beta Sheet structure in amyloid beta fibrils and vibrational dipolar coupling.

Fibrils formed by amyloid beta proteins were labeled with 13C at various positions and examined by infrared spectroscopy to detect vibrational dipolar coupling, implying close physical proximity. The results support key features of several recently proposed models for amyloid fibril structure, but they also add some important caveats. For instance, they support the conclusion that the beta structure is parallel; however, the coupling is not as strong as expected when residues are in register. This may be explained by out-of-register alignment of adjacent strands, or nonstandard parallel sheet structure that yields suboptimal alignment of labeled dipole moments. The data also point to a significant structural difference between fibrils formed by the 40-residue amyloid beta protein and fibrils formed by residues 10-35.