| Summary: | Exchange dynamics between molecules free in solution and bound to the surface of a large supramolecular structure, a polymer, a membrane or solid support are important in many phenomena in biology and material science. Here we present a novel and generally applicable solution NMR technique, known as Dark-state Exchange Saturation Transfer (DEST), to probe such exchange phenomena with atomic resolution. This is illustrated by the exchange reaction between amyloid β (Aβ) monomers and polydisperse, NMR invisible ('dark') protofibrils, a process of significant interest since the accumulation of toxic, aggregated forms of Aβ, from small oligomers to very large assemblies, have been implicated in the etiology of Alzheimer's disease1–6. The 15N-DEST experiment imprints with single residue resolution dynamic information on the protofibril-bound species in the form of 15N transverse relaxation rates (15N-R2) and exchange kinetics between monomers and protofibrils onto the easily observed two-dimensional 1H-15N correlation spectrum of the monomer. The exchanging species on the protofibril surface comprise an ensemble of sparsely-populated states where each residue is either tethered to (via other residues) or in direct contact with the surface. The first eight residues exist predominantly in a mobile tethered state`, while the largely hydrophobic central region and part of the C-terminal hydrophobic region are in direct contact with the protofibril surface for a significant proportion of the time. The C-terminal residues of both Aβ40 and Aβ42 display lower affinity for the protofibril surface indicating that they are likely to be surface exposed rather than buried as in structures of Aβ fibrils7–10, and may therefore comprise the critical nucleus for fibril formation11,12. The N15−R2tethered values, however, are significantly larger for the C-terminal residues of Aβ42 than Aβ40 which may explain the former’s higher propensity for rapid aggregation and fibril formation13,14.
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