J 820 spectropolarimeter

Circular dichroism (CD) and UV-Vis absorption spectra of the KBr-pelleted samples were acquired by CCS at room temperature.^{6 (link)} The CD and UV-Vis absorption spectra of PMMA- or myo-IPU-film-dispersed states and CHCl₃ solutions were recorded with a JASCO J-820 spectropolarimeter at room temperature. The KBr-pelleted and PMMA (or myo-IPU) films were prepared as described for the samples used for solid-state PL and CPL spectroscopy. A 1 mm path length was used for solution-phase spectroscopy.

CD spectra were measured using the J-820 spectropolarimeter (JASCO, Tokyo, Japan) with a 1-mm path length quartz cuvette. The single strand DNA sequences of G4 (5′-GGGGTGGGGCCCTGCGAGGGCGGG-3′) and the sequence with mutation (5′-GGAGTGGAGCCCTGCGAGAGCGAG-3′) were purchased from Fasmac Co. (Kanagawa, Japan). Single strand DNAs at a concentration of 10 μM were dissolved in 50 mM Tris-HCl (pH 7.4) containing 100 mM KCl, heated at 95 °C for 5 min and then slowly cooled to 25 °C. CD spectra from 220 to 320 nm were scanned at 25 °C as follows: 100 nm/min scanning speed, 1-nm band width, 0.5-nm data interval and 1-s response.

Circular dichroism spectra of peptides were acquired under in 50 mM TBS (50 mM Tris, 50 mM NaCl, pH 7.4, 25 °C). The circular dichroism spectra of HAL-2, HAL-B, HAL-C and HAL-D were measured at 25 °C with a J-820 spectropolarimeter (Jasco, Tokyo, Japan) equipped with a rectangular quartz cell with a path length of 0.3 cm. Spectra were recorded at a scanning speed of 10 nm min^–1 from 190 nm to 260 nm. An average of three scans were collected for HAL-2, HAL-B, HAL-C and HAL-D.
The CD spectra were then converted to mean residue ellipticity using the following equation:^{24 (link)}[θ]_obs = [θ]₂₂₂ × 1000/clnwhere [θ]_obs is mean residue ellipticity [deg cm² dmol^–1], [θ]₂₂₂ is the observed ellipticity corrected for the buffer at a given wavelength of 222 nm [mdeg], c is the peptide concentration [mM], l is the path length [mm] and n is the number of amino acids.
The average helical content of the peptide was calculated according to the formula:^{25 (link)}% helix = 100([θ]_obs – [θ]0222)/[θ]100222where, [θ]0222 = estimated ellipticity of a peptide with 0% helicity (–1000 deg cm² dmol^–1) and [θ]100222 = estimated ellipticity of a 100% helical peptide (–36 500 deg cm² dmol^–1).

Secondary structure analysis of the peptide was performed using a J-820 spectropolarimeter (Jasco, Tokyo, Japan) equipped with a 1-mm-path-length quartz cell at 25°C. The peptides were dissolved in 10-mM phosphate-buffered saline (PBS) (pH 7.4) or 30-mM sodium dodecyl sulfate (SDS) (Sigma-Aldrich, St. Louis, MO, United States) to give a final concentration of 150 μM. CD spectra were collected at a wavelength ranging from 190 to 250 nm with a scan rate of 100 nm/min, and each spectrum was the average of three scans. The following equation calculated the mean residue ellipticity (θ_M, deg cm² dmol^–1):
where θ_obs is the observed ellipticity (mdeg), c is the concentration (mM) of peptide solution, l is the path length (mm), and n is the number of peptide residues.

CD spectra were recorded on a J-820 spectropolarimeter (Jasco, Tokyo, Japan). The spectra were measured between 190–260 nm (in 0.2-nm steps) at 25 °C with a 1-mm-pathlength cell. Samples were dissolved in a 0.2 M phosphate buffer (pH 7.0) or 50% TFE in a 0.2 M phosphate buffer (pH 7.0) at a concentration of 10 µM. Five scans were averaged for each sample. The percentages of secondary structures were estimated from the spectra using the DichroWeb server [45 (link)]. The helical wheel projection was performed using the online software HeliQuest (https://heliquest.ipmc.cnrs.fr, accessed on 14 July 2022).

Far-UV CD spectra of proteins and peptides in soluble and insoluble states were measured with a J-820 spectropolarimeter (Jasco, Japan) using a cell with a light path of 1 mm at each condition. Individual Aβ42 solutions were prepared at 10 μM for CD measurements. The CD signals between 195 and 250 nm were expressed as mean residue ellipticity [θ] (deg cm² dmol^–1). Temperature regulation was carried out using a PFD-425S Peltier-unit (Jasco, Japan).