NMR Spectroscopy in Biology and Medicine PDF

Basic principles of nuclear magnetic resonance. NMR spectroscopy in biology and medicine The text under the slides was written by Andrea Dóczy-Bodnár 2019 This instructional material was prepared for the biophysics lect...

Basic principles of nuclear magnetic resonance. NMR spectroscopy in biology and medicine The text under the slides was written by Andrea Dóczy-Bodnár 2019 This instructional material was prepared for the biophysics lectures held by the Department of Biophysics and Cell Biology Faculty of Medicine University of Debrecen Hungary https://biophys.med.unideb.hu 1 The slide summarizes the main aims and topics of the lecture. The phenomenon of Nuclear Magnetic Resonance (NMR) is the consequence of the interaction of atomic nuclei possessing magnetic moment with a static external magnetic field. In addition to spectroscopy applications, the NMR phenomenon is utilized in Magnetic Resonance Imaging (MRI), a non-invasive diagnostic method with high importance in medical practice. This lecture discusses the key concepts needed to understand Nuclear Magnetic Resonance. You are going to get introduced to the basic principles of NMR spectroscopy and its biochemical/biological and medical uses as well. Details of Magnetic Resonance Imaging will be discussed in a later lecture. 2 The slide shows topics covered in previous biophysics lectures and other courses (including high school) that are necessary for understanding this lecture. Textbook chapters related to the material being discussed are also listed. 3 The NMR phenomenon is based on the magnetic moment of certain nuclei associated with the nuclear spin. Similar to electrons, nucleons (proton, neutron) also possess intrinsic angular momentum or spin. To obtain a tangible characterization of spin, which also helps to understand the details of many phenomena related to NMR, it is useful to introduce the so- called gyroscope model, which derives the existence of the intrinsic angular momentum by supposing a rotational motion of elementary particles (electrons, nucleons, nuclei) (Figure A). Spin is a vector quantity, its magnitude can be described with Eq. 1, where I is the spin quantum number. The spin quantum number uniquely determines the magnitude of spin; therefore, it is used to characterize the particle instead of the absolute size of the vector. Nucleons (similar to electrons) are fermions or half-spin particles (I=1/2). With the exception of 1H consisting of a single proton, all the nuclei are composed of protons and neutrons: the net spin (spin quantum number) is determined by the composition of the nucleus (see next slide). Both protons and neutrons are built up of quarks. Proton is a positively charged particle. Neutron has no charge, but due to the asymmetric distribution of quarks relative to the axis of rotation it possesses electric dipole moment. (Figure B depicts the nucleus of deuterium (2H) atom, where the quarks are also shown.) As a result of the joint presence of 4 charge/dipole and spin (spinning charge/electric dipole) the particle will have an intrinsic magnetic moment, the magnitude of which depends on the size of the spin (Eq. 2). Nuclei where the net spin is different from zero will also possess magnetic moment. Magnetic moment and the magnetic field generated by the elementary particles and atomic nuclei can be modeled with a tiny bar magnet and its field (Figure A). The magnitude of the magnetic moment is determined by the gyromagnetic ratio (N), which can be determined as the ratio of the magnetic moment and the spin of the particle/nucleus (gyro- is coming from gyroscope). The value of gyromagnetic ratio depends on the type of the particle/nucleus. It is determined by the nuclear g-factor (another quantity depending on the type of the particle/nucleus) and the nuclear magneton (Eq. 3). The nuclear magneton (µN) is a material constant: it is the unit of the magnetic dipole moment of heavy particles (e.g. nucleons, nuclei). The value of N shows “how strong” the magnetic field of the particle is. Magnetic moment is a vector quantity. For the proton it is parallel to the spin, whereas for the neutron its direction is opposite to that of the spin vector. (The direction of the magnetic moment vector is from the south pole to the north pole of the magnet.) 5 The net spin of atomic nuclei is determined by the overall number of protons and neutrons (see Table). According to the nuclear shell modell protons and neutrons fill their respective shells in a pairwise fashion. Pauli’s exclusion principle is also valid in the nucleus: the spins of two protons or two neutrons in a pair have opposite directions; therefore, they cancel out each other. As a conseqeunce, if both types of nucleons are present in an even number (e.g. 4He), the net spin of the nucleus is zero, i.e. it does not have a magnetic moment. If both nucleons are present in an odd number (1 unpaired proton + 1 unpaired neutron), the net spin will be an integer number (e.g. for 2H I=1). If only one of the nucleons is present in an odd number, the net spin will be an odd integer multiple of 1/2 (e.g. for 13C I=1/2, for 17O I=5/2). Supplementary information: In the case of nuclei the spin and the orbital angular momentum cannot be separated from each other; therefore, the quantity we address as nuclear spin is the total angular momentum (spin + orbital) of the nucleus. Taking it simply, that is the reason that the net value of I can be larger than ½ or 1. 6 The table displays some of the NMR compatible nuclei used in biology and medicine along with their gyromagnetic ratios. The second column shows the absolute values of N in rad/Ts units (radian/Tesla×seconds). In the NMR field, however, the gyromagnetic ratio is often presented in /2 units (3rd column, unit of measurement will be MHz/Tesla). For some elements the gyromagnetic ratio is negative. If N>0, the direction of the magnetic moment is the same as that of the spin. If N