G wider/Progress in Nuclear Magnetic Resonance Spectroscopy 32(1998)193-2 3.3. 1. The transmitting path 3.3. 2. The receiving path 3.3.3. The lock system 3.4. The magnet 3.5. The prol 3.5. 1. The radiofrequency coil 3.5. 2. The magnetic field gradient coil 228 3.5.3. The variable temperature operation 3.6. Stability of the sys stel 4. Basic segments of pulse sequences 230 4. 1. Evolution segments 4.2. Transfer segments 235 4.2. 1. Homonuclear through-bond transfer 4.2.2. Homonuclear through-space transfer 4.2.3. Heteronuclear transfer 43 Decoupling sequences 4.4. Pulsed magnetic field gradients 4.5. Combinations of basic segments 4. 5. 1. The HsQC and the hmQc scheme 4.5.2. Concatenating basic segment: 4. 6. Artefact reduction 5.2. NMR and hydration 5.3. Basic experiments 5.3. 1. NOEs between water and protein protons 53. 2. HYDRA 5.3.,3. Measurement of exchange rates using diffusion filter experiments 5.3. 4. Relaxation dispersion experiments 262 5.4. Artefacts in hydration studies at high magnetic fields 5.4.1. Radiation damping and demagnetizing field effect 5.4.2. Minimizing artefacts in hydration measurements 266 5.4.3. Consequences of radiation damping and demagnetizing field effects Acknowledgements Appendix A: The Bloch equations Appendix B: The product operator formalism 270 References Keywords: Protein NMR: Instrumentation; Hydration; Multidimensional NMR 1. Introduction macromolecules has been a growing field in research and applications [2]. The capability to observe signals The first nuclear magnetic resonance (NMR) from individual atoms in complex biological macro- spectrum of a protein was published some forty molecules in solution makes possible the measure- years ago [l] and ever since, NMR of biological ments of parameters that can be analysed in terms of