Nanorheology measurement on single circularly permuted green fluorescent protein molecule

The force measurement mode of an atomic force microscope (AFM) has enabled us to measure the mechanical properties of biological materials at the single molecular level. In a conventional quasi-static force measurement on a single circularly permuted green fluorescent protein (cpGFP), we could unfold it by unraveling several sub-domains in a distinct sawtooth pattern at a slow stretching speed. In order to elucidate more detailed conformational changes at each extension length, we further measured dynamic relax-stress response of cpGFP molecules. In this measurement, several cycles of sinusoidal movement were applied to the sample during the stretching process. We found the protein molecule showed in-phase response to the sinusoidal input in most case of measurements.