*3.7. Fourier Transform Infrared Spectroscopy (FTIR)*

The infrared absorption characteristics of collagen were obtained by an FTIR spectrometer (Tensor 27, BRUKER, Bremen, Germany). First, one-mg collagen samples were mixed with potassium bromide (KBr) at a ratio of 1:100 and pressed into pellets with a manual mechanical squeezing device. The spectra were recorded with a wavenumber range from 4000 to 400 cm−<sup>1</sup> at the resolution of 2 cm<sup>−</sup>1.

#### *3.8. Sodium Dodecyl Sulphate-Polyacrylamide Gel Electrophoresis (SDS-PAGE)*

The protein molecular mass analysis was studied by SDS-PAGE according to Laemmli [74] with 7.5% resolving gel and 5% stacking gel and 120 voltage using a Bio-Rad electrophoresis. The protein bands were stained with Coomassie Blue R250 and destained with 30% (*v*/*v*) methanol and 10% (*v*/*v*) acetic acid. The molecular mass of the subunits was analyzed by the location of the bands.
