1. Introduction 328

2. Basic concepts of neutron scattering 329

2.1 Introduction 329

2.2 Neutron-scattering functions 331

2.3 Coherent and incoherent neutron scattering. The particular role of hydrogen in incoherent scattering 332

2.4 Total elastic scattering, EISF and mean square displacement (MSD) 333

2.5 Quasielastic scattering and relaxation function 334

2.6 Inelastic scattering and density of states 335

3. Experimental aspects and instruments 335

3.1 Energy and space resolution 335

3.2 General sample aspects 335

3.3 Potential effects of D2O on dynamics 336

3.4 Experimental 2H (deuterium) labelling 336

4. Physics of protein dynamics 336

4.1 Models 336

4.2 The dynamical transition 338

4.3 Effective force constants 339

5. Dynamics of hydrated protein powders 339

5.1 First experiments on myoglobin 340

5.2 Dynamical transitions in other proteins 340

5.3 The role of hydration water 341

5.4 Influence of the solvent 344

5.5 Diffusional motions within proteins by QENS 346

5.6 Inelastic neutron scattering and vibrational spectra 347

5.7 Conclusions 351

6. Membranes 352

6.1 Lipid bilayers 353

6.2 BR and the purple membrane (PM) 353

6.2.1 The dynamical transition in the PM 353

6.2.2 QENS from oriented PM 354

6.2.3 Hydration dependence of PM motions 355

6.2.4 Local dynamics in PM studied by isotope labelling 356

6.2.5 Dynamics of different BR conformations 357

7. Protein solutions 358

7.1 From powders to solutions 358

7.2 Water dynamics and solvent dependence of the dynamical transition in proteins 359

8. Comparing neutron scattering with other techniques 359

9. Biological relevance 360

9.1 Dynamics–activity relations 360

9.2 Dynamics–stability relations (adaptation to extreme environments) 360

9.3 Protein folding 361

10. Acknowledgements 364

11. References 364

This review of protein dynamics studied by neutron scattering focuses on data collected in the last 10 years. After an introduction to thermal neutron scattering and instrumental aspects, theoretical models that have been used to interpret the data are presented and discussed. Experiments are described according to sample type, protein powders, solutions and membranes. Neutron-scattering results are compared to those obtained from other techniques. The biological relevance of the experimental results is discussed. The major conclusion of the last decade concerns the strong dependence of internal dynamics on the macromolecular environment.