The effects of desiccation and rehydration cycles encountered by extracellular enzymes in soils are studied on alpha-chymotrypsin adsorbed on montmorillonite. The controlled hygrometric FTIR cell used in this study enables to monitor drying and rehydration steps undergone by the alpha-chymotrypsm-montmorillomte suspension or by the enzyme alone. Relative humidity (RH) determines the amount of deuterated water in the FTIR cell atmosphere. The molar water/protein ratio (W/P) as well as the conformational and solvation states of the enzyme have been determined using H/D exchange monitored by FTIR-transmission spectroscopy. When the W/P ratio decreases from 3500 to similar to400, unfolding of beta-secondary structure in three different domains involves about 8% of the polypeptide backbone with respect to the most solvated states. Desiccation induces beta-unfolding, which opens channels allowing free vapor water molecules to diffuse into the enzyme at 15% RH. On drying to 0% RH, displacements of internal water (H2O) in the enzyme are demonstrated by reverse peptide isotopic exchanges (COND double right arrow CONH). Specific beta-structures, only formed in highly solvated states, sequester around 20 internal H2O molecules. Indeed, most of the unfolded secondary structures during the drying step are refolded at W/P similar to 1000 during rehydration. However, self-association hinders the recovery of the initial closed tertiary structure. The pD-dependent structural changes controlling inward and outward water diffusion are suppressed, whether the protein is initially in an adsorbed state or in solution. Changes in secondary structures encountered during desiccation/rehydration cycle are similar for the protein either free or in the adsorbed state. Thus domains that are unfolded by adsorption are not concerned by the desiccation/rehydration cycle. (C) 2004 Elsevier Inc. All rights reserved.