Hydrogen-Deuterium (H/D) Exchange Mapping of A super( beta ) sub(1-40) Amyloid Fibril Secondary Structure Using Nuclear Magnetic Resonance Spectroscopy

We describe here details of the hydrogen-deuterium (H/D) exchange behavior of the Alzheimer's peptide A beta sub(1-40), while it is a resident in the amyloid fibril, as determined by high-resolution solution NMR. Kinetics of H/D exchange in A beta sub(1-40) fibrils show that about half the backbone amide protons exchange during the first 25 h, while the other half remain unexchanged because of solvent inaccessibility and/or hydrogen-bonded structure. After such a treatment for 25 h with D sub(2)O, fibrils of super(15)N-enriched A beta were dissolved in a mixture of 95% dimethyl sulfoxide (DMSO) and 5% dichloroacetic acid (DCA) and successive heteronuclear super(1)H- super(15)N HSQC spectra were collected to identify the backbone amides that did not exchange in the fibril. These studies showed that the N and C termini of the peptide are accessible to the solvent in the fibril state and the backbone amides of these residues are readily exchanged with bulk deuterium. In contrast, the residues in the middle of the peptide (residues 16-36) are mostly protected, suggesting that that many of the residues in this segment of the peptide are involved in a beta structure in the fibril. Two residues, G25 and S26, exhibit readily exchangeable backbone amide protons and therefore may be located on a turn or a flexible part of the peptide. Overall, the data substantially supports current models for how the A beta peptide folds when it engages in the amyloid fibril structure, while also addressing some discrepancies between models.