  The Rouse model with internal friction (RIF), a widely used theoretical
framework to interpret the effects of internal friction on conformational
transitions in biomolecules, is shown to be an approximate treatment that is
based on preaveraging internal friction. By comparison with Brownian dynamics
simulations of an exact coarse-grained model that incorporates fluctuations in
internal friction, the accuracy of the preaveraged model predictions is
examined both at and away from equilibrium. While the two models predict
intrachain autocorrelations that approach each other for long enough chain
segments, they differ in their predictions for shorter segments. Furthermore,
the two models differ qualitatively in their predictions for the chain
extension and viscosity in shear flow, which is taken to represent a
prototypical out-of-equilibrium condition.
