Quantitative evaluation of water content in a solid protein by deuterium NMR

A method for evaluating absolute water content in a solid protein based on deuterium NMR measurements in solution is described. By dissolving the hydrated solid protein, which has been specifically deuterium-labeled, into deuterium-depleted water and by comparing the deuterium NMR signal intensity of water (1H2HO) with that of the protein, the amount of water contained in the solid protein is evaluated quantitatively. The method requires a heat pretreatment of the protein sample in water of an enriched (e.g. 2%) deuterium composition for complete hydrogen exchange of the labile protons, and hence is applicable to a protein with a reasonably good reversibility of thermal unfolding. By utilizing this method, the absolute content of the bound water in a protein, Streptomyces subtilisin inhibitor (SSI), lyophilized for 8 h was determined to be 9.2%. The extent of hydration of solid SSI during its exposure to a deuterium-enriched water vapor could also be followed from the deuterium NMR signals in solution. In addition, solid state deuterium NMR measurements of SSI suggested that direct measurement of the natural abundance deuterium signal can give a reasonable estimate of the water content in a solid protein.