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Two main amyloid-β peptides of different length (Aβ 40 and Aβ 42 ) are involved in Alzheimer's disease. Their relative abundance is decisive for the severity of the disease and mixed oligomers may contribute to the toxic species. However, little is know about the extent of mixing. To study whether Aβ 40 and Aβ 42 co-aggregate, we used Fourier transform infrared spectroscopy in combination with 13 C-labeling and spectrum calculation and focused on the amide I vibration, which is sensitive to backbone structure. Mixtures of monomeric labeled Aβ 40 and unlabeled Aβ 42 (and vice versa ) were co-incubated for ∼20 min and their infrared spectrum recorded. The position of the main 13 C-amide I' band shifted to higher wavenumbers with increasing admixture of 12 C-peptide due to the presence of 12 C-amides in the vicinity of 13 C-amides. The results indicate that Aβ 40 and Aβ 42 form mixed oligomers with a largely random distribution of Aβ 40 and Aβ 42 strands in their β-sheets. The structures of the mixed oligomers are intermediate between those of the pure oligomers. There is no indication that one of the peptides forces the backbone structure of its oligomers on the other peptide when they are mixed as monomers. We also demonstrate that isotope-edited infrared spectroscopy can distinguish aggregation modulators that integrate into the backbone structure of their interaction partner from those that do not. As an example for the latter case, the pro-inflammatory calcium binding protein S100A9 is shown not to incorporate into the β-sheets of Aβ 42 .

Amyloid β-peptides 1–40 and 1–42 form oligomers with mixed β-sheets