Fig. 3.

In contrast to the apparent denaturant independence of the dimensions and random coil behavior of dye-free PEG, the observed efficiency of energy transfer across the dye-labeled polymer is denaturant-dependent. Shown is transfer across 3 kDa PEG labeled at the termini with Alexa-488 and -594, the most commonly used smFRET dye pair, as a function of (Left) GuHCl or (Right) urea. The peaks at near-zero transfer efficiency arise because of molecules lacking a functional acceptor dye. smFRET data collected on 5 kDa PEG exhibit similar denaturant dependence (Fig. S8).