Single molecules time-dependent signals from

Figure 6.13 Time-dependent signals from single molecules showing slow folding or unfolding transitions.

Instead of measuring the lifetime of a single molecule with pulsed excitation the time dependent intensity of a single molecule under stationary excitation can be measured. Fig. 4.8 shows the first fluctuation of the single DNA molecule. As evident from Fig. 4.8 the data are almost identical to a patch clamp signal from the experiments of Neher and Sackmann showing single ion channels and the fluctuation of the ion current. 

Figure 1.2 Illustration of the concept of measuring the fluorescence from an immobilized single molecule, (a) A molecule, which can undergo a reversible transition between folded and unfolded conformations, is labelled with a dye at one terminus and a quencher at the other. In the folded (native) conformation the fluorescence from the dye is quenched. In the unfolded (denatured) conformation the fluorescence is enhanced. (b)The molecule is immobilized (tethered) onto a solid substrate and the fluorescence signal from a small volume near the surface monitored as a function of time. (c)The same molecule can be monitored for a considerable length of time and the stochastic transitions (the number of which depend on the height of the energy barrier for the transition) can be obsen/ed. Eventually (at around 9.5 s in this simulated example), photobleaching of the dye occurs to a non-fluorescent state, at which point no more information can be extracted from this molecule.

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