Heteronuclear Double-Quantum-Coherence Selection with Magnetic-Field Gradients in Diffusion Experiments

A 3.5-fold reduction in the magnitude of the magnetic-field-gradient pulses was achieved in a ³¹P NMR pulsed-field-gradient spin-echo experiment, used to measure the diffusion coefficient of a phosphorus-containing solute; this reduction involved a variant of a heteronuclear ′inverse-detection′ pulse sequence. The attendant eddy currents, set up in the conducting elements of the probe and magnet were reduced because of the smaller current required in the field-gradient coils. Linear magnetic-field gradients were also used in the pulse sequence to select heteronuclear double-quantum coherences, thus obviating the requirement for extensive phase cycling and potential dynamic range problems, because any signal that arose from magnetization that had not been double-quantum coherent was "spoiled" in the experiment prior to signal detection. The utility of the pulse sequence and the necessary modifications to the standard Stejskal and Tanner analysis that are required for data from the experiment are illustrated with the heteronuclear-coupled, ¹H-³¹P, system of neutralized phosphorus acid [HPO(OH)₂] in water.     

Heteronuclear Multiple-Quantum Coherence NMR Spectroscopy of Im-cyt C

Some ¹³C chemical shifts of the CH_n groups in the aliphatic side chains of Im-cyt c have been determined for the first time based on the H chemical shifts of their attached protons with the aid of heteronuclear multiple-quantum coherence (HMQC) spectroscopy. Comparison of chemical shifts of these specifically assigned ¹³C and H resonances from Im-cyt c with those from cyt c indicates that ¹³C-NMR spectra may provide an opportunity to probe the electronic structure and conformational changes induced by axial ligand substitution.     

High-Resolution Heteronuclear Correlation between Quadrupolar Nuclei

Multiple-quantum magic angle spinning is successfully incorporated with double-quantum cross-polarization between quadrupolar nuclei, producing a two-dimensional 11B[27Al] high-resolution heteronuclear correlation spectrum for a magnesium aluminoborate glass. It is shown that the six-coordinate aluminum site (AlO6) preferentially coordinates to the tetrahedral boron site (BO4).     

Heteronuclear Double-Quantum MAS NMR Spectroscopy in Dipolar Solids

A new pulse sequence for high-resolution solid-state heteronuclear double-quantum MAS NMR spectroscopy of dipolar-coupled spin-12 nuclei is introduced. It is based on the five-pulse sequence known from solution-state NMR, which is here applied synchronously to both spin species. The heteronuclear double-quantum (HeDQ) spinning-sideband patterns produced by this experiment are shown to be sensitive to the heteronuclear distance, as well as the relative orientations of the chemical-shift and dipolar tensors. In particular, it is shown that the HeDQ patterns exhibit an enhanced sensitivity to the chemical shielding tensors as compared with the single-quantum spinning-sideband patterns. The detection of HeDQ patterns via the I and S spins is discussed. The isolated (13)C-(1)H spin pair in deuterated ammonium formate with (13)C in natural abundance was chosen as a model system, and the perturbing influence of dipolar couplings to surrounding protons on the (13)C-(1)H DQ coherence is discussed. The pulse sequence can also be used as a heteronuclear double-quantum filter, hence providing information about heteronuclear couplings, and thus allowing the differentiation of quaternary and CH(n) bonded carbons. The elucidation of (13)C-(1)H dipolar proximities is presented for a sample of bisphenol A polycarbonate with (13)C in natural abundance, recorded with a broadband version of the synchronized five-pulse sequence.     

Magic Angle Spinning NMR Spectroscopy with Composite RF Pulses

Using numerical optimization procedures it is shown that it is possible to design composite 180 degrees RF pulses for MAS NMR spectroscopy by explicitly taking into account the variation of the resonance offset of each crystallite during the application of the RF pulses. When using composite RF pulses in experiments such as TOSS, where the delays between the RF pulses have to be critically adjusted, an optimization of these delays can lead to the desired performance characteristics.     

Water–Surfactant Contact Studied byF–H Heteronuclear Overhauser Effect Spectroscopy

Intermolecular¹⁹F–¹H cross-relaxation is measured using heteronuclear Overhauser effect NMR spectroscopy (HOESY) in the micellar solution of cesium pentadecafluorooctanoate. The results are analyzed in terms of a weak¹H–¹⁹F cross-relaxation between the water protons and the fluorines in the fluoroalkyl chain and a strong¹⁹F–¹⁹F cross-relaxation within the fluoroalkyl chain. The water–surfactant cross-relaxation indicates a water approach to the first CF₂segment in the order of 2.0 Å and a short (?ns) water residence time. Evidence of fluorine hydration further inside the micelle is presented.     

Fluorescence Lifetime Correlation Spectroscopy

This article explains the basic principles of FLCS, a genuine fusion of Time-Correlated Single Photon Counting (TCSPC) and Fluorescence Correlation Spectroscopy (FCS), using common terms and minimum mathematics. The usefulness of the method is demonstrated on simple FCS experiments. The method makes possible to separate the autocorrelation function of individual components of a mixture of fluorophores, as well as purging the result from parasitic contributions like scattered light or detector afterpulsing.     

Two-Dimensional CW-ESR Correlation Spectroscopy

In the present paper, a new method called two-dimensional continuous-wave electron spin resonance (2-D CW-ESR) correlation spectroscopy is introduced. This method is based on mathematical analysis of ESR data obtained from common CW-ESR spectra. It is different from conventional 2-D ESR, which is built on multiple-pulse sequences. Synchronous and asynchronous 2-D CW-ESR correlation spectra defined by two independent magnetic field variable axes are generated by a correlation analysis of dynamic fluctuations of ESR signals induced by an external perturbation. The basic property of 2-D ESR correlation spectra is described. Several examples are investigated and discussed on applications of 2-D ESR correlation spectra in the spin self-trapping of 2-methyl-2-nitrosopropane (MNP), the mixture of spin adducts generated by self-trapping of MNP and 5,5-dimethyl-1-pyrroline N-oxide, and the coordination reaction of benzo-15-crown-5 with cupric ion in boron trifluoride-ether solution. The potential applications of this new 2-D CW-ESR method are discussed. Authors' address: Yong Li, Department of Chemistry, Tsinghua University, Beijing 100084, China     

Low-Concentration Photon Correlation Spectroscopy

Photon correlation spectroscopy (PCS) is a technique to measure rapidly particle size in the sub-micrometre region. The use of PCS is, however, limited by concentration. The upper limit is due to multiple scattering of the incident light and the lower limit is determined by the fact that fluctuations of the number of particles in the measuring zone have a significant influence on the apparent diffusion coefficient. In this paper a signal processing method is described which differentiates this influence. With this system the lower limit is no longer limited to about 100 particles in the measuring volume corresponding to a concentration of 10⁹ particles/cm³. The limitation is now the intensity of the scattered light, which becomes too weak at a concentration of about 50 particles/cm³. As a consequence of this work, a revision to the basic theory of PCS may be necessary. Moreover, the new processing method also permits the measurement of the particle concentration in the sample.     

Raman磁共振谱中强单量子信号的压抑

通常单量子信号的强度在Raman磁共振谱中要比其它多量子信号的强度大得多,因而易引起接收机饱和,使信号发生变形,不利于多量子信号的检测,因此压抑强的单量子峰是必要的.积算符理论分析表明,异核体系Raman磁共振谱的单,多量子峰的共振频率及其信号强度关于频偏满足对称关系:当频偏由正变为负时,单量子信号的强度亦随之改变了符号.因此,对正负频偏(数值相等)的两次Raman磁共振实验的FID信号进行累加,就可压抑强的单量子峰,而对于多量子峰则在大的频偏条件下不会相互抵消.因此,这样的实验累加可以保持多量子信号的强度而压抑掉强的单量子峰.作为实验验证,我们给出了异核AX_n体系(CHCl₃,CH₂Cl₂,CH₃OH)的按上述正负频偏循环累加采样方法得到Raman磁共振¹³C实验谱.实验结果同理论预言完全一致.     

Evolution of Two-Dimensional Correlation Spectroscopy

A number of useful techniques associated with two-dimensional correlation spectroscopy(2DCOS)to improve its performance and utility have been developed in the last 30years.Evolution of these 2DCOS techniques,including some of the very recent developments,is reviewed with examples.Topics include merged or modified asynchronous 2Dcorrelation spectrum,two-dimensional codistribution spectroscopy(2DCDS),Pareto scaling,and null-space projection treatment of spectral dataset.     

HSQC-based Methyl Group Selection via Gradients in Multidimensional NMR Spectroscopy of Proteins

A quadruple-quantum filtered HSQC (QQF-HSQC) for the selection of methyl groups which minimizes the line-broadening effects of proton homonuclear couplings is presented. The scheme uses gradients forn-quantum coherence selection and solvent suppression. In contrast to the heteronuclear quadruple-quantum coherence (HQQC) approach, the QQF-HSQC allows for long constant-time (CT) evolution, making use of the generally favorable relaxation properties of methyl groups. The increase in resolution and concomitant gain in sensitivity is discussed in theory and demonstrated in practice on the 14-kDa human nonpancreatic synovial phospholipase A2 (hnps-PLA2). The constant-time version is particularly useful for obtaining high-resolution spectra as demonstrated on hnps-PLA2. The applicability of the CT-QQF-HSQC module in multidimensional experiments is demonstrated using a 3D CT NOESY-QQF-HSQC spectrum of the 31-kDa homodimeric IIAMan domain of the mannose transporter ofE. coli.     

Multi-band Acousto-Optical Filters for Correlation Spectroscopy

Optics and Spectroscopy - The problem of creating optical filters with an arbitrary transmission curve for correlation spectroscopy problems is considered. Experiments showed that acousto-optical...     

Fluorescence Correlation Spectroscopy of Gold Nanoparticles

Diffusion dynamics of gold nanoparticles (GNPs) was studied by fluorescence correlation spectroscopy (FCS). The fluorescence was studied by exciting the particles by green laser (532 nm), which is far from longitudinal plasmon band of nanorods. Transmission electron microscope (TEM) and UV-Vis-NIR spectrometer were used to characterize the gold nanoparticles. Despite their low quantum yields, GNPs possess the native fluorescence. The excellent antiphotobleaching behavior of gold nanorods leads to prospects of using FCS for its detailed studies. Using FCS, dynamic information can be extracted from the fluorescence fluctuations in the system by autocorrelation function. Maximum entropy method (MEMFCS) was used to identify the number of distinct components present in the system. The particle sizes obtained from FCS were found to be higher (by few orders of magnitude) compared to TEM analysis. This might be due to the possible contributions from cetyltrimethyl ammonium bromide (CTAB) capping in the system.