FIGURE 1.

Variance reveals that force-clamp experiments are not in thermal equilibrium in the Gibbs ensemble. (A) A single dextran molecule was stretched (blue) and relaxed (red) consecutively by linearly varying the force. The figure shows the first three out of 108 cycles. In the transition region, between 800 and 1000 pN, the pyranose rings in dextran are thought to flip from a chair to a boat conformation, corresponding to a 20% increase in length (25). The noise in the length increases markedly in the transition region. (B) The experimental (green) and predicted (pink) length variances are highly discrepant, indicating that the molecule is not in thermal equilibrium in the Gibbs ensemble.