Background: Little tertiary structure information is available for the toxic intermediates in the amyloid- (A) fibrillation process. Results: A protofibrils show tertiary contacts between Glu-22 and Ile-31, which are not present in mature fibrils. Conclusion: A protofibrils share tertiary structure features with oligomers but not with mature fibrils. Significance: A protofibrils must undergo a major structural reorientation in the development of mature A fibrils.We have studied tertiary contacts in protofibrils and mature fibrils of amyloid- (A) peptides using solid-state NMR spectroscopy. Although intraresidue contacts between Glu-22 and Ile-31 were found in A protofibrils, these contacts were completely absent in mature A fibrils. This is consistent with the current models of mature A fibrils. As these intramolecular contacts have also been reported in A oligomers, our measurements suggest that A protofibrils are structurally more closely related to oligomers than to mature fibrils. This suggests that some structural alterations have to take place on the pathway from A oligomers/protofibrils to mature fibrils, in agreement with a model that suggests a conversion of intramolecular hydrogen-bonded structures of A oligomers to the intermolecular stabilized mature fibrils (Hoyer, W., Grönwall, C., Jonsson, A., Ståhl, S., and Härd, T. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 5099 -5104).Alzheimer disease is characterized by extracellular deposition of plaques of amyloid- (A) 2 peptides in the brain (1). These protein aggregates are composed of mature A fibrils, which represent the end product of a long, complex, and not well understood fibrillation process (2, 3). The fibrillation pathway initiates with soluble unstructured monomeric A peptides, which are converted into oligomers, protofibrils, and finally into mature fibrils (4 -6). Recently, interest in the transient A intermediate structures has been growing rapidly because these species are considered to represent the cytotoxic intermediates in Alzheimer disease (7). In addition to the well studied structure (8 -12) and dynamics (13) of mature A fibrils, a growing amount of data for oligomers (14 -19) and protofibrils (20, 21) has become available. With regard to the secondary structure elements, these studies revealed that oligomers and protofibrils already exhibit the characteristic two -strand sections connected by a short loop also present in mature A fibrils. However, there are several significant differences between theses species. For instance, the first -strand of A oligomers and protofibrils is significantly shorter than that in mature A fibrils and has to elongate during the conversion from protofibrils to mature fibrils (20). In addition, many questions about the tertiary structure, the fibrillation process, and the conversion from one intermediate to another are still unanswered.Härd and co-workers have resolved the structure of A(1-40) oligomers stabilized by an Affibody and also proposed a model for the arrangement of t...