Random coil negative control reproduces the discrepancy between scattering and FRET measurements of denatured protein dimensions

Small-angle scattering studies generally indicate that the dimensions of unfolded single-domain proteins are independent (to within experimental uncertainty of a few percent) of denaturant concentration. In contrast, single-molecule FRET (smFRET) studies invariably suggest that protein unfolded states contract significantly as the denaturant concentration falls from high (similar to 6 M) to low (similar to 1 M). Here, we explore this discrepancy by using PEG to perform a hitherto absent negative control. This uncharged, highly hydrophilic polymer has been shown by multiple independent techniques to behave as a random coil in water, suggesting that it is unlikely to expand further on the addition of denaturant. Consistent with this observation, small-angle neutron scattering indicates that the dimensions of PEG are not significantly altered by the presence of either guanidine hydrochloride or urea. smFRET measurements on a PEG construct modified with the most commonly used FRET dye pair, however, produce denaturant-dependent changes in transfer efficiency similar to those seen for a number of unfolded proteins. Given the vastly different chemistries of PEG and unfolded proteins and the significant evidence that dye-free PEG is well-described as a denaturant-independent random coil, this similarity raises questions regarding the interpretation of smFRET data in terms of the hydrogen bond-or hydrophobically driven contraction of the unfolded state at low denaturant.