High Resolution NMR An Overview

When we now speak of nuclear magnetic resonance, we are discussing the kind of NMR discovered by Bloch and Purcell, that is, nuclear magnetic resonance in bulk materials. The early work in NMR was concentrated on the elucidation of the basic phenomena (much of which we cover in Chapters 2 and 8) and on the accurate determination of nuclear magnetic moments, which were of interest in elucidating aspects of the structure of the atomic nucleus. 

NMR attracted little attention from chemists until, in 1950, it was discovered that the precise resonance frequency of a nucleus depends on the state of its chemical environment. Proctor and Yu12 discovered accidentally that the two nitrogen nuclei in NH4N03 had different resonance frequencies, and similar results were reported independently for fluorine and hydrogen.13-15 In 1951, Arnold, Dharmatti, and Packard found separate resonance lines for chemically different protons in the same molecule.16 The discovery of this so-called chemical shift set the stage for the use of NMR as a probe into the structure of molecules. 

It is found that chemical shifts are very small, and in order to observe such effects one must study the material under suitable conditions. In solids, where intermole-cular motion is highly restricted, internuclear interactions cause such a great broadening of resonance lines that chemical shift differences are masked (as we discuss in detail in Chapters 2 and 7). In liquids, on the other hand, rapid molecular tumbling causes these interactions to average to zero, and sharp lines are observed. Thus, in the early days of NMR studies, there came to be a distinction 

An NMR spectrum is obtained by placing a sample in a homogeneous magnetic field and applying electromagnetic energy at suitable frequencies to conform to Eq. 1.1. In Chapter 2 we examine in detail just how NMR spectra arise, and in Chapter 3 we delve into the procedures by which NMR is studied. Before we do so, however, it may be helpful to see by a few examples the type of information that can be obtained from an NMR spectrum. 

Several important features are illustrated in Fig. 1.3. First, the chemical shift is clearly demonstrated, for the resonance frequencies depend on the chemical environment (as we study in detail in Chapter 4). Second, the areas under the lines are different and, as we shall see when we examine the theory in Chapter 2, the area of each line is proportional to the number of nuclei contributing to it. Third, the widths of the lines are different in particular, the line due to the OH is considerably broader than the others. (We examine the reasons for different line widths in Chapters 2 and 8.) 

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In this article, an overview is given of this field, with an emphasis on the development of new or inq)roved techniques and methodologies. In order to narrow the scope, only solution and solid state NMR techniques are covered. The polymeric systems chosen as examples are taken from the literature and particularly from the papers included in this synposium volume (42-71). Also included are selected preprints taken firom tiie papers presented at the international symposium on High Resolution NMR Spectroscopy of Polymers held at the ACS National Meeting in April, 2001 in San Diego, CA (72-87). 

A general description is given here of the way in which a modern NMR spectrometer operates, of the various components that go into making a complete system, and the particular role that they play. Today, with the level of computer control present in modern spectrometers, this naturally includes a description of both the hardware and software. An overview diagram of the components of a high-resolution NMR spectrometer is given in Figure 1. 

Obviously, it is highly desirable to develop techniques capable of recovering weak dipolar coupling constants from high-resolution NMR spectra obtained under MAS conditions. The focus of the present discussion will be to provide an overview of the basic RR scheme. First, the theory of RR will be outlined, followed by a discussion of the most important experimental techniques employed to measure dipolar coupling constants under conditions of RR. Finally, some examples that illustrate the applications and limitation of the techniques will be described. 

Highly sophisticated pulse sequences have been developed for the extraction of the desired information from ID and multidimensional NMR spectra [172]. The same techniques can be used for high-resolution 1-NMR, s-NMR and NQR. Pulse experiments are commonly used for the measurement of relaxation times [173], for the study of diffusion processes [174] and for the investigation of chemical reactions [175]. Davies et al. [176] have described naming and proposed reporting of common NMR pulse sequences (IUPAC task group). An overview of pulse sequence experiments has been given [177],... 

An overview of high-resolution solid state NMR applications to polypeptides and membrane proteins has been presented by Luca et a/. The importance of the MAS based techniques at ultrahigh magnetic fields for the studies of insoluble or noncrystalline molecules at the atomic level is highlighted. Recently developed NMR methods suitable for the study of multiply or uniformly [ C, N]-labelled polypeptides and proteins are discussed. In addition, latest biophysical applications are reviewed. 

High-resolution proton NMR spectroscopy can be used to obtain an overview on the phase behavior. The hnewidth and the presence or absence of peaks provides information on the dynamic state of the chain molecules. As an example, the proton spectra of the different samples of SDS/CA/D2O obtained at 30 and 70 °C, which are shown in Fig. 4, are discussed [19]. The signal of D2O, which has a temperature dependent resonance frequency, was used as lock signal and chemical shift reference. Therefore the other signals have temperature-dependent chemical shifts. The signal intensities of all spectra are scaled to obtain equal heights of the large CH2 peaks, which occur at about 1.3 and 1.7 ppm in the spectra at 30 and 70 °C, respectively. 

The evolution of hop chemistry over the last 100 years has paralleled the development of modern chemistry. New chemical tools or insights were rapidly and successfully applied. As an example, NMR spectrometry allowed the elucidation of the structural aspects of hops chemistry in the years 1960-1970. Today, modern liquid chromatography has an increasing impact on the field. Analysis of hops and beer bitter acids has always been associated with the evolution of separation techniques. Therefore, recent developments in high resolution chromatographic techniques is of great value. It is not our intention to provide an exhaustive overview of all known methods for the analysis of hops and beer bitter acids rather the current status and future outlook will be emphasized. 

A field of research, on which solid-state NMR spectroscopy had a tremendous impact during the past decade, is the structural study of amyloid fibrils. Solid-state NMR spectroscopy can provide information on several aspects of the cross-p core structure of amyloid fibrils such as the localization of p-strands within the amino acid sequence and their relative arrangement within protofilaments and at the protofilament interface. Even high-resolution structures for the fibril core have been determined. An overview over emerging central motifs for amyloid structure is given in Fig. 4. 

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