Thermal stability and conformational structure of salmon calcitonin in the solid and liquid states

Salmon calcitonin (sCT) was selected as a model protein drug for investigating its intrinsic thermal stability and conformational structure in the solid and liquid states by using a Fourier transform infrared (FT-IR) microspectroscopy with or without utilizing thermal analyzer. The spectral correlation coefficient (r) analysis between two second-derivative IR spectra was applied to quantitatively estimate the structural similarity of sCT in the solid state before and after different treatments. The thermal FT-IR microspectroscopic data clearly evidenced that sCT in the solid state was not effected by temperature and had a thermal reversible property during heating-cooling process. Moreover, the high r value of 0.973 or 0.988 also evidenced the structural similarity of solid-state sCT samples before and after treatments. However, sCT in H(2)O exhibited protein instability and thermal irreversibility after incubation at 40 degrees C. The temperature-induced conformational changes of sCT in H(2)O was occurred to transform the alpha-helix/random coil structures to beta-sheet structure and also resulted in the formation of intramolecular and intermolecular beta-sheet structures.