## 3 Results and Discussion
### Study of the Physicochemical Properties of Aqueous IgG Colloid of Different Concentrations
The increment changes in the study had a wide range of variation from 0.153 to 0.272 depending on the buffer used. In general, the discrepancy between experimental data and theory requires further and more careful study. The specific conductivity of water and aqueous IgG protein colloids is shown in Figure 3a. It has been shown that the conductivity of protein colloids (IgG concentration < 1 mg/mL) does not differ much from the conductivity of water. The significant increase in specific electrical conductivity of solutions is observed at IgG concentration of 10 mg/mL. Figure 3b shows the molecular oxygen content of water and aqueous colloids with different protein concentrations. It has been shown that significant differences in dissolved oxygen are visible only at the protein concentration of 10 mg/mL. Possibly, when the concentration of IgG is significantly high, the process of binding molecular oxygen with polar, positively charged amino acids in the protein (lysine, arginine, histidine) becomes noticeable. An alternative explanation is the displacement of gas molecules from water by dissolved molecules [50]. Figure 3c shows the redox potential values of water and aqueous colloids with different protein concentrations. At different concentrations of protein molecules, the values of the redox potential are observed, both higher and lower compared to the values of the control water. Multidirectional concentration trends can be explained by different surface charges of proteins. For IgG colloid, a \"bell-shaped\" dependence on protein concentration is observed. This form is most likely due to the fact that with increasing concentration, the protein passes the isoelectric point, which for IgG corresponds to pH \(\approx\) 7.0 [51]. Therefore, for \"small\" concentrations, the protein surface is positively charged, and for \"high\" concentrations, it is negatively charged. Although the differences are visible to the naked eye, the large dispersion of data characteristic of this method makes it possible to distinguish between water and a 0.1 mg/mL IgG solution. Obviously, to confirm the assumption made about the influence of pH on the values of the redox potential, it is necessary to measure the impact of the concentration of IgG molecules on the pH values of the aqueous IgG colloid. Figure 3d shows the pH changes in water and in aqueous protein colloids. It has been shown that with increasing concentration of IgG in an aqueous colloid, the pH value increases. In the range of protein concentrations of 1-10 mg/mL, the pH value becomes more than 6.5; that is, the protein colloid passes the isoelectric point. Figure 2: Refractive index of aqueous colloid as a function of IgG concentration. (**a**) Refractive index of IgG colloids with protein concentrations of 0.0001–0.1 mg/mL measured at a wavelength of 632.9 nm (M \(\pm\) SD, n = 6). (**b**) Differential refractive indexes of different samples relative to the solvent according to the data from Figure 3a. An increment of 0.19 g/mL is set aside as a “theory”. *—statistically significant difference relative to samples not containing protein (\(t\)-test, \(p<0.05\)). Normalized 3D fluorescence spectra of aqueous solutions with different IgG concentration are shown in Figure 4. It has been shown that the protein is reliably detected in an aqueous colloidal solution up to concentrations of 10\({}^{-4}\) mg/mL. In this case, it is recorded as fluorescence (\(\sim\)\(\lambda_{\text{ex}}\) = 280 nm, \(\sim\)\(\lambda_{\text{em}}\) = 330 nm) caused by aromatic amino acid residues. Thus, it has been established that most simple general laboratory methods can clearly distinguish between water and an aqueous colloid of antibodies with a concentration of at least 0.1-1.0 mg/mL; the only exception to the rule is fluorescence. Since the manuscript is devoted to the research of methods for control of quality of protein drugs in aqueous colloids, it was necessary to establish the possible influence of third-party physical influences present in pharmaceutical production.