Flexibility and Solvation of Amyloid- Hydrophobic Core

Amyloid fibril deposits found in Alzheimer disease patients are composed of amyloid- (A) protein forming a number of hydrophobic interfaces that are believed to be mostly rigid. We have investigated the s-ms time-scale dynamics of the intra-strand hydrophobic core and interfaces of the fibrils composed of A(1-40) protein. Using solid-state H-2 NMR line shape experiments performed on selectively deuterated methyl groups, we probed the 3-fold symmetric and 2-fold symmetric polymorphs of native A as well as the protofibrils of D23N Iowa mutant, associated with an early onset of Alzheimer disease. The dynamics of the hydrophobic regions probed at Leu-17, Leu-34, Val-36, and Met-35 side chains were found to be very pronounced at all sites and in all polymorphs of A, with methyl axis motions persisting down to 230-200 K for most of the sites. The dominant mode of motions is the rotameric side chain jumps, with the Met-35 displaying the most complex multi-modal behavior. There are distinct differences in the dynamics among the three protein variants, with the Val-36 site displaying the most variability. Solvation of the fibrils does not affect methyl group motions within the hydrophobic core of individual cross- subunits but has a clear effect on the motions at the hydrophobic interface between the cross- subunits, which is defined by Met-35 contacts. In particular, hydration activates transitions between additional rotameric states that are not sampled in the dry protein. Thus, these results support the existence of water-accessible cavity recently predicted by molecular dynamics simulations and suggested by cryo-EM studies.