Measurement of protein hydration shells using a quartz microbalance

The protein myoglobin is dried on the surface electrodes of a 5-MHz quartz crystal oscillator. As the humidity above the film increases, the protein absorbs water and develops a hydration shell. The oscillator frequency shift measures the mass of absorbed water. Incremental decreases in the amplitude of vibration are also observed, corresponding to a softening of the protein structure and subsequent energy dissipation (damping) of the oscillations. The transition of a protein from a dry, more rigid structure to a semifluid hydrated state is observed to occur in discrete steps.