Application of ATR-FTIR spectroscopy to the study of thermally induced changes in secondary structure of protein molecules in solid state

FTIR spectroscopy in combination with ATR sampling technique is the most accessible analytical technique to study secondary structure of proteins both in solid and aqueous solution. Although several studies have demonstrated the applications of ATR-FTIR to study conformational changes of solid dried proteins due to dehydration, there are no reports that demonstrate the application of ATR-FTIR in the study of thermally induced changes of secondary structure of biomolecules directly on the solid state. In this study, four biomolecules of pharmaceutical interest, lysozyme, myoglobine, chymotripsin and human growth hormone (hGH), were studied on the solid state before and after different thermal treatments in order to relate changes of secondary structure to partial or total thermal denaturation processes. The results obtained provide experimental evidence that protein thermal denaturation in the solid state can be detected by displacement of carbonyl bands which correspond to conformational transformations between α-helix to β-sheet or intermolecular β-sheet; the molecules studied undergo this transformation when exposed to a temperature close to their denaturation temperature which may become irreversible depending on the extent of the heating treatment. These findings demonstrate that ATR-FTIR is an effective and time efficient technique that allows the monitoring of the protein thermal denaturation process of solid samples without further reconstitution or prior sample preparation.