This work was carried out to compare the effectiveness of the methods of infrared spectroscopy in the amide I region and UV circular dichroism to the analyze the protein secondary structure by the example of linker histone H1 and bovine serum albumin (BSA). It has been shown that the application of a diamond ATR cell quantifies the proportion of α-helices and β-structures in a good agreement with UV circular dichroism spectroscopy. It has been shown that histone H1 is able to aggregate, which results in considerable changes in its secondary structure.