Infrared spectroscopy a tool for protein characterization

Abstract: Infrared (IR) spectroscopy, which belongs to vibrational spectroscopy, detects the vibrations of molecules, for example, proteins. The absorption of the peptide group gives rise to 9 characteristic bands in the infrared region, named A, B, I-VII, with a decreasing energy or wavenumber (cm-1). Among the 9 bands, amide I, which is mainly caused by C=O stretching vibration, is most sensitive to backbone structure and environment, and therefore can be used for structural analysis. In this thesis, a membrane protein sarcoplasmic reticulum Ca2+-ATPase (SERCA1a) and a self-assembling peptide was studied with IR spectroscopy.  In the first two papers, IR spectroscopy was used to assess the quality of a recombinant SERCA1a. A yeast-based expression system was applied to express recombinant SERCA1a, and the reaction cycle as well as the structure was analysed with IR spectroscopy. Different reaction intermediates were accumulated under different buffer conditions upon the release of ATP. The results showed that the recombinant protein shared similar IR features compared to the native protein. However, two SERCA1a preparations showed a difference around 1640 cm-1 in the amide I region. Using curve fitting, the band was assigned to β structure, and further investigation indicated that the difference in this region originates from protein aggregation. In the third paper, a co-fitting approach was tested and showed to be a more reliable method for structural analysis, and it can be applied in the biological IR spectroscopy. In the fourth paper, a peptide was computational designed and was predicted to self-assemble to amyloid fibrils, the formation of the fibril was confirmed with both electron microscopy and X-ray diffraction. IR spectroscopy was used to analyze further the structural details and the results support our structural predication.