1. Determination of the contact point in force curves.
Determination of the contact point in force curves. (A) A PeakForce QNM (2 kHz) force curve recorded in the centre of an NPC (cytoplasmic side). (B) The force curve is segmented into many partitions (initially 20, shown by the dotted grey lines). The standard deviation of each segment is determined, and the smallest is taken as a measure of the baseline noise. This number is multiplied by a factor (3 in this study) and saved as a threshold value. The difference in the mean between neighbouring partitions is calculated and compared with this threshold: if the difference is less than the threshold, it is assigned a 0; if greater than the threshold, it is assigned a 1. If this produces a series of 0s and 1s with only one transition, the force curve is cropped from the end of the baseline to the first segment after the transition point (green dotted line is the transition point and blue line is the segment of force curve for cropping). If there is more than one transition, the number of segments is reduced by one, and the process restarts. The rationale behind this procedure is that it yields a force curve over an indentation that is so small that the increase can be approximated by a linear function. (C) A piecewise linear function is fitted to the cropped force curve using a least squares regression. (D) The knot in the piecewise function is defined as the contact point (z0; red cross).
George J Stanley et al. LSA 2018;1:e
© 2018 Stanley et al.