Nuclear Magnetic Resonance of Biomolecules

Abstract

Nuclear magnetic resonance (NMR) spectroscopy exploits a property of the nucleus known as nuclear spin, which exhibits angular momentum and as such generates a local magnetic field that can be influenced by a larger static external field. In the presence of a large external magnet, the nuclear spins align with the field, with a slight excess opposed to the field. That small excess can be flipped to alignment with the field in the presence of a radiofrequency (RF) pulse of appropriate frequency. Relaxation of those spins back to the ground state results in emission of RF radiation whose frequency is an indication of the local electron density of the molecule in which the spin resides. Such information can ultimately be directly related to molecular structure. NMR spectroscopy is unique in that it can be used to image the whole human body at one extreme, and determine the three‐dimensional (3D) structure of a biomolecule within the body at the other extreme. The principal advantages of NMR are that: (i) the complete molecular structure can be determined; (ii) the technique spans all principal states of matter (solid, liquid, gas); (iii) the method is nondestructive and can be used noninvasively, as in a clinical setting. The principal disadvantages of NMR are that: (i) the technique is very insensitive (typically samples below 50 µM are difficult to study, although recent advances in nanoprobe and cryprobe technology are rapidly revising these numbers); (ii) in liquids there are molecular weight limits of around 50 kDa – although there are new methods being developed that may have a significant impact on this; (iii) in solids only limited structural information can be obtained to date.