Abstract
We have developed a new force ramp modification of the atomic force microscope (AFM) which can control multiple unfolding events of a multi-modular protein using software-based digital force feedback control. With this feedback the force loading rate can be kept constant regardless the length of soft elastic linkage or number of unfolded polypeptide domains. An unfolding event is detected as a sudden drop in force, immediately after which the feedback control reduces the applied force to a low value of a few pN by lowering the force set point. Hence the remaining folded domains can relax and the subsequent force ramp is applied to relaxed protein domains identically in each case. We have applied this technique to determine the kinetic parameters xu, which is the distance between the native state and transition state, and α0, which is the unfolding rate constant at zero force, for the mechanical unfolding of a pentamer of I27 domains of titin. In each force ramp the unfolding probability depends on the number of folded domains remaining in the system and we had to take account of this effect in the analysis of unfolding force data. We obtained values of xu and α0 to be 0.28 nm and 1.02 × 10−3 s−1, which are in good agreement with those obtained from conventional constant velocity experiments. This method reveals unfolding data at low forces that are not seen in constant velocity experiments and corrects for the change in stiffness that occurs with most mechanical systems throughout the unfolding process to allow constant force ramp experiments to be carried out. In addition, a mechanically weak structure was detected, which formed from the fully extended polypeptide chain during a force quench. This indicates that the new technique will allow studies of the folding kinetics of previously hidden, mechanically weak species.
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