In this study, we examined the human myeloma second-class immunoglobulins, LOM and SIN, and their Fc fragments, by a number of physical methods, such as scanning calorimetry, fluorescence spectroscopy and analytical centrifugation. In addition, we obtained and carried out a separate analysis of their hFc fragments, which contain not only the lower portion of the hinge region, but its complete core peptide, Cys-Cys-Val-Glu-Cys-Pro-Pro-Cys. Joint analysis of calorimetric and optical melting curves revealed that only the first low-temperature heat absorption peak in all of the melting curves corresponded to the melting of the two С_Н2 domains. Thus, we demonstrate that the С_Н2 domains of the intact IgG2 are present in a less compact conformation compared to their state within the hFc and Fc fragments.