Recent progress in the understanding of hydrogen exchange and its use in studies of the fluctuation of the protein structure are reviewed. By means of an infrared spectroscopic observation of the hydrogen exchange reaction, a simple and reliable method of determining the number of the peptide-peptide hydrogen bonds in a given protein molecule in its aqueous solution is proposed. Then, the mechanisms of hydrogen exchange in protein are critically examined comparing hydrogen exchange kinetics with other physical method for several proteins. The graphic method of evaluation of the fluctuation amplitude at various levels and the results of its application to myoglobin structure are shown. In addition, the fluctuation of the side chain is also discussed.
