*2.6. Fourier Transform Infrared Spectroscopy (FTIR)*

The FTIR peak of blue shark skin collagen was presented in Figure 3B. As shown, the primary peaks such as Amide-A and Amide-B were observed at 3302.67 and 3301.20 cm−1, and 2972.57 and 2972.36 cm−<sup>1</sup> for ASC and PSC, respectively. Amide-A and Amide-B peaks are generally produced by N-H stretch joined with the H2 bond in a carbonyl group of peptide chains and an asymmetric stretching vibration of NH3 <sup>+</sup> with =C-H, respectively. The major spectral wavelength observed in blue shark skin collagen was similar to earlier studies [17,20,22,23]. In this study, the shifting of Amide-A peak in PSC to lower wavelength was due to the hydrogen bond vibration of the NH group.

The other major peaks like Amide-1 and Amide-2 occurred at 1632.28–1632.06 cm−<sup>1</sup> and 1547.89–1547.25 cm−<sup>1</sup> for blue shark skin ASC and PSC, respectively. The secondary structure of collagen is closely represented by Amide-1 peak, which is due to the C=O stretching vibration of peptides. Therefore, any shifting in the amide-1 peak directly reflects the changes in the secondary structure of a protein. In this study, the amide-1 peak of PSC shifted to a lower wavelength compared to ASC, which denotes the secondary structural changes of PSC by pepsin digestion.

**Figure 3.** UV absorption (**A**) and Fourier transform infrared spectroscopy (FTIR) pattern (**B**) of blue shark skin collagen. ASC-Acid soluble. collagen, PSC-Pepsin soluble collagen.
