## 3 Results and Discussion  
### Study of the Physicochemical Properties of Aqueous IgG Colloid of Different Concentrations  
Absorption spectra of aqueous IgG solutions are shown in Figure 1a as a function of concentration. The wavelengths attributed to local minima and maxima are as follows: 250 nm and 280 nm, 285 nm. It has been established that detailed optical absorption spectra are observed at a minimum concentration of IgG molecules of 0.1 mg/mL. The spectrum still remains characteristic at IgG concentration of 0.01 mg/mL. It is shown that when protein concentration is 0.001 mg/mL, the peak that corresponds to aminoacidic residues (280 nm) is no longer observed. The optical density of IgG solutions is minimal in the wavelength range 310-800 nm, but the optical density increases with increasing concentration of IgG molecules. It is known that the molecules of most proteins, including antibodies, do not absorb in the wavelength region 450-800 nm [31]. Typically, changes in optical density in such wavelengths are associated with light scattering rather than absorption [32], which in turn is associated with either denaturation of molecules or their aggregation [33]. To answer this question, the change in the hydrodynamic diameter of light scatterers in aqueous IgG colloids with concentrations of 0.01-10 mg/mL was studied using the DLS method. Figure 1b shows measurements of the hydrodynamic particle diameter in a protein colloid containing different concentrations of IgG molecules. It has been shown that using the DLS method, in colloids with protein concentrations of 1 and 10 mg/mL, particles with a hydrodynamic diameter of \(\sim\)10 nm are observed, corresponding in size to individual IgG molecules (immunoglobulin G molecules have sizes of about 10 nm (15 \(\times\) 8 \(\times\) 4 nm) [34]. Also, a fraction of 30-50 nm is observed in the size distribution; these are probably antibody aggregates. When the protein concentration is reduced to 0.1 and 0.01 mg/mL, no peak associated with individual antibody molecules is observed in the size distribution. This detects a fraction with sizes of 100-200 nm. Obviously, individual IgG molecules do not disappear. At low concentrations, scattering from individual molecules is a \"mask\" by  
Figure 1: Changes in the physical and chemical properties of an aqueous colloid depending on the concentration of IgG. (**a**) Optical density changes of IgG solutions depending on the concentration of IgG. Internal figure shows the optical absorption in the wavelength range 400–800 nm. (**b**) Measurement with DLS method of the hydrodynamic diameter of light scatterers (IgG solutions) (concentrations: 0.01–10 mg/mL). (**c**) Changes in dynamic viscosity of aqueous IgG colloids with different content of antibodies at different shear stresses (M \(\pm\) SD, n = 6). (**d**) Study of the frequency ratio in a measuring cell with an aqueous IgG colloid (concentration: 0.01–1 mg/mL) and a reference cell with water using ultrasonic spectroscopy (M \(\pm\) SD, n = 6). *—statistical difference relative to samples not containing protein (_t_-test, \(p\) < 0.05). larger objects. To illustrate this thesis, we repeated the experiment with measuring the size of immunoglobulin, but in this case, to reduce the number of aggregates, the protein colloid was filtered using a membrane filter with an average pore size of 200 nm. The intensity particle size distribution results and autocorrelation functions for these measurements are presented in Appendix A (Figure A1). It is well-known that the intensity of light scattering from this fraction is several orders of magnitude less than from any fraction in a colloid containing 10 mg/mL protein. There are no light scatterers in freshly distilled water. Usually, the experiments used water that had been in contact with atmospheric gases for quite a long time. In such water, there are peaks in the region of 150-200 nm, with low light scattering intensity.