## 3 Results and Discussion
### Amino Acid Analysis  
In general, collagen has a high content of glycine, alanine, hydroxyproline, and proline amino acid, with the standard amino acid sequence in the triple helix region being Gly-X-Y, where X and Y are often proline and hydroxyproline. Accordingly, the amino acid composition of the extracted collagens is particularly rich in glycine, alanine, and proline (87), with glycine being the most abundant amino acid in _COLRp_1_, _COLRp_1_, and _COLRp_1_ collagens, accounting for 33.8%, 35.0%, and 32.4% of the total amino acid composition, respectively. These values are in agreement with the percentage reported by [PERSON] et al. [62] for silver carp collagen. Furthermore, hydroxyproline and proline (pyrrolidine acids) are known to influence the temperature stability of collagen, with marine-origin collagens exhibiting typically a lower denaturation temperature and viscosity than terrestrial mammals collagens [63]. Greenland halibut collagens contained 153 (_COLRp_1_), 145 (_COLRp_1_), and 140 (_COLRp_1_) residues of pyrrolidine acids, similar to the values detected for other  
Figure 3: Physicochemical properties of Greenland halibut skin collagens. (**A**) Rheological properties of _COLRp_1_, _COLRp_1_, and _COLRp_1_ collagens presented as the temperature dependence of \(\eta^{*}\) (Pa.s) and tan \(\delta\). (**B**) The amino acid contents of collagens. (**C**) Micro-DSC thermograms. (**D**) Moisture regain (%) of extracted collagens. Statistically significant differences were determined using one-way ANOVA following a post hoc Tukey ’s test. The asterisks (”) denote a statistically significant difference (\(p<0.05\)). Values represent the mean \(\pm\) SD of at least three individual measurements.  
marine collagens such as Atlantic cod (_Gadus morhua_) [64] or carp (_Cyprinus carpio_) [65]. Furthermore, the hydroxylation degree is a relevant parameter to evaluate the collagen thermal stability and helix structure [1]. The hydroxylation degree was 43.1% for _COLRp_I_, 40% for _COLRp_II_, and 39.2% for _COLRp_III_. Indeed, the degree of the hydroxylation of _COLRp_I_ collagen was the highest, at least 3% which is directly linked to the higher thermal stability of this collagen molecule [66]. These results point out that the change in NaCl concentration during collagen precipitation has a relevant role in the thermal stability of the isolated protein. It has been reported that collagen thermal stability is dependent on the presence of different ion concentrations [67; 68; 69], which also influence the rheological properties of collagen solution. According to [PERSON] et al., for NaCl concentration above 0.1 M, collagen solutions exhibited a salting-out effect, with their pseudo-plastic behavior becoming stronger, without a significant change in thixotropy [70].  
The variation of both the pyrrolidine acids contents and hydroxylation degree observed in the extracted collagens is consistent with the variation in melting temperature, supporting the relationship between both features in collagen. Interestingly, the serine amino acid content was 74, 70, and 69 residues for _COLRp_I_, _COLRp_II_, and _COLRp_III_, respectively, being an amino acid reported in the literature as playing a role in enhancing the moisturizing capability of the skin [71], which might be relevant for the development of cosmetic applications using the produced collagens.