Showing papers in "Annual reports on NMR spectroscopy in 1995"

Magnetic Resonance Microscopy

Abstract: Publisher Summary Magnetic resonance (MRI) can produce three-dimensional images of foods non-invasively. The images represent true 3D information of the interior of the material. No slicing or sectioning of the material is necessary to prepare it for imaging, and the material may be subjected to processing conditions during imaging. Various techniques can be used to study different molecules with the same basic instrumentation. Advances in MRI technology have increased the spatial resolution of images, allowing for the microscopic study of small regions of interest. Magnetic resonance (MR) microscopy is the terminology used for these high-resolution MRI techniques that yield images with spatial resolution of 100 μ m or less. The purpose of this chapter is to provide an overview of MR microscopy, including the theory and instrumentation behind the technology, as well as a summary of published studies and possibilities for future food applications.

One-Bond 13C-13C Spin-Spin Coupling Constants

Abstract: Publisher Summary Developments in nuclear magnetic resonance (NMR) instrumentation combined with the introduction of the Fourier transform (FT) pulse technique allows to measure 13C-13C couplings at natural abundance of 13C isotope. The pulse sequences, such as the one- and two-dimensional incredible natural abundance double quantum transfer experiment (INADEQUATE) technique and its further improvements and/or substitutes, simplify the interpretation of the spectra considerably. This has made 1J (CC) couplings easily accessible even in the case of large molecules. In spite of the initial assumptions, one-bond carbon-carbon couplings undergo strong variations upon substitution and complexation and are, in this way, a sensitive measure of the electronic structure of carbon-carbon bonds. This chapter reviews the factors that determine the 1J (CC) magnitude. Two factors are taken into consideration to estimate the unknown one-bond CC coupling value. These are: (i) hybridization of the bonding orbitals involved and (ii) the ring size of the compound. The electronegativity of the substituents is another important factor that determines the magnitude of one-bond CC couplings. One of the most useful applications of both one- and two-dimensional INADEQUATE experiments concerns the determination of carbon-carbon connectivity of the carbon network.

Carbon-13 NMR Spectra of Sesquiterpene Lactones

Abstract: Publisher Summary This chapter presents the carbon-13 nuclear magnetic resonance (NMR) data of sesquiterpene lactones. The structure classification and methods of structure determination are also described in the chapter. The sesquiterpene lactones are successfully used in chemotaxonomical studies and there are various biological activities of sesquiterpene lactones that form a basis for the structure–activity relation studies. Large numbers of sesquiterpene lactones are isolated from the plant material. The sesquiterpenoids are C15-compounds formed of three isoprenoid units. Based on the biogenetic assumptions, sesquiterpene lactones arise from a common precursor, farnesyl pyrophosphate, by various modes of cyclization followed in many cases by skeletal rearrangements. Their classification in general is based on the carbocyclic skeleton. The combination of various spectral methods is the most common way to determine the structure of the sesquiterpene lactones. The most important methods discussed are: mass spectra, infrared, ultraviolet, NMR, and X-ray. The 13C NMR spectra of sesquiterpene lactones are recorded with proton broad-band decoupling that eliminates the splitting from 13C-lH spin interactions and hence, the 13C signals appear as singlets. The structural assignments of the signals can be determined by chemical shifts, signal multiplicities, relaxation times, or shift reagents.

Stable Isotope Analysis of Food and Beverages by Nuclear Magnetic Resonance

Abstract: Publisher Summary Nuclear magnetic resonance is the only analytical method that is able to detect simultaneously the different isotopomers of a given chemical species, provided the isotope under consideration has a magnetic moment and the chemical shift discrimination is large enough. In 1981, it was shown that deuterium atoms at natural abundance exhibit large deviations with respect to a statistical distribution among the different sites of a molecule. In order to estimate the accuracy of the NMR determinations an isotopic dilution technique can be used. The NMR performances are then compared to those of the balance and it is possible to appraise the linearity of the NMR response and therefore the consistency of possible systematic errors. The ability of the method to detect adulteration by exogenous sugars is improved when environmental conditions are taken into account. More than 800 carbohydrate samples of starches or sugars extracted from cereals, tubers and leguminosae (maize, sorghum, rice, wheat, barley, potato, bean), from fruits (pineapple, citrus, apple, soft fruits), and from sugar plants (beet, cane) were studied in order to constitute a confident database.

NMR Applications to Porous Solids

Abstract: Publisher Summary This chapter discusses the nuclear magnetic resonance (NMR) applications to porous solids. One major NMR application to porous solids is the investigation of the structure of the solid and to gain information about the solid surface, where possible. Another area is investigating the behavior of the adsorbed species. These may be probe molecules that reveal something about the structure of the solid. Porous materials can be divided into the classification of microporous (pore diameters 500 A). The materials containing both micropores and mesopores are also discussed in the chapter. Because of their great commercial applications, zeolites are extensively studied by NMR spectroscopy. The 29 Si MAS NMR spectrum provides indirect information about the Si-O H -AI Bronsted acid sites in the hydrogen-exchanged form of the zeolite. Mesoporous materials are of considerable importance as sorbents and catalyst supports. They give rise to fairly broad NMR peaks, reflecting the range of local environments present. The NMR of silica, alumina, and as a probe of pore size are described in the chapter. There are a large number of NMR investigations into heterogeneous catalyst systems supported on silica and/or alumina. 129 Xe NMR is useful in identifying more than one adsorption zone inside a zeolite, as in the case of mordenite, offretite, and zeolite rho, provided that xenon exchange between the zones is not fast on the chemical shift times.

NMR of Lipids

Abstract: Publisher Summary Nuclear magnetic resonance spectroscopy (NMR) is now a standard analytical technique that has been applied in a number of ways to the study of lipids. This chapter describes the use of low-resolution 1H NMR and of high-resolution 13C NMR spectroscopy. The use of imaging techniques is described elsewhere in this book. High-resolution 1H NMR spectra are of value in determining the structure of individual organic compounds but have found only limited use in the study of fat mixtures. The CH3 signal for n-3 acids differs from that for other unsaturated and for saturated acids; In the period 1955-70 dilatometry was the most widely accepted method for the characterization of fats. The development of a continuous-wave (wide-line) instrument (Newport Analyser) was investigated in detail from 1968 to 1975. It offered certain advantages regarding speed and ease of operation. However, serious drawbacks remained. No efforts were pursued to employ pulsed NMR for solid fat determination until 1973 because of the unavailability of cheap instruments. Bruker was the first to produce commercially a fully automated instrument for this purpose, followed by the Praxis Corporation USA. Pulsed low-resolution NMR is becoming a standard technique for the determination of solid fat content (SFC). Other applications that have found a positive response in the food industry include studies of fat polymorphism, studies of fatty emulsions, determination of oil content of seeds and meals, and determination of humidity in a variety of foodstuffs.

Miscibility, Morphology and Molecular Motion in Polymer Blends

Abstract: Publisher Summary This chapter discusses the miscibility, morphology, and molecular motion in 13C high-resolution solid-state nuclear magnetic resonance (NMR) measurements applied to a blend of two polymers Since miscibility is very important for a blend, various NMR techniques are developed The chapter also explains how to specify the origin of interpolymer interactions between the component polymers Without such interactions, most of the polymer pair does not mix well NMR approaches to study the heterogeneity in a blend are also focused in the chapter It also includes the interface of domains and phase separation The molecular motion in a blend is discussed in the chapter For some polymer pairs, microscopic mixing at the molecular level is achieved Macroscopic properties of a blend are influenced largely by its microscopic degree of mixing In most NMR methods, microheterogeneity manifests itself in the magnetic relaxation phenomena, such as spin diffusion and spin-lattice relaxation Some techniques to examine cross-relaxation phenomena induced by heteronuclear dipole interactions are summarized in this chapter One of the typical cross-relaxation phenomena is the transient nuclear overhauser effect (NOE) that is a function of interspin distance However, all the techniques rely on a dipole-dipole (DD) interaction that is a function of the internuclear distance The morphology can be determined by NMR imaging, lineshape, microscopic heterogeneity, or thermally induced morphological change

The Use of ESR Spectroscopy for the Identification of Irradiated Food

Abstract: Publisher Summary The use of ionizing radiation for the preservation of food is not a new technology. About 90 years ago, a patent was issued in the United Kingdom that detailed the use of the process for the preservation of foods, especially cereals and their products. Despite this initial interest in the technology, progress was hindered because of the limited availability of suitable sources of ionizing radiation. About 1950, 60 Co that generated gamma photons and machines producing high-energy electrons began to become available and as a consequence extensive research programmes started in the United States. Since then, many countries throughout the world have been involved in evaluation of the technology for the preservation of a wide range of foods and the process is used commercially in a number of countries. In combination with good hygienic practices, the process is effective in enhancing food safety by reducing the numbers of pathogenic microorganisms including Listeria, Salmonella, and Campylobacter, which are often implicated in food poisoning outbreaks involving poultry, meat, fish, and shellfish. The process also kills spoilage microorganisms and so the shelf life of these same foods can be extended. Ripening of fruits can be delayed by irradiation and the technology provides an alternative to chemicals that have been or are still being used to decontaminate spices and herbs, disinfest cereals and tropical fruit and inhibit sprouting in tubers such as potatoes.

On-line Applications in Food Science

Abstract: Publisher Summary On-line process control can often improve productivity and quality in the food industry by allowing a rapid identification of the raw materials used as ingredients and by adjusting the blending to keep the final composition of the product constant. For these reasons on-line process control is a rapidly growing branch of analytical chemistry. On-line sensors must be able to collect chemical or physical information from a sample and convert them into an interpretable signal for regulation within a short time according to the downstream process. On-line sensors for use in the food industry must meet special requirements such as food compatibility of the material in contact with the food sample and the ability to be easily cleaned and sterilized. For these reasons, the on-line use of sensors, which are sensitive to the chemical composition of the product, is poorly developed in the food industry. Biosensors, despite their high sensitivity and selectivity are often unsuitable for industrial purposes as they are unsterilizable, unreliable, and invasive. Among physicochemical sensors, only infrared techniques have been introduced in industrial production and until now very few examples describe the use of an NMR sensor particularly in the food industry.

NMR in Context

Abstract: Publisher Summary The materials encountered in food science may be examined by a number of spectroscopic techniques In order to understand the relationship between magnetic resonance spectroscopy and other spectroscopies it is useful to consider the range of frequencies available in the electromagnetic spectrum Moving to lower energy, hard X-rays are associated with nuclear events Softer X-rays cause transitions among core electron levels With ultraviolet and visible light the distance scale increases again and transitions among valence electrons are caused The near-infrared and mid-infrared regions operate over distance scales greater than a single atom and are typically associated with vibrational transitions Thus they move from atomic to molecular distances At lower energy still, the far-infrared causes transitions in very low-energy vibrations such as polymer backbone motions and hydrogen bonds Both the far-infrared and the microwave regions can cause rotational transitions As chemistry is largely associated with valence electrons and molecular structure, it is only those spectroscopies that report on appropriate length scales that are expected to respond to these effects

NMR Studies in Meat

Abstract: Publisher Summary Low-field nuclear magnetic resonance (NMR) affords a cheap, reliable, and rapid method of measuring fat content, solid/liquid ratios, and water binding in many foods. Until 1980 the use of NMR in meat science was almost entirely restricted to the assay and the dynamics of water. In 1974, it was reported that informative 31 P NMR spectra could be obtained from intact muscle. About 3040% of the weight of the live animal consists of skeletal muscle. Bones and other connective tissues provide support for the skeletal muscle system. Adipose tissue provides a storage site for energy. The meat industry needs to know the relative quantities of muscle, bone, and fat in the carcass. The main constituents of muscle are water (=75%), protein (18-20%), carbohydrates (1%), and fat ( ∼ 2 % ) . Fat and protein are also to be determined accurately and quickly in meat products. Fat content decreases as water content increases and vice versa. The water-holding capacity of muscle is particularly relevant to meat quality and loss of water during storage is an important economic factor. Water-holding capacity is very sensitive to structural changes in protein and many NMR studies provide a better understanding of the water-protein interaction.

Calculation and Prediction of Structural Nmr Shifts in Respiratory Proteins

Abstract: Publisher Summary Nuclear magnetic resonance (NMR) is a significant tool in the determination of three-dimensional protein structures, driven by the advances in the multi-dimensional NMR techniques. These techniques exploit the nuclear overhauser effect (NOE) to extract distances between particular nuclei. These distances can then be used as constraints in conjunction with energy minimization to determine the native protein structure in the solution. This chapter describes the calculating chemical shifts in macromolecules and prediction of structural NMR shifts in respiratory proteins. Various approaches used are taken to calculate the structural shifts in macromolecules. Empirical relations for the experimentally observed chemical shifts in proteins of known structure are discussed in the chapter. Theoretical methods are developed; ab initio methods for calculation are limited to molecules that are quite small relative to proteins. Small changes in electronic structure are readily manifested in the chemical shielding. These small changes that result from non-covalent interactions, such as electrical perturbation, ring currents, and magnetic anisotropy can be described by the classical approaches. These long-range influences are also discussed in this chapter. However, variation in the covalent bonding, such as changes in bond lengths and bond angles, are described quantum mechanically. The property correlations in respiratory proteins are also discussed in this chapter.

Conformational Characterization of Polysaccharides as Studied by High-resolution 13C Solid-state NMR

Abstract: Publisher Summary The physical properties of a polysaccharide as well as its biological response may be strongly related to its secondary structures in the solid, gel, or solution states. However, the elucidation of the three-dimensional structure of a polysaccharide, either in the solid or solution state, is not as straightforward as for proteins or nucleic acids. This is because crystallization of polysaccharides for X-ray diffraction study is extremely difficult and the number of nuclear Overhauser effects between sugar residues is usually not sufficient for the purpose of model building. A possible disruption of secondary structures on solubilization should also be taken into account in the interpretation of solution-state NMR data. Many polysaccharides that are in common use are not entirely crystalline and may appear to be amorphous as viewed by the X-ray diffraction method. It should be emphasized, however, that even such amorphous materials retain certain secondary structures that can be analysed by 13 C NMR and can be converted to other forms by a variety of physical treatments. The advantage of the NMR approach for this sort of conformational characterization is its inherent sensitivity to a conformation of short-range order as compared with X-ray diffraction.

Probing Water Relations in Foods Using Magnetic Resonance Techniques

Abstract: Publisher Summary Water is an essential and pervasive component of life. It is abundant in all three physical states-solid, liquid, and gas. Water serves a sundry of vital functions: as an environmental and body regulator, as a transportation and solvent medium for cellular functions, as a stabilizer of biopolymer conformation, as a reactant, product and solvent for many chemical and biological reactions, as a major constitution of most foods, and countless other functions. The importance of water in foods begins with the hydrological cycle, that is the continuous circulation of water by evaporation from the hydrosphere and its subsequent precipitation back from the atmosphere. This cycle provides the necessary water for the fundamental elements of the world's food supply, crop, and animal production. Water continues to be a vital component throughout the various stages of food production, such as harvest or slaughter, addition of other ingredients, processing, and storage. NMR techniques have been successfully used in a number of disciplines, such as chemistry, polymer science, and food science, to study a variety of phenomena ranging from molecular structure to water dynamics to chemical composition. Based on the principles of NMR, Lauterbur developed an imaging method to map the spatial distribution of select nuclei within an object, which he called NMR zeugmatography.

Wide-line and High-resolution 1H NMR of Food Materials

Abstract: Publisher Summary Objectives of NMR spectroscopy of foods include non-destructive chemical analysis and characterization of molecular state relevant to functionality. To achieve these objectives, NMR signals from specific components must be obtained on intact material. Molecules in liquid-like states normally produce narrow NMR resonances (tenths to a few Hz) with molecular structure-dependent frequencies or chemical shifts. Thus it should be easy to identify specific components in liquids by NMR. Molecules in a solid-like state generally produce a much broader resonance (1000 to 100 000 Hz), which is inhomogeneousthat is, since magnetic interactions between nucleus and environment are orientation-dependent , the multiple orientations of molecules in a sample produce many resonances with slightly different frequencies. Thus a broad resonance contains information about local structure and motion. The dominant broadening interaction affecting protons in organic materials is the homonuclear magnetic dipolar interaction. Molecular motion exchanges orientations and hence frequencies in the inhomogeneous solid resonance, leading to narrowing of broad resonances. Analysis of lineshapes produces information about such restricted mobility.

Applications of NMR to Drying

Abstract: Publisher Summary NMR and NMR imaging have a promising future with respect to the study of the fundamentals of drying and the development of on-line sensors for moisture In this chapter, the role that NMR can play in understanding the fundamental processes of drying is addressed With NMR imaging, changes in these quantities with respect to space and time are available and they provide a potentially valuable resource for the study of transport phenomena during drying and for the study of moisture transport in multi-phase materials in general The field of drying is a diverse one, unified under the general topic of moisture transport in and interaction with solids As such, the field ranges from the study of two-phase flow in rocks, to equilibrium measurement of water in foods, to measurements of ink-drying rates for jet printers As this chapter emphasizes NMR and foods, the examples cited predominantly involve food materials; however there are many instances where results from other fields are helpful in understanding how NMR is useful in food research

Studies of Protein Structure by NMR Spectroscopy

Abstract: Publisher Summary A description of NMR studies of proteins can conveniently be broken into three parts: the magnetic properties and behavior of spins in proteins, the structural information that can be obtained from NMR studies, and the NMR experiments themselves. To a large extent, the developments in the last have been driven by the promise of the second and have fuelled new progress. This contribution will describe the physical basis for NMR experiments and the general attributes of NMR spectra, a blend of the first two categories, and then describe some applications to protein systems. The description of applications is divided into two parts. The first represents a rough historical development, which is appropriate because NMR experimentation has developed from very simple experiments to the rather complex experiments possible today. The second part is a more detailed account of specific applications to problems of direct interest to food science. In many cases, detail is sacrificed in order to save space. This is justified by the fact that clear and complete descriptions of theory already exist. The fundamentals of nuclear magnetism are given in the authoritative work by Abragam.