Raman Spectroscopy of Insulin under Ambient and Extreme Conditions

Raman spectroscopy has been used to investigate different conformational states of bovine pancreatic insulin: the native form and several structurally modified states with different extent of denaturation induced by thermo-chemical treatment and by applying very high pressure (up to 8 GPa) using a diamond anvil cell. High-pressure results confirm the peculiar strength-tovolume compression of insulin and largely extend the pressure range of its structural stability (0–4.2 GPa). Above 4.2 GPa, insulin undergoes an irreversible structural transition that, once pressure is released, leaves the sample in a new conformational state. The protein secondary structure after the pressure treatment results to be somehow intermediate between the native and the thermo-chemical fibrillar samples. The analysis of the pressure dependence of the Raman spectrum and of several specific spectroscopic markers allows us to follow the path from the native to new pressure-denatured protein conformation. Morover a refolding studies has been conducted on fibrillar samples of insulin and lysozyme by means of ammonium based ionic liquids.