*2.4. Viscosity and Solubility*

Upon increasing temperature, the viscosity of ASC and PSC had decreased gradually (Figure 2B). The viscosity of PSC was gradually decreased in respect to temperature, which was similar to other fish collagen [19,20], whereas the viscosity of ASC had a sharp decline at 20–30 ◦C. The collagens had lost their viscosity against the temperature due to the breaking of the collagen molecule hydrogen bonds that lead separate triple helix chains into random coils or distinct chains during heating at high temperature. Therefore, the secondary structure highly influences the viscosity of collagen. In this study, PSC was more viscous than ASC due to the high content of imino acid, which is believed to be responsible for the stability of collagen [19].

**Figure 2.** Electrophoretic profile (**A**), viscosity (**B**) and solubility against pH (**C**) and salt (**D**) of blue shark skin collagen. ASC-Acid soluble collagen, PSC-pepsin soluble collagen.

The solubility of collagen against pH was presented in Figure 2C. The results showed that ASC had high solubility in acidic conditions at pH 4.0, whereas the maximum solubility PSC was observed at pH 6.0. The solubility of collagen against salt, NaCl, showed that the maximum solubility of PSC and ASC was observed at 3% and 4% NaCl concentration, respectively (Figure 2D). Similar findings on salt solubility (3–4%) of eel fish skin collagen were earlier reported by Veeruraj et al. [17]. In general, the collagen tends to precipitate at their iso-ionic point (pH 5–8), which leads to decrease the solubility [18]. Also, the solubility of collagen against pH is closely related with the molecular composition and ionic state of the functional groups present in collagen [15].

#### *2.5. UV Absorbance*

The UV absorbance pattern of blue shark skin collagen was presented in Figure 3A. Both collagens, PSC and ASC, had a UV maximum absorption at 225 nm. It was reported that the major functional

groups (-COOH, C=O and CO-NH2) of collagen tend to absorb UV light [21]. An earlier study revealed that the purity of collagen was indirectly measured by the absence of UV absorption at 280 nm since the non-collagen proteins tend to absorb maxima at this particular UV wavelength [16].
