Commonly-used FRET fluorophores promote collapse of an otherwise disordered protein.

The dimensions that unfolded proteins, including intrinsically disordered proteins (IDPs), adopt at low or no denaturant remains controversial. We recently developed an innovative analysis procedure for small-angle X-ray scattering (SAXS) profiles and found that even relatively hydrophobic IDPs remain nearly as expanded as the chemically denatured ensemble, rendering them significantly more expanded than generally inferred using fluorescence resonance energy transfer (FRET) measurements. We show here that fluorophores typical of those employed in FRET can contribute to this discrepancy. Specifically, we find that addition of Alexa488 to a normally expanded IDP causes contraction of its ensemble. In parallel, we also tested the recent suggestion that FRET and SAXS results can be reconciled if, in contrast to homopolymers, the radius of gyration (Rg) of an unfolded protein chain can vary independently from its end-to-end distance (Ree). To do so, we developed an analysis procedure that can accurately extract both Rg and Ree from SAXS profiles even if they are decoupled. Using this procedure, we find that Rg and Ree remain tightly coupled even for heteropolymeric IDPs. We thus conclude that, when combined with improved analysis procedures for both SAXS and FRET, fluorophore-driven interactions are sufficient to explain the preponderance of existing data regarding the nature of polypeptide chains unfolded in the absence of denaturant.