Coherent ultrafast core-hole correlation spectroscopy: x-ray analogues of multidimensional NMR

We propose two-dimensional x-ray coherent correlation spectroscopy for the study of interactions between core-electron and valence transitions. This technique may find experimental applications in the future when very high intensity x-ray sources become available. Spectra obtained by varying two delay periods between pulses show off-diagonal crosspeaks induced by coupling of core transitions of two different types. Calculations of the N1s and O1s signals of aminophenol isomers illustrate how novel information about many-body effects in electronic structure and excitations of molecules can be extracted from these spectra.     

Multiple-time correlation functions in spin-3/2 solid-state NMR spectroscopy

Stimulated echo spectroscopy of nonselectively excitable I = 3/2 nuclei offers new perspectives for the investigation of ultraslow motions predominantly in inorganic solids and solid-like materials. Conditions for the generation of pure, quadrupole modulated multipolar spin orders and for the detection of two- and four-time correlation functions are discussed. The case of spins I > 3/2 is also briefly considered. Copyright 2000 Academic Press.     

Indirectly detected heteronuclear correlation solid-state NMR spectroscopy of naturally abundant 15N nuclei

Two-dimensional indirectly detected through-space and through-bond ¹H{¹⁵N} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse ¹H decoupling are evaluated. Remarkable efficiency of polarization transfer can be achieved at a MAS rate of 40 kHz by both cross-polarization and INEPT, which makes these methods applicable for routine characterizations of natural abundance solids. The first measurement of 2D ¹H{¹⁵N} HETCOR spectrum of natural abundance surface species is also reported.     

A thermometer for nonspinning solid-state NMR spectroscopy

We discuss a method to determine temperature in a static NMR experiment from the temperature variation of the lead nitrate peak shift.     

Multidimensional solid-state NMR spectroscopy of plant cell walls

Plant biomass has become an important source of bio-renewable energy in modern society. The molecular structure of plant cell walls is difficult to characterize by most atomic-resolution techniques due to the insoluble and disordered nature of the cell wall. Solid-state NMR (SSNMR) spectroscopy is uniquely suited for studying native hydrated plant cell walls at the molecular level with chemical resolution. Significant progress has been made in the last five years to elucidate the molecular structures and interactions of cellulose and matrix polysaccharides in plant cell walls. These studies have focused on primary cell walls of growing plants in both the dicotyledonous and grass families, as represented by the model plants Arabidopsis thaliana, Brachypodium distachyon, and Zea mays. To date, these SSNMR results have shown that 1) cellulose, hemicellulose, and pectins form a single network in the primary cell wall; 2) in dicot cell walls, the protein expansin targets the hemicellulose-enriched region of the cellulose microfibril for its wall-loosening function; and 3) primary wall cellulose has polymorphic structures that are distinct from the microbial cellulose structures. This article summarizes these key findings, and points out future directions of investigation to advance our fundamental understanding of plant cell wall structure and function.     

Three-dimensional electrophoretic NMR correlation spectroscopy

A novel method of three-dimensional electrophoretic NMR correlation spectroscopy (3D EP-COSY) has been proposed, developed, and implemented. It has a demonstrated potential of facilitating simultaneous structural assignments of multiple proteins in mixtures. The principle is to add a pulsed DC electric field that introduces a new dimension of electrophoretic flow, in which resonances of different molecules can be separated by their electrophoretic migration rates without physical separation. As a result, two COSY spectra were simultaneously obtained in a single 3D EP-COSY experiment from a mixture of 150 mM l-aspartic acid and 148 mM 4, 9-dioxa-1,12-dodecanediamine with concurrent resolution of their chemical shifts and J-coupling constants. This approach creates a new horizon of multidimensional electrophoretic NMR. The technical advance opens doors for structure characterization of complex protein systems and protein interactions, which are at the basis of biochemical mechanisms and the phenomena of living systems.     

Methods of reconstruction of spectra in multidimensional NMR spectroscopy

In this review, some methods for speeding up the performance of multidimensional nuclear magnetic resonance (NMR) experiments are discussed. It is shown that, at a sufficiently high spectral sensitivity, which does not require multiple scanning with averaging, two-dimensional proton-correlation experiments (COSY and TOCSY) can be performed in less than one minute. A multifold decrease in the time of multidimensional experiments can be achieved by various methods, for example, by the direct excitation of resonance signals with a set of different frequencies obtained using the Hadamard matrix. Methods for reconstructing multidimensional NMR spectra based on the inverse Radon transform and a number of other promising methods are also considered     

Three-dimensional 13C shift/1H-15N coupling/15N shift solid-state NMR correlation spectroscopy.

Triple-resonance experiments capable of correlating directly bonded and proximate carbon and nitrogen backbone sites of uniformly 13C- and 15N-labeled peptides in stationary oriented samples are described. The pulse sequences integrate cross-polarization from 1H to 13C and from 13C to 15N with flip-flop (phase and frequency switched) Lee-Goldburg irradiation for both 13C homonuclear decoupling and 1H-15N spin exchange at the magic angle. Because heteronuclear decoupling is applied throughout, the three-dimensional pulse sequence yields 13C shift/1H-15N coupling/15N shift correlation spectra with single-line resonances in all three frequency dimensions. Not only do the three-dimensional spectra correlate 13C and 15N resonances, they are well resolved due to the three independent frequency dimensions, and they can provide up to four orientationally dependent frequencies as input for structure determination. These experiments have the potential to make sequential backbone resonance assignments in uniformly 13C- and 15N-labeled proteins.     

NH-NH vector correlation in peptides by solid-state NMR

We present a novel solid-state magic angle-spinning NMR method for measuring the NH(i)-NH(i+1) projection angle θ(i,i+1) in peptides. The experiment is applicable to uniformly (15)N-labeled peptides and is demonstrated on the chemotactic tripeptide N-formyl-l-Met-l-Leu-l-Phe. The projection angle θ(i,i+1) is directly related to the peptide backbone torsion angles φ(i) and psi(i). The method utilizes the T-MREV recoupling scheme to restore (15)N-(1)H interactions, and proton-mediated spin diffusion to establish (15)N-(15)N correlations. T-MREV has recently been shown to increase the dynamic range of the (15)N-(1)H recoupling by gamma-encoding, and permits an accurate determination of the recoupled NH dipolar interaction. The results are interpreted in a quasi-analytical fashion that permits efficient extraction of the structural parameters.     

Experimental aspects of proton NMR spectroscopy in solids using phase-modulated homonuclear dipolar decoupling

In this paper we demonstrate experimentally that the continuously phase-modulated homonuclear decoupling sequence DUMBO-1 is suitable for high-resolution proton NMR spectroscopy of rigid solids. Over a wide range of experimental conditions, we show on the model sample L-alanine as well as on small peptides that proton linewidths of less than 0.5 ppm can be obtained under DUMBO-1 decoupling. In particular the DUMBO-1 sequence yields well resolved proton spectra both at slow and fast MAS. The DUMBO-1 decoupling scheme can in principle be inserted in any multi-nuclear or multi-dimensional solid-state NMR experiment which requires a high-resolution 1H dimension. An example is provided with the 13C-1H MAS-J-HMQC experiment.     

Spatially resolved multidimensional NMR spectroscopy within a single scan

We have recently demonstrated that the spatial encoding of internal nuclear magnetic resonance (NMR) spin interactions can be exploited to collect multidimensional NMR spectra within a single scan. Such experiments rely on an inhomogeneous spatial excitation of the spins throughout the sample, and lead to indirect-domain peaks via a constructive interference among the spatially resolved spin-packets that are thus created. The shape of the resulting indirect-domain echo peaks approaches a Sinc function when the chemical's distribution is uniform, but will depart from this function otherwise. It is hereby shown that a Fourier analysis of either the diagonal- or the cross-peaks resolved in these single-scan two-dimensional (2D) NMR experiments can in fact provide a weighted spatial distribution of the analyte originating such peak, thus opening up the possibility of completing spatially resolved multidimensional NMR measurements within a fraction of a second. Principles of this new mode of analysis are discussed, and examples where the potential of spatially resolved ultrafast 2D NMR spectroscopy is brought to bear are presented. Potential extensions of this approach to higher dimensions are also briefly addressed.     

Through-bond heteronuclear single-quantum correlation spectroscopy in solid-state NMR, and comparison to other through-bond and through-space experiments

A new through-bond carbon-proton correlation technique, the MAS-J-HSQC experiment, is described for solid-state NMR. This new pulse scheme is compared experimentally with the previously proposed MAS-J-HMQC experiment in terms of proton resolution on a model sample of powdered L-alanine. We show that for natural abundance compounds, the MAS-J-HMQC and MAS-J-HSQC experiments give about the same proton resolution, whereas, for (13)C-labeled materials, narrower proton linewidths are obtained with the MAS-J-HSQC experiment. In addition we show that in scalar as well as in dipolar heteronuclear shift correlation experiments, when the proton chemical shift is encoded by the evolution of a single-quantum coherence, the proton resolution can be enhanced by simply adding a 180 degrees carbon pulse in the middle of the t(1) evolution time.     

Sensitivity and resolution enhancement in solid-state NMR spectroscopy of bicelles

Magnetically aligned bicelles are becoming attractive model membranes to investigate the structure, dynamics, geometry, and interaction of membrane-associated peptides and proteins using solution- and solid-state NMR experiments. Recent studies have shown that bicelles are more suitable than mechanically aligned bilayers for multidimensional solid-state NMR experiments. In this work, we describe experimental aspects of the natural abundance (13)C and (14)N NMR spectroscopy of DMPC/DHPC bicelles. In particular, approaches to enhance the sensitivity and resolution and to quantify radio-frequency heating effects are presented. Sensitivity of (13)C detection using single pulse excitation, conventional cross-polarization (CP), ramp-CP, and NOE techniques are compared. Our results suggest that the proton decoupling efficiency of the FLOPSY pulse sequence is better than that of continuous wave decoupling, TPPM, SPINAL, and WALTZ sequences. A simple method of monitoring the water proton chemical shift is demonstrated for the measurement of sample temperature and calibration of the radio-frequency-induced heating in the sample. The possibility of using (14)N experiments on bicelles is also discussed.