The relationship between amino acid sequence, three-dimensional structure and biological function of proteins is one of the most intensely pursued areas of molecular biology and biochemistry. In this context, the three-dimensional structure has a pivotal role, its knowledge being essential to understand the physical, chemical and biological properties of a protein (Branden & Tooze, 1991; Creighton, 1993). Until 1984 structural information at atomic resolution could only be determined by X-ray diffraction techniques with protein single crystals (Drenth, 1994). The introduction of nuclear magnetic resonance (NMR) spectroscopy (Abragam, 1961) as a technique for protein structure determination (Wüthrich, 1986) has made it possible to obtain structures with comparable accuracy also in a solution environment that is much closer to the natural situation in a living being than the single crystals required for protein crystallography.