BACKGROUND:THz experiments have been used to characterize the picosecond time scale fluctuations taking place in the model, globular protein crambin.RESULTS:Using both hydration and temperature as an experimental parameter, we have identified collective fluctuations (<= 200 cm(-1)) in the protein. Observation of the protein dynamics in the THz spectrum from both below and above the glass transition temperature (Tg) has provided unique insight into the microscopic interactions and modes that permit the solvent to effectively couple to the protein thermal fluctuations.CONCLUSIONS: Our findings suggest that the solvent dynamics on the picosecond time scale not only contribute to protein flexibility but may also delineate the types of fluctuations that are able to form within the protein structure.