A comparison of infrared spectra of proteins in solution and crystalline forms

Fourier transform infrared spectroscopy has been used to compare the structure of a range of proteins in solution and in the form of single crystals. An infrared microscope was used to record the spectra of single crystals of the proteins. The proteins studied in this way were hen egg white lysozyme, bovine pancreatic ribonuclease A, bovine gamma-II crystallin, human serum amyloid P component, Endothia parasitica pepsin and Mucor pusillus pepsin. The amide I and amide II bands in the FTIR spectra of these proteins were analysed using derivative procedures thereby providing information on the secondary structure. The crystals were held under a vapour of mother liquor to reduce the effects of dehydration. A comparison of the spectra revealed that spectra recorded from crystals of lysozyme, ribonuclease A and gamma-II crystallin are nearly identical to those recorded from the proteins in solution. However, differences are observed between the spectra of serum amyloid P component, Endothia parasitica pepsin and Mucor pusillus pepsin in solution compared with that of the crystalline form These differences are suggested to be due to rearrangements of turn structures within the protein structure.