Figure 3.

Kt-7 molecules are bound to L7Ae in a predominantly folded conformation in the presence of divalent metal ions. (A) I_D and I_A versus time for one representative molecule in the presence of 2 mM Mg²⁺ ions, at 33-ms frame rate. Cy5 was photobleached at 125 s. Near the end of the trace (labeled dissoc) the intensities of both fluorophores fall to background levels; this is most likely due to dissociation of the complex from the surface, although it could result from Cy3 photobleaching. (B) Histogram of E_FRET values for Kt-7 molecules bound to L7Ae in 2 mM Mg²⁺ ions (1185 molecules). Gaussian mixture parameters of data distribution were estimated using an expectation maximization algorithm and maximum likelihood estimator in MATLAB, giving a mean value of E_FRET = 0.83. (C) Cross-correlation analysis of a time-trace of I_D and I_A for a single k-turn molecule in 2 Mg²⁺, calculated from data recorded at 33-ms frame rate. The fit is linear and almost horizontal with an intercept that is very close to zero, indicating the absence of cross-correlation between donor and acceptor emission down to the timescale of the data collection.