FTIR Analysis of Aβ42 Oligomers with Ni(II) Acetate

Fourier-transformed infrared (FTIR) spectra of the Aβ₄₂ oligomers prepared with 0–500 µM Ni(II) acetate, as described in the materials section, were recorded on a Tensor 37 FTIR spectrometer (Bruker Optics, Germany) operating in transmission mode at room temperature and equipped with a liquid nitrogen-cooled MeCdTe detector and a sample shutter. During the measurements, the instrument was continuously purged with dry air. 8–10 µL of the 80 µM Aβ₄₂ oligomer samples were placed between two flat CaF₂ discs, which were separated by a 50 µm plastic spacer that had been covered with vacuum grease at the periphery. The mounted IR cuvette was put in a holder inside the sample chamber, and was then allowed to sit for at least 20 min after the chamber lid was closed, to remove H₂O vapor. FTIR spectra were recorded between 1900 and 800 cm⁻¹, at a resolution of 2 cm⁻¹ and with 6 mm aperture. The IR intensity above 2200 cm⁻¹ was blocked with a germanium filter, and that below 1500 cm⁻¹ with a cellulose membrane, to increase the light intensity in the relevant spectral range^{91 (link)}. The OPUS 5.5 software was used for analysis and plotting of the spectra. Second derivatives were computed with a smoothing factor of 17.