Six years of protein structure determination by NMR spectroscopy: what have we learned?

Nuclear magnetic resonance (NMR) spectroscopy in solution is a second technique, in addition to X-ray diffraction in single crystals, for the determination of three-dimensional protein structures at atomic resolution. Structures of proteins derived by NMR have now been with us for six years, and here I entertain the following question: what information have we gained that would not be available if X-ray crystallography were still the only method for protein structure determination? Answers include that NMR structures are available of proteins that have not been crystallized, that the two techniques afford different insights into internal mobility of proteins, and that one gets different views of protein hydration and hence the molecular surface when using NMR spectroscopy or X-ray diffraction.