G wider/Progress in Nuclear Magnetic Resonance Spectroscopy 32(1998)193-275 technical aspects of NMR spectroscopy. Section 3 Hertz(Hz) the symbols w or f are used for the former introduces those parts of a modern NMR spectrometer and v for the latter which critically influence the performance of NMR The basis of all NMR experiments is the nuclear experiments. Section 4 concentrates on the basic ich can be interpreted as a magnetic momer experimental segments from which most of the vast A spin i nucleus in this view forms a small number of experiments available today are con- This dipole orients either parallel(a state)or structed. Section 5 discusses hydration studies with allel(B state) to a magnetic field leading to a small NMR and serves two purposes. First, it gives exam- energy difference Ae between the two states ples of experiments using the segments introduced in Section 4 and the les discussed in Section 2. AE=:Bo=hyBo Second, it introduces the technical aspects of a ver interesting application of NMR which allows detaile where Bo is a large externally applied homogeneous studies of individual water molecules in the hydration magnetic field, h is Planck's constant (h h/(2T), shell of a protein. and y the gyromagnetic ratio which is a property of the nucleus and can have a positive or a negative value. Table I lists the gyromagnetic ratio and some 2. Basic principles other properties of nuclei important in NMR of bio- logical macromolecules. from the two states of a L. Th favourable and thus possesses a higher population This section presents some basic theoretical aspects than the p state. Transitions between adjacent energy of nMR which are relevant to a technical discussio levels can be induced by small additional magnetic of the principles and experimental procedures used fields perpendicular to B, which oscillate with a fre- when studying biological macromolecules in solution quency vo fulfilling the resonance condition v,= AE/ by NMR. A rigorous discussion of the theoretical h. The frequency vo typically lies in the radio- foundation of NMR can be found in many textbooks, frequency range and is often referred to as the Larmor e.g. [16, 19, 24, 26, 29]. Only a very limited theoretical frequency. In the equilibrium state the boltzmann foundation is necessary for the technically oriented distribution favours the lower energy states. Thus, discussion of NMR methods in the following sections, the sum of all contributing nuclear magnetic moments The concept of energy levels, the Bloch equations and of the individual nuclei leads to a resulting macro- the product operator formalism are sufficient in most scopic magnetization M along the homogeneous cases. NMR is intimately related with frequencies and external field Bo. In the framework of classical to obtain a clear distinction between angular frequen- physics the behaviour of this magnetization under cies with units rad s and technical frequencies in the action of time-dependent magnetic fields can be Properties of selected nuclei Nucleus y710 Ts Natural abundance/% Relative sensitivit H 99985 H 4.106625 0015 9.65×10 H 6.72828 1.59x10 193378 0.037 ×10-2 10.8394 6.64×10 y, gyromagnetic ratio: 2=1/(2 For an equal number of nuclei to protons