Accounting for molecular mobility in structure determination based on nuclear magnetic resonance spectroscopic and X-ray diffraction data

This chapter describes accounting for molecular mobility in structure determination based on nuclear magnetic resonance (NMR) spectroscopic and x-ray diffraction data. It discusses procedures to obtain an ideal representation of a biomolecule, with an eye to appropriately accounting for molecular mobility and flexibility. In NMR spectroscopy, the primary measured data are resonance chemical shifts σ and intensities, nuclear Overhauser effect (NOE) intensities I_NOE, relaxation times T₁ and T₂, and vicinal J coupling constants or J values. As in X-ray crystallography, these primary data cannot be directly transformed into a molecular structure with mobility. As before, the inverse relations that express the primary measured quantities in terms of a molecular structure or a set of structures must be used to calculate them from an estimated molecular structure, which is improved in an iterative manner by minimizing the difference between the calculated and observed primary data. So, also in this case, both molecular structure and mobility are derived quantities.