## 2 Results and Discussion
### Amino Acid Analysis of Gelatin and Hydrolyzed Gelatin Peptides  
The amino acid compositions of G and HGPs extracted from unsorted mixed dehydrated raw tuna side streams are reported in Figure 3 and are expressed in the following format: amino acid/100 amino acids (%).  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Gelatin (G)** & **Hydrolyzed Gelatin Peptides (HGPs)** \\ \hline \multirow{3}{*}{CIELab 1} & L* = 88.66 \(\pm\) 0.01 & L* = 88.62 \(\pm\) 0.01 \\  & a\({}^{\star}\) = 0.41 \(\pm\) 0.01 & a* = 0.46 \(\pm\) 0.01 \\  & b* = 5.89 \(\pm\) 0.01 & b* = 7.71 \(\pm\) 0.01 \\ \hline \hline \end{tabular} \({}^{\dagger}\) Results are expressed as means \(\pm\) standard errors (\(n\) = 3).  
\end{table}
Table 2: Color parameter of gelatin (G) and hydrolyzed gelatin peptides (HGPs).  
Figure 3: Amino acid compositions of gelatin (G) and hydrolyzed gelatin peptides (HGPs).  
The most abundant amino acids in collagen are glycine, proline, alanine, and hydroxproline. The glycine content was 28 and 26% for G and HGPs, respectively, while the quantities of the imino acids proline and hydroxproline constituted, for both samples, around 16%, as was also observed by [PERSON] et al. in gelatin extracted from skate (_Raja kenojei_) skins [19] and by [PERSON] for gelatin hydrolysate coming from bones of the skipjack tuna (around 18%) [13]. In comparison with mammalian gelatin, the marine gelatin content in amino acids (proline and hydroxproline) is lower, and this is the main reason why they present different physical properties [20] (viscosity, gel strength, and gelling point). Alanine and glutamic acid content represented about 10% in both samples of G and HGPs, followed by arginine at 7%.  
Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy Analysis of Gelatin and Hydrolyzed Gelatin Peptides  
The secondary structures of G and HGPs were evaluated through Attenuated Total Reflectance FTIR (ATR-FTIR), and the corresponding spectra are reported in Figure 4.  
Collagen and gelatin have characteristic FTIR spectra absorption bands that represent amide vibrational modes (Amides A, B, I-III) and can provide information regarding the secondary structures of proteins [21]. Amide A, found at 3283/3286 cm\({}^{-1}\), reflects NH stretching and, more precisely, it is believed to originate from Fermi resonance between the NH-stretching frequency and the first overtone of amide II [22]. Amide B, with low intensity peaks at 2938/2934 cm\({}^{-1}\), corresponds to both NH\({}_{3}\) and the asymmetric stretching vibrations of =C H. [23]. Amide I, indicative of \(\alpha\)-helix chains [24], is represented at 1634/1644 cm\({}^{-1}\) with C=O stretching [25] coupled with CN stretching, CCN deformation, and in-plane NH bending modes [26]. In collagen, this peak is found at 1655 cm\({}^{-1}\) while, in the case of hydrolysis and denaturation, a shift occurs at 1645-1630 cm\({}^{-1}\) due to the vibrations of proline and hydroxproline [27]. The Amide II bands were both found at 1532 cm\({}^{-1}\), associated with CN stretching and NH bending vibrations [28]. Amide III was represented by the peaks at 1237/1239 cm\({}^{-1}\); the intensity of this band is related to the triple-helical structure [29]. When comparing denatured collagen to pure collagen, the IR absorption ratio 1235/1450 cm\({}^{-1}\) of the latter is 1.00, and it is linked with the triple-helix integrity [30]; in this case, the ratio was equal to 0.99 for G and 0.96 for HGPs, meaning that the structure of the triple helix seemed almost totally to have been maintained in both.