Figure 3.

Ensemble absorption and fluorescence spectra of Alexa 488 and Alexa 594 labeled His-GB1^K10C/C57. Interactions between donor and acceptor dyes result in (A) a significant decrease in acceptor absorbance and (B) fluorescence quenching (λ_Ex = 470 nm) in the folded state (0 M GdHCl or urea; from (19)). (C) Two different donor fluorescence spectra in 5 M urea averaged over segments of trajectories of immobilized single molecules, distinguished by the fraction of donor photons detected in the acceptor channel with a 600-nm long-pass filter (from (19)).