Probing the microelastic properties of nanobiological particles with tapping mode atomic force microscopy

We have studied untreated photosystem II (PSII) membrane using tapping mode atomic force microscopy (AFM). The individual PSII particles distribute randomly in the membrane. Near the center of each particle, our AFM reveals an intramolecular cavity which confirms the previous electron microscopy of stained samples. The cavity can be reversibly enlarged from a few nm to as many as 40 nm in diameter by increasing the force on the AFM tip. A study of the particleʹs apparent height and cavity size under various forces provides unique information about the microelastic properties of single PSII particles.