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Protein structure in solution fluctuates over a wide range of conformation, but the reality of the fluctuation has been little explored. In this article, we present an approach to this problem using variable pressure NMR spectroscopy. The method enables us to explore protein conformation over a wide range of conformational space than hitherto explored by conventional NMR method and brings about a new dynamic view on protein structure. The highlight of the method is the determination of the structural coordinates of semi-stable higher energy conformers that exist in low population in dynamic equilibrium with the stable "native" conformer under physiological conditions. Available evidence suggests that such high energy conformers play crucial roles in function in many proteins.

Although the conformation of a protein is considered largely fixed in crystal, it fluctuates amply in solution among ensembles of conformers differing in partial molar volume. Applying pressure causes a shift of equilibrium among conformers so that the system reaches a new equilibrium in which the population of a lower volume conformer is favored over a larger volume conformer. Because of the nearly parallel relationship between the conformational order of a protein and its partial molar volume, a less ordered (= higher energy) conformer has a lower volume than that of the stable, basic folded conformer. We apply variable pressure NMR to detect and analyze structures of these higher energy conformers in proteins. We find a number of alternate structures in various proteins, which have hitherto been overlooked spectroscopically. These structures differ from the stable, basic folded ones, often in regions of the molecule where the binding with a ligand or with other macromolecules takes place. These findings have led us to a new dynamic view of proteins such that proteins in solution fluctuates among multiple conformations, which are required for function. Furthermore, we have developed a notion that atom defects or cavities are major sources of conformational fluctuation leading to multiple conformations.