FT NMR

8 Key excerpts on "FT NMR"

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  eBook - ePub

  Compendium of Biomedical Instrumentation

  • Raghbir Singh Khandpur(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  FT‐NMR spectrometers have an integration algorithm included in the spectrometer software, and the resonance peak areas may be presented graphically as stepped lines or tabulated as numeric values. Sensitivity enhancement is the major attraction of the Fourier transform technique, which has great importance in biochemical field.

  Figure 341.3 shows a typical FT‐NMR installation Fourier 300 from M/s Bruker, which is an ideal system for chemical education and routine analysis. The software packages that come with this instrument can expand its use to include structure elucidation, structure consistency checks, analysis of mixtures, and absolute quantification experiments.

  Figure 341.3 Typical 300 MHz FT‐NMR spectrometer.

  Source: Photo reproduced with permission from M/s Bruker, UK.

  Specifications

  FT‐NMR spectrometer

  • Magnet: 400 MHz, superconducting magnet at 9 T
  • Magnet drift: <4 Hz/h
  • Cooling: Liquid helium, hold time of >300 days
  • Shims: At least 20 room temperature shims
  • Gradient coils: For gradient shimming
  • Frequency resolution: <0.005 Hz or better
  • NMR software: For 1D and 2D acquisition and processing
  • Temperature: Variable below and above ambient (−140 to +150 °C)

  Applications

  NMR spectroscopy finds applications in the study of the structure of molecules, the interaction of various molecules, the kinetics or dynamics of molecules, and the composition of mixtures of biological materials. Biochemical information can also be obtained from living tissue with this technique. The magnetic resonance imaging (MRI

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  eBook - ePub

  Understanding Bioanalytical Chemistry

  Principles and Applications

  • Victor A. Gault, Neville H. McClenaghan(Authors)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  In the NMR spectrum, T 2 time determines the width of the NMR signal, where nuclei with large T 2 times give rise to sharp signals and those with shorter T 2 times have broader signals. Relaxation effects have structural and dynamic applications, where T 1 measurements are useful for dynamic (motional) studies, while the so-called nuclear Overhauser effect can give structural information (such as internuclear separations). Together these approaches can give lots of information about the molecules being studied. Principles of NMR spectroscopy NMR spectroscopy exploits the magnetic properties of nuclei to give important qualitative and quantitative information on biological samples. There are various different forms of this important technique, but in each case NMR spectroscopy relies on fundamental NMR theory discussed earlier. There are three essential requirements for NMR recordings: (i) an intense and static (stable) magnetic field with defined field strength; (ii) a source of RF radiation (high frequency transmitter) to irradiate and excite nuclei in the sample; and (iii) a method or device to detect the NMR signal (record resonance energy). A typical NMR spectrometer is outlined in Figure 11.3. The earliest form of NMR spectroscopy relied on CW methods, where experiments were conducted with the RF field present throughout, bringing nuclei with different chemical shifts into resonance by either keeping the electromagnetic frequency fixed and sweeping the magnetic field, or vice versa. However, the origin of pulse methods including FT-NMR soon superseded CW methods. Pulse methods depend on applying a short intense burst of RF radiation to a sample to set nuclear moments spinning around the line of the magnetic field in a motion called precession. As such, each nucleus precesses around the static field direction at a frequency called the Larmor frequency

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  eBook - ePub

  Polymer Characterization

  Physical Techniques, 2nd Edition

  • Dan Campbell, Richard A. Pethrick, Jim R. White(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  6.2.5 .

  Early NMR spectrometers operated in a continuous wave mode. Experimentally this may be carried out by either fixing the frequency (v) and varying the applied field (B) until the resonance condition is satisfied, or by varying the frequency at constant field strength. In practice both methods have been employed. However, continuous wave spectrometers have been largely superceded by machines employing Fourier Transform techniques and these will be considered in the following sections.

  6.2.3 Fourier Transform Spectroscopy

  Fourier transform spectroscopy relies on the response of the magnetic nuclei to a broad band frequency source which simultaneously excites the spin system with a range of frequencies. The resultant signal is transformed to give the NMR spectrum. The broad band of frequencies is achieved by applying to the sample a strong RF pulse having a frequency close to the Larmor frequency. Application of such a pulse for a duration r gives an effective range of frequencies of 1/τ . For example, for a pulse duration of 5 ps, the bandwidth is 200 kHz. This is equivalent to simultaneous illumination of the sample with a range of frequencies. Provided that the resonance frequencies of the nuclei in the sample lie within this frequency range, they will be excited to their upper energy state. In practice it is desirable to select τ −1 to be one or two orders of magnitude greater than the chemical shift range (Section 6.2.6 ) under investigation.

  Pulse techniques may be treated by considering the vector model of nuclear spin angular momentum (Fig 6.2 ). In the absence of a magnetic field the spins are oriented randomly with equal numbers of α and β spins. When a magnetic field is applied the spins precess about the field or z direction and there is a net magnetization (M ) in the direction of the field. This is because there is a slight excess of nuclear spins in the lower energy state given by the Boltzmann distribution. The application of a radiofrequency pulse, having a field strength B 1 at right angles to the main magnetic field (in the x–y plane) for a certain time τ (typically of the order of microseconds) causes the magnetization vector to rotate towards the x–y plane. The nutation angle θ , in radians, through which it rotates is given by θ = γ B 1 τ . When θ = 90° this is termed a 90° pulse and the magnetization vector is then rotating in the x–y plane with the Larmor frequency. (A 180° pulse is equivalent to inversion of the populations of the α and β energy levels.) It is sometimes more convenient to consider the phenomenon by using a rotating frame of reference. With the reference frame rotating with the Larmor frequency, the application of the radiofrequency (B 1 ) may be said to rotate the magnetization vector into the x–y

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  eBook - ePub

  Data Processing Handbook for Complex Biological Data Sources

  • Gauri Misra(Author)
  • 2019(Publication Date)
  • Academic Press

    (Publisher)

  The chapter offers a practical tool concerning the essential procedures related to principle, data collection, analysis, and interpretation of nuclear magnetic resonance (NMR) applied to biological sciences. The primary purpose is to introduce the reader with a brief history and the main applications of the technique, besides presenting an essential explanation of the physical principles required for understanding the subject. An informative theoretical base enables the user to acquire a critical approach to deep discussions and problem solving to biological objectives. The various examples delineate the different dimensions of the technique with the help of vivid illustrations that are routinely used for structural understanding of various macromolecules and biological phenomena. The chapter outlines the instructions for the beginners, describing the preparation of the NMR samples, and machine usage for accurate measurements. The sample NMR spectra, real NMR data, and stepwise descriptive interpretation are presented for better data analysis and correlations with the user experience. It helps to clarify the fundamental concepts presented in the literature. Several examples of well-established online tools and software are offered to give a wide range of options, helping in advanced data processing and analysis. An overview of informative instructions and aspects of each tool is provided for the reader to make a better choice. Further literature references are provided that are mandatory to develop the skills of the user in this powerful technique.

  Keywords

  Nuclear magnetic resonance; chemical shift; 1D NMR; 2D NMR; data analysis; NMR spectrum acquisition

  5.1 Introduction

  The nuclear magnetic resonance (NMR) technique applied to biological systems is a powerful tool for structural determination and dynamics of both small organic molecules and biopolymers. The dynamic aspects and inherent flexibility play a central role in the facets of biological functionality [1 ,2] . Regarding natural biopolymers, such as proteins, nucleic acids, and carbohydrates, the structural fluctuations of these molecules can range from nanoseconds to hours, referred to as molecular dynamics [3 –5] .

  The currently available NMR experiments cover a broad spectrum of the observable time-scales phenomena showed by both small molecules and biopolimers (Fig. 5.1 ). It is possible to monitor structural fluctuations, which may offer information about fast or slow movements. The cellular environment experienced by the molecule includes a wide range of pH, salinity, solutes, and temperature, which exhibits a significant effect on the structure of these molecules [6 –10] .

  Figure 5.1 General scheme of the NMR techniques and the phenomena associated.

  The history of NMR starts in the early 1940s, specifically in 1943, with Dr. Otto Stern at the Carnegie Institute of Technology, United States. He received the Nobel Prize in 1944 for the description of the angular momentum of quantized subatomic particles. He proposed electrons and atoms can act as rotational punctual charges and thus, generate a magnetic field on their surroundings. This work, therefore, opened the doors for further studies of the magnetic properties of nuclei [11]

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  eBook - ePub

  Spectroscopy for Materials Characterization

  • Simonpietro Agnello, Simonpietro Agnello(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  10 Nuclear Magnetic Resonance Spectroscopy Alberto Spinella1 and Pellegrino Conte2 1 Advanced Technologies Network Center (ATeN Center), University of Palermo, Palermo, Italy 2 Department of Agriculture, Food and Forestry Sciences, University of Palermo, Palermo, Italy

  10.1 Introduction

  Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique used in many fields from basic to applied sciences in order to characterize both molecular and supramolecular structures of organic and inorganic compounds. Among the various spectroscopic techniques, NMR is one of the most versatile due to its ability to unveil three‐dimensionality of molecular systems in each of the three different physical states (liquid, solid, and gas) and regardless of sample crystallinity.

  Purcell, Torrey, and Pound at Harvard University [1] and Bloch, Hansen, and Packard at Stanford University [2] performed the first NMR experiments in condensed matter independently of each other in 1945. In particular, they obtained NMR signals from protons of paraffin wax and liquid water, respectively. However, since those pioneering days, NMR has become a well‐established spectroscopic technique. In particular, both liquid and solid‐state NMR spectroscopies are largely exploited for molecular structure determination and dynamics in several chemistry fields (e.g. protein chemistry, materials science, cultural heritage, food science, and so on), while NMR imaging is recognized as a very powerful tool in modern medicine in order to monitor biological tissues and living functions.

  In the following, an overview of the basic principles of NMR spectroscopy and the main topics that may be useful to researches of different areas will be given. A detailed and in‐depth introduction to NMR spectroscopy can be found in several textbooks [3 –5 ].

  10.2 NMR General Concepts

  10.2.1 Nuclear Spin and Magnetic Moment

  All elementary particles have an intrinsic property referred to as spin. This property has been introduced to explain the deflection of the elementary particles when they pass through an inhomogeneous magnetic field. In fact, according to the experiment by Stern and Gerlach, an elementary particle, such as an electron, being a moving electric charge, should behave like a small linear magnet when crossing a magnetic field gradient. Therefore, a continuous distribution should be revealed on the surface of a detector. Conversely, the image obtained by elementary particles crossing the magnetic field gradient is an ensemble of discrete points of accumulation. This suggests that all the elementary particles have an intrinsic spin angular momentum which can have only discrete values given by:

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  eBook - ePub

  Practical Approaches to Biological Inorganic Chemistry

  • Robert R. Crichton, Ricardo O. Louro(Authors)
  • 2019(Publication Date)
  • Elsevier

    (Publisher)

  Chemical exchange  177
• Multidimensional nuclear magnetic resonance  179
  • How do the correlations arise and how are cross-peaks generated?  180
  • The COSY  181
  • The NOESY  181
  • The HSQC  182
• Metals in biomolecular nuclear magnetic resonance spectra  183
  • Transition metals and interaction with the unpaired electron(s)  184
• Hyperfine scalar coupling  184
• Dipolar coupling  184
  • Relaxation  186
• Contact relaxation  186
• Dipolar relaxation  186
• Curie relaxation  188
• Residual dipolar couplings  188
• Nuclear magnetic resonance of (semi-)solid samples  188
  • Direct observation of metals by nuclear magnetic resonance  189
• In-cell nuclear magnetic resonance  190
• An nuclear magnetic resonance spectrometer: measuring macroscopic magnetization and relaxation  191
• Care in obtaining nuclear magnetic resonance spectra of paramagnetic samples  192
  • Water eliminated Fourier transform and super-water eliminated Fourier transform sequences: catching up with fast relaxing samples  193
  • Evan’s method: measuring magnetic susceptibility  195
• Conclusions  197
• Further reading  197
• Useful physical constants  198
• Exercises  198
• Answers  199