonance offsets for the irradiated nucleus. The heteronuclear dipolar recoupling efficiencies were quantitatively determined based on the experimentally obtained rf homogeneity. As a result, SFAM retains higher recoupling efficiency (>95%) at an 85% effective nutation frequency, and its recoupling efficiency is gradually reduced at lower effective nutation frequencies. On the other hand, although REDOR retains higher recoupling (>95%) efficiency at high (>92%) effective nutation frequency with an XY-8 compensation pulse sequence, the recoupling efficiency is dramatically decreased when the effective nutation frequency is below 90%. Over all, SFAM has significant advantages for insensitivity to carrier frequency offset and rf inhomogeneity.     

Compensating for pulse imperfections in REDOR

Rotational-echo double resonance (REDOR) is a magic-angle spinning technique for measuring heteronuclear dipolar couplings. Rotor-synchronized pi pulses recouple the dipolar interaction. The accuracy of a REDOR determination of distance or orientation depends totally on the quality of the dephased (recoupled) and full-echo spectra. We present a scheme for measuring and compensating for the effects of pulse imperfections in REDOR experiments. No assumptions are made about the quality of the pi pulses, and no pulses are added or taken away in implementing the compensation for incomplete REDOR dephasing by imperfect pi pulses.     

Rotor-encoded heteronuclear MQ MAS NMR spectroscopy of half-integer quadrupolar and spin I=1/2 nuclei

A new two-dimensional heteronuclear multiple-quantum magic-angle spinning (MQ MAS) experiment is presented which combines high resolution for the half-integer quadrupolar nucleus with information about the dipolar coupling between the quadrupolar nucleus and a spin I=1/2 nucleus. Homonuclear MQ coherence is initially created for the half-integer quadrupolar nucleus by a single pulse as in a standard MQ MAS experiment. REDOR recoupling of the heteronuclear dipolar coupling then allows the creation of a heteronuclear multiple-quantum coherence comprising multiple- and single-quantum coherence of the quadrupolar and spin I=1/2 nucleus, respectively, which evolves during t1. Provided that the t1 increment is not rotor synchronized, rotor-encoded spinning-sideband patterns are observed in the indirect dimension. Simulated spectra for an isolated IS spin pair show that these patterns depend on the recoupling time, the magnitude of the dipolar coupling, the quadrupolar parameters, as well as the relative orientation of the quadrupolar and dipolar principal axes systems. Spectra are presented for Na2HPO4, with the heteronuclear 23Na-1HMQ MAS experiments beginning with the excitation of 23Na (spin I=3/2) three-quantum coherence. Coherence counting experiments demonstrate that four- and two-quantum coherences evolve during t1. The heteronuclear spinning-sideband patterns observed for the three-spin H-Na-H system associated with the Na(2) site are analyzed. For an IS2 system, simulated spectra show that, considering the free parameters, the spinning-sideband patterns are particularly sensitive to only, first, the angle between the two IS internuclear vectors and, second, the two heteronuclear dipolar couplings. It is demonstrated that the proton localization around the Na(2) site according to the literature crystal structure of Na2HPO4 is erroneous. Instead, the experimental data is consistent with two alternative different structural arrangements, whereby either there is a deviation of 10 degrees from linearity for the case of two identical Na-H distances, or there is a linear arrangement, but the two Na-H distances are different. Furthermore, the question of the origin of spinning-sidebands in the (homonuclear) MQ MAS experiment is revisited. It is shown that the asymmetric experimental MQ sideband pattern observed for the low-C(Q) Na(2) site in Na(2)HPO4 can only be explained by considering the 23Na chemical shift anisotropy.     

Towards a Fundamental Understanding of the Mechanics of Crystal Agglomeration: A Microscopic and Molecular Approach

A novel experimental apparatus has been developed which enables the measurement of adhesion forces between two crystals suspended in a supersaturated solution and allowed to agglomerate over a fixed time period. The geometry of the crystal surfaces at the contact points and the dynamic development of the bond are captured on video and characterised using an image analysis technique. The experimental apparatus has been designed to allow control of supersaturation, orientation of crystal faces, distance between crystals, relative movement of crystals and contact time. The experimental results show that the agglomerate bond strength, expressed as the agglomerate adhesion force per unit contact area, increases with increasing supersaturation and is higher for faster growing faces than for slower growing faces. In addition, a qualitative comparison has been made between the measured force and a theoretical estimation of the interaction force between crystal faces, determined through molecular modelling. It is shown that the speed of approach of two opposing crystal faces is a key parameter in the nature of the subsequent bond, as is their atomic structure.     

Recoupling of heteronuclear dipolar interactions with rotational-echo double-resonance at high magic-angle spinning frequencies

Heteronuclear dipolar recoupling with rotational-echo double-resonance (REDOR) is investigated in the rapid magic-angle spinning regime, where radiofrequency irradiation occupies a significant fraction of the rotor period (10-60%). We demonstrate, in two model (13)C-(15)N spin systems, [1-(13)C, (15)N] and [2-(13)C, (15)N]glycine, that REDOR DeltaS/S(0) curves acquired at high MAS rates and relatively low recoupling fields are nearly identical to the DeltaS/S(0) curve expected for REDOR with ideal delta-function pulses. The only noticeable effect of the finite pi pulse length on the recoupling is a minor scaling of the dipolar oscillation frequency. Experimental results are explained using both numerical calculations and average Hamiltonian theory, which is used to derive analytical expressions for evolution under REDOR recoupling sequences with different pi pulse phasing schemes. For xy-4 and extensions thereof, finite pulses scale only the dipolar oscillation frequency by a well-defined factor. For other phasing schemes (e.g., xx-4 and xx-4) both the frequency and amplitude of the oscillation are expected to change.     

Selective refocusing pulses in magic-angle spinning NMR: characterization and applications to multi-dimensional protein spectroscopy

Band-selective pulses are frequently used in multi-dimensional NMR in solution, but have been used relatively less often in solid-state NMR applications because of the complications imposed by magic-angle spinning. In this work, we examine the frequency profiles and the refocusing efficiency of several commonly employed selective general rotation pi pulses through experiments and numerical simulations. We demonstrate that highly efficient refocusing of transverse magnetization can be achieved, with experiments that agree well with numerical simulations. We also show that the rotational echo is shifted by a half rotor period if a selective pulse is applied over an integer number of rotor periods. Appropriately synchronizing indirect evolution periods with selective pulses ensures proper phasing of cross peaks in 2D spectra. The improved performance of selective pulses in multi-dimensional protein spectroscopy is demonstrated on the 56-residue beta1 immunoglobulin binding domain of protein G (GB1).     

A practical radio frequency fingerprinting scheme for mobile phones identification

Radio frequency fingerprint (RFF) has attracted a remarkable surge of attention to wireless transmitter identification due to the inimitable hardware imperfection. However, most RFF-based schemes devised in controlled environments are unable to reach the claimed performance in real-world scenarios. This paper proposes a practical RFF identification method for mobile phones in Global System for Mobile (GSM) system. Specifically, the instantaneous amplitude of near-transient part in normal burst (NB) is regarded as RFF that extracted from up-link communications. In addition, an anomaly filtering and stacking (AFS) method is introduced to obtain stable RFF in Time Division Multiple Access (TDMA) system. Lastly, the impacts of frequency point and sending power variations on RFF are fully dissected in comparison experiments. Experiments on 10 mobile phones show that the proposed RFF scheme yields 99.17% True Acceptance Rate (TAR) in real wireless environments. Experiments also show that the varying transmission power decreases the accuracy of RFF identification.     

橡胶硫化促进剂一硫化四甲基秋兰姆制备及光谱分析

合成出了一硫化四甲基秋兰姆(TMTM),采用溶剂蒸发法培养出了单晶TMTM,通过XRD单晶衍射仪、FTIR、TG-DSC对其进行检测和表征,揭示了单晶TMTM的微观结构和内在规律性。XRD单晶衍射从衍射数据、部分健长和键角, 揭示TMTM的微观结构,单晶TMTM高度有序化的定向排列结构,决定了其高效的橡胶硫化促进性能。TG-DSC从热效应分析TMTM的性质,FTIR、TG-DSC检测结果表明,单晶TMTM中蕴含很少许的二硫化碳,FTIR揭示了TMTM内部的化学键键型,和单晶衍射仪从不同侧面检测的结果一致,为确定TMTM的分子结构提供了实验依据。TMTM的分解温度偏高,为采用硫化仪研究TMTM的橡胶硫化性能提供参考。本研究为企业选定工作标准品,对TMTM工业化生产进行跟踪检测,评判TMTM的产品性能指标,填报立项TMTM化工行业标准的申报,撰写标准草案,完成国家工业和信息化部的标准制修订项目计划,提供基础实验数据。     

Characterization of local environments in crystalline borophosphates using single and double resonance NMR

(11)B and (31)P magic-angle spinning as well as (11)B{(31)P} and (31)P{(11)B} rotational echo double resonance (REDOR) NMR have been applied to characterize the local environments in the crystalline borophosphates K(3)[BP(3)O(9)(OH)(3)], NH(4)[ZnBP(2)O(8)] and Rb(3)[B(2)P(3)O(11)(OH)(2)]. Dipolar second moment values extracted from the REDOR curves at short evolution times (DeltaS/S(0) < or = 0.2) are in reasonable agreement with those calculated from the internuclear distances in the corresponding crystal structures. In particular, the method is found to be useful for distinguishing between boron and phosphorus local environments with different numbers of B-O-P connectivities, making REDOR a well-suited tool for medium-range order investigations in glasses.     

Threonine side chain conformational population distribution of a type I antifreeze protein on interacting with ice surface studied via ¹³C-¹⁵N dynamic REDOR NMR

Antifreeze proteins (AFPs) provide survival mechanism for species living in subzero environments by lowering the freezing points of their body fluids effectively. The mechanism is attributed to AFPs' ability to inhibit the growth of seed ice crystals through adsorption on specific ice surfaces. We have applied dynamic REDOR (Rotational Echo Double Resonance) solid state NMR to study the threonine (Thr) side chain conformational population distribution of a site-specific Thr ¹³C_γ and ¹⁵N doubly labeled type I AFP in frozen aqueous solution. It is known that the Thr side chains together with those of the 4th and 8th Alanine (Ala) residues commencing from the Thrs (the 1st) in the four 11-residue repeat units form the peptide ice-binding surface. The conformational information can provide structural insight with regard to how the AFP side chains structurally interact with the ice surface. χ-squared statistical analysis of the experimental REDOR data in fitting the theoretically calculated dynamic REDOR fraction curves indicates that when the AFP interacted with the ice surface in the frozen AFP solution, the conformations of the Thr side chains changed from the anti-conformations, as in the AFP crystal structure, to partial population in the anti-conformation and partial population in the two gauche conformations. This change together with the structural analysis indicates that the simultaneous interactions of the methyl groups and the hydroxyl groups of the Thr side chains with the ice surface could be the reason for the conformational population change. The analysis of the theoretical dynamic REDOR fraction curves shows that the set of experimental REDOR data may fit a number of theoretical curves with different population distributions. Thus, other structural information is needed to assist in determining the conformational population distribution of the Thr side chains.     

Atomic refinement with correlated solid-state NMR restraints

The orientation data provided by solid-state NMR can provide a great deal of structural information about membrane proteins. The quality of the information provided is, however, somewhat degraded by sign degeneracies in measurements of the dipolar coupling tensor. This is reflected in the dipolar coupling penalty function used in atomic refinement, which is less capable of properly restraining atoms when dipolar sign degeneracies are present. In this report we generate simulated solid-state NMR data using a variety of procedures, including back-calculation from crystal structures of alpha-helical and beta-sheet membrane proteins. We demonstrate that a large fraction of the dipolar sign degeneracies are resolved if anisotropic dipolar coupling measurements are correlated with anisotropic chemical shift measurements, and that all sign degeneracies can be resolved if three data types are correlated. The advantages of correlating data are demonstrated with atomic refinement of two test membrane proteins. When refinement is performed using correlated dipolar couplings and chemical shifts, perturbed structures converge to conformations with a larger fraction of correct dipolar signs than when data are uncorrelated. In addition, the final structures are closer to the original unperturbed structures when correlated data are used in the refinement. Thus, refinement with correlated data leads to improved atomic structures. The software used to correlate dipolar coupling and chemical shift data and to set up energy functions and their derivatives for refinement, CNS-SS02, is available at our web site.