J 715 circular dichroism spectrophotometer

Circular dichroism was performed to determine the secondary structure of the self-assembled peptides. A 400 mg/mL stock of each peptide was prepared in DMSO and diluted to a final concentration of 0.4 mg/mL in deionized water (final concentration of DMSO was 0.1% (v/v)), and then incubated at room temperature (21 °C), undisturbed, for 24 h. Spectroscopic measurements were completed using the Jasco J-715 Circular Dichroism spectrophotometer (Jasco Corp., Tokyo, Japan), with a quartz cuvette (path length = 1 mm) at room temperature under nitrogen gas. Three scans were recorded at a wavelength range of 190–250 nm and scanning speed of 100 nm/sec and then averaged. Baseline subtraction was done using a deionized water and DMSO (0.1% v/v) blank and the results were converted to mean residue ellipticity (deg × cm² × dmol⁻¹) using a mean residue weight of 110.74 Da and peptide concentration of 0.4 mg/mL on the CDToolX software [54 (link)]. Plotting and smoothing of the resulting spectra were performed using GraphPad Prism version 9.2.0 for Windows (GraphPad Software, La Jolla, CA, USA). Secondary structure approximation was achieved using the CD fitting software, BeStSel [32 (link)], using the wavelength range, 190–250 nm.

The secondary structure of IAPP was analyzed using the Jasco J-715 Circular Dichroism spectrophotometer (Jasco Corp., Tokyo, Japan). Each sample consisted of 20 µM IAPP and 20 µM tetrapeptide in 10 mM phosphate buffer (pH 7.4). Two time points, initial (0 h) and final (48 h), were measured using a quartz cuvette with a path length of 1 mm at room temperature in nitrogen gas. Samples were incubated at 37 °C in between the time point measurements. Three scans were recorded and averaged at a wavelength range of 200–250 nm at a scanning speed of 100 nm/sec. Baseline subtraction was done using a phosphate buffer blank and results were converted to mean residue ellipticity (deg × cm² × dmol⁻¹) using a mean residue weight of 105.49 Da and IAPP concentration of 0.078 mg/mL. All data processing was performed using CDToolX [39 (link)]. Plotting and smoothing of resulting data was achieved with GraphPad Prism version 9.2.0 for Windows (GraphPad Software, La Jolla, CA, USA). Calculation of the secondary structure contents was performed using the CD fitting software, BeStSel [40 (link)].