## 3 Results and Discussion
### Change in the Secondary Structure of MPs  
CD spectra were used to present changes in the secondary structure of MPs-fermentation-stinky compound complexes. As shown in Figure 5A-D, all MPs-fermentation-stinky compound complexes displayed two negative peaks with 208 nm and 222 nm, which illustrated that the n-\(\pi\)* of peptide bonds in an \(\alpha\)-helical structure transformed [30]. As the concentration of fermentation-stinky compounds increased, the \(\alpha\)-helical structure changed dramatically. Compared to the effects of different fermentation-stinky compounds on the secondary structure of MPs, the abundance of the negative peaks of isovaleric acid (Figure 5B) and dimethyl disulfide (Figure 5C) presented a trend change with concentration.  
Figure 4: Synchronous fluorescence spectra of fermentation-stinky compounds-MPs complexes at (**A–D**) \(\Delta\lambda\) = 15 nm and (**E–H**) \(\Delta\lambda\) = 60 nm in different concentrations (0, 2, 5, and 8 mg/L).  
The abundance of the negative peak at 5 mg/L for indole and dibutyl phthalate was comparable to that at 8 mg/L.  
Under the addition of 8 mg/L fermentation-stinky compounds, Figure 5E observed that \(\alpha\)-helix content was enhanced, with the most significant increase appearing as a result of indole treatment. Their \(\beta\)-sheets were completely transformed, while their random coil content was increased. The more secondary structures of MPs randomly coiled by isovaleric acid or dimethyl disulfide binding, the more their content increased from 39.47% to 48.00% and 48.40%, respectively. The results suggested that interactions between all fermentation-stinky compounds and MPs altered the initial secondary structure of MPs, allowing the \(\beta\)-sheet to fully expand and add \(\alpha\)-helix and random coils. Hydrophobic bonding promotes \(\beta\)-sheet formation, while hydrogen bonding dominates the helix [10].  
Figure 5: Changes in the circular dichroism spectra of (**A**) myosin-indole complexes, (**B**) myosin-isovaleric acid complexes, (**C**) myosin-dimethyl disulfide complexes, and (**D**) myosin-dibutyl phthalate complexes in different concentrations (0, 2, 5, and 8 mg/L). (**E**) Changes in the secondary structure content of mandarin fish MPs. Different letters indicate significant differences between groups (_p_ < 0.05).  
Thus, this indicated that the transition from the \(\beta\)-sheets to the \(\alpha\)-helix or random coil in the structure of MPs was dominated by hydrogen bond interactions.  
The correlation heatmap (Figure 6) showed that the adsorption capacity of fermentation-stinky compounds and \(\alpha\)-helix percent presented a positive correlation, whereas the percentage of \(\beta\)-sheet, \(\beta\)-turns, and random coil and fluorescence intensity at 280 nm, \(\Delta\lambda\) = 15 nm, and \(\Delta\lambda\) = 60 nm were negatively related to them. Differently to this, random coil percent and the adsorption capacity of dimethyl disulfide presented a high positive correlation (r = 1.0). Therefore, the change in the secondary structure of MPs and the reduction in fluorescence intensity could be attributed to the binding between fermentation-stinky compounds and MPs.