Broadband selective excitation radiofrequency pulses for optimized localization in vivo.

PURPOSE: The aim of the study is to optimize the performance of localized 1 H MRS sequences at 3T, using the entire spin system of N-acetyl aspartate (NAA) as an example of the large chemical shift spread of all the metabolites routinely detected in vivo, including the amide region. We specifically focus on the design of the suitable broadband excitation radiofrequency (RF) pulses to minimize chemical shift artifacts. METHODS: The performance of the excitation and refocusing pulse shapes is evaluated with respect to NAA localization. Two new excitation RF pulses are developed to achieve optimized performance in the brain using single-voxel 1 H MRS at 3T. Numerical simulations and in vivo experiments are carried out to demonstrate the performance of the RF pulses. RESULTS: New excitation RF pulses with the same B1 requirements but larger excitation bandwidth (up to a factor of 2) are shown to significantly reduce localization artifacts. The large frequency spread of the entire NAA spin system necessitates the use of broadband excitation and refocusing pulses for MRS at 3T. CONCLUSION: To minimize chemical shift artifacts of metabolic compounds with spins in the amide area (>5 ppm) at 3T it is important to use broadband excitation and refocusing pulses.

Proton-driven spin diffusion in rotating solids via reversible and irreversible quantum dynamics.

Proton-driven spin diffusion (PDSD) experiments in rotating solids have received a great deal of attention as a potential source of distance constraints in large biomolecules. However, the quantitative relationship between the molecular structure and observed spin diffusion has remained obscure due to the lack of an accurate theoretical description of the spin dynamics in these experiments. We start with presenting a detailed relaxation theory of PDSD in rotating solids that provides such a description. The theory applies to both conventional and radio-frequency-assisted PDSD experiments and extends to the non-Markovian regime to include such phenomena as rotational resonance (R(2)). The basic kinetic equation of the theory in the non-Markovian regime has the form of a memory function equation, with the role of the memory function played by the correlation function. The key assumption used in the derivation of this equation expresses the intuitive notion of the irreversible dissipation of coherences in macroscopic systems. Accurate expressions for the correlation functions and for the spin diffusion constants are given. The theory predicts that the spin diffusion constants governing the multi-site PDSD can be approximated by the constants observed in the two-site diffusion. Direct numerical simulations of PDSD dynamics via reversible Liouville-von Neumann equation are presented to support and compliment the theory. Remarkably, an exponential decay of the difference magnetization can be observed in such simulations in systems consisting of only 12 spins. This is a unique example of a real physical system whose typically macroscopic and apparently irreversible behavior can be traced via reversible microscopic dynamics. An accurate value for the spin diffusion constant can be usually obtained through direct simulations of PDSD in systems consisting of two (13)C nuclei and about ten (1)H nuclei from their nearest environment. Spin diffusion constants computed by this method are in excellent agreement with the spin diffusion constants obtained through equations given by the relaxation theory of PDSD. The constants resulting from these two approaches were also in excellent agreement with the results of 2D rotary resonance recoupling proton-driven spin diffusion (R(3)-PDSD) experiments performed in three model compounds, where magnetization exchange occurred over distances up to 4.9 Å. With the methodology presented, highly accurate internuclear distances can be extracted from such data. Relayed transfer of magnetization between distant nuclei appears to be the main (and apparently resolvable) source of uncertainty in such measurements. The non-Markovian kinetic equation was applied to the analysis of the R(2) spin dynamics. The conventional semi-phenomenological treatment of relxation in R(2) has been shown to be equivalent to the assumption of the Lorentzian spectral density function in the relaxatoin theory of PDSD. As this assumption is a poor approximation in real physical systems, the conventional R(2) treatment is likely to carry a significant model error that has not been recognized previously. The relaxation theory of PDSD appears to provide an accurate, parameter-free alternative. Predictions of this theory agreed well with the full quantum mechanical simulations of the R(2) dynamics in the few simple model systems we considered.

SPINEVOLUTION: a powerful tool for the simulation of solid and liquid state NMR experiments.

Exact numerical simulations of NMR experiments are often required for the development of new techniques and for the extraction of structural and dynamic information from the spectra. Simulations of solid-state magic angle spinning (MAS) experiments can be particularly demanding both computationally and in terms of the programming required to carry them out, even if special simulation software is used. We recently developed a number of approaches that dramatically improve the efficiency and allow a high degree of automation of these computations. In the present paper, we describe SPINEVOLUTION, a highly optimized computer program that implements the new methodology. The algorithms used in the program will be described separately. Although particularly efficient for the simulation of experiments with complex pulse sequences and multi-spin systems in solids, SPINEVOLUTION is a versatile and easy to use tool for the simulation and optimization of virtually any NMR experiment. The performance of SPINEVOLUTION was compared with that of another recently developed NMR simulation package, SIMPSON. Benchmarked on a series of examples, SPINEVOLUTION was consistently found to be orders of magnitude faster. At the time of publication, the program is available gratis for non-commercial use.

NMR determination of the torsion angle psi in alpha-helical peptides and proteins: the HCCN dipolar correlation experiment.

Several existing methods permit measurement of the torsion angles phi, psi and chi in peptides and proteins with solid-state MAS NMR experiments. Currently, however, there is not an approach that is applicable to measurement of psi in the angular range -20 degree to -70 degree, commonly found in alpha-helical structures. Accordingly, we have developed a HCCN dipolar correlation MAS experiment that is sensitive and accurate in this regime. An initial REDOR driven (13)C'--(15)N dipolar evolution period is followed by the C' to C(alpha) polarization transfer and by Lee--Goldburg cross polarization recoupling of the (13)C(alpha)(1)H dipolar interaction. The difference between the effective (13)C(1)H and (13)C(15)N dipolar interaction strengths is balanced out by incrementing the (13)C--(15)N dipolar evolution period in steps that are a factor of R(R approximately omega(CH)/omega(CN)) larger than the (13)C--(1)H steps. The resulting dephasing curves are sensitive to variations in psi in the angular region associated with alpha-helical secondary structure. To demonstrate the validity of the technique, we apply it to N-formyl-[U-(13)C,(15)N] Met-Leu-Phe-OH (MLF). The value of psi extracted is consistent with the previous NMR measurements and close to that reported in diffraction studies for the methyl ester of MLF, N-formyl-[U-(13)C,(15)N]Met-Leu-Phe-OMe.

Accurate determination of interstrand distances and alignment in amyloid fibrils by magic angle spinning NMR.

Amyloid fibrils are structurally ordered aggregates of proteins whose formation is associated with many neurodegenerative and other diseases. For that reason, their high-resolution structures are of considerable interest and have been studied using a wide range of techniques, notably electron microscopy, X-ray diffraction, and magic angle spinning (MAS) NMR. Because of the excellent resolution in the spectra, MAS NMR is uniquely capable of delivering site-specific, atomic resolution information about all levels of amyloid structure: (1) the monomer, which packs into several (2) protofilaments that in turn associate to form a (3) fibril. Building upon our high-resolution structure of the monomer of an amyloid-forming peptide from transthyretin (TTR(105-115)), we introduce single 1-(13)C labeled amino acids at seven different sites in the peptide and measure intermolecular carbonyl-carbonyl distances with an accuracy of ~0.11 A. Our results conclusively establish a parallel, in register, topology for the packing of this peptide into a ß-sheet and provide constraints essential for the determination of an atomic resolution structure of the fibril. Furthermore, the approach we employ, based on a combination of a double-quantum filtered variant of the DRAWS recoupling sequence and multispin numerical simulations in SPINEVOLUTION, is general and should be applicable to a wide range of systems.

High-resolution solid-state NMR structure of alanyl-prolyl-glycine.

We present a de novo high-resolution structure of the peptide Alanyl-Prolyl-Glycine using a combination of sensitive solid-state NMR techniques that each yield precise structural constraints. High-quality (13)C-(13)C distance constraints are extracted by fitting rotational resonance width (R(2)W) experiments using Multimode Multipole Floquet Theory and experimental chemical shift anisotropy (CSA) orientations. In this strategy, a structure is first calculated using DANTE-REDOR and torsion angle measurements and the resulting relative CSA orientations are used as an input parameter in the (13)C-(13)C distance calculations. Finally, a refined structure is calculated using all the constraints. We investigate the effect of different structural constraints on structure quality, as determined by comparison to the crystal structure and also self-consistency of the calculated structures. Inclusion of all or subsets of these constraints into CNS calculations resulted in high-quality structures (0.02A backbone RMSD using all 11 constraints).

High-performance selective excitation pulses for solid- and liquid-state NMR spectroscopy.

Computer-optimized selective pulses are routinely used in solution-state NMR spectroscopy. At the same time, their utility and importance for solid-state applications has yet to be fully realized. We suggest a new computational approach that makes the design of soft selective pulses with desired properties relatively straightforward. By applying this technique to the generic selective excitation problem, we have arrived at a family of high performance selective excitation pulses, dubbed E-Family, that allows more flexibility and better performance than analogous pulses previously reported in the literature. The new pulses have been successfully tested in both solid- and solution-state NMR experiments. A theoretical treatment of the effects of chemical shift anisotropy (CSA) on the selective excitation in magic-angle spinning (MAS) experiments in solids is presented. The set of heuristics that comprise our new strategy were incorporated into a general NMR simulation program SPINEVOLUTION.