Constant force is applied for different (1,2,3,4,5) loading cycles.(a)The complete force vs displacement curve:(b)Force vs number of cycles.(c)Force vs radius variations in figure 4. The discontinuity in the loading sequence is due to switching off the force generator after lowering the load, the subsequent (1,3) and (4,5) curves are then obtained by repeating these loading processes. Same as 4a for the (1,4), and (2,5) curves. Of course when the number of cycles is 3, the load profile looks different (due to load reversals) but the mechanism works consistently. The numbers on the loading curve (green) represent (Insert: Free body diagram of loaded TFP, is binding force due to RA).
The number of molecules per unit length in a micro-bead is approximated as that of the free chainmolecules multiplied by the avid binding of free chains to the pilus, Nbeads=Nbases*NB, (NBav = 1.4) is the avidity factor that takes into account multiple bonds securing the whole pilus. Dilution of free basease concentration with a higher avidity factor (as well as higher concentration of bases) results in accumulation of bases at the base of the pilus. Expression of Nbeads in terms of Nb's (total number of bases including bound and free) is shown in fig 5. NB signifies the number of bases at the base of the pilus (given by the BM15 conditioned mutant as Nb147). NBav is in fact a lower bound on the avidity factor that could be achieved by taking into account multiple bonds securing the whole pilus. The binding energy of NB is the difference between the free energy of a base pair at low force and that of a base pair bound to the pilus. The binding energy at zero deformationis 1.8 ergs/base pair and is assumed to saturate below 2ergs/base pair. NBav is of the order of 7. The characteristic saturation of the force-molecule count curve inset in Fig. 3 thus reflects the saturation of the base in the binding region. Adhered bases are pulled out by elastic force and are not released by the applied force. d2c66b5586