Abstract
In NMR spectroscopy, paramagnetic relaxation enhancement (PRE) can be used to probe transiently formed long-range contacts in macromolecular structures. This is particularly powerful for the structural characterization of intrinsically disordered and unfolded proteins. Here we assess the structural information encoded in PRE data, and its use in describing ensemble representations of disordered states. We seek to determine both the optimal number of PRE labels for structural determination as well as their capabilities in reproducing contact regions and contact populations. We first constructed several ensembles of a model protein with a variety of imposed tertiary contacts, and then attempted to reproduce the ensembles through simulations restrained by theoretical PRE intensities. Almost universally, the PRE data is able to recapture the contacts of the synthetic ensembles provided that one PRE label is placed every 10-20 residues. However, the degeneracy inherent in ensemble-averaged PRE data, compounded by the radius to the minus sixth dependency, makes it difficult to distinguish between contact distances and populations. As a consequence, while the contacts themselves remain visible, their populations are largely determined by the size of the simulated ensemble.
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