The sequential growth mechanism of a protein monolayer at the air-water interface

The growth mechanism of a protein monolayer at the air-water interface was investigated using dynamic surface tension, diffuse X-ray scattering and X-ray fluorescence techniques. To ensure the surface sensitivity, the grazing angle was kept smaller than the critical angle of the total external reflection for water. The protein solution was inserted into liquid in a Langmuir trough and the surface was monitored with time-resolved measurements which demonstrate a sequential mechanism involving adsorption and relaxation processes characterized with different time-scales. X-ray measurements provide evidence for sudden time-scale shifts which could not be observed with surface pressure measurements. We propose a mechanism to separate adsorption and relaxation processes by simultaneously measuring surface pressure and X-ray scattering. The X-ray fluorescence technique, which is not sensitive to in-plane structural organisation at the surface, supports the X-ray scattering results.