The different molar absorptivities of the secondary structure types in the amide I region: an attenuated total reflection infrared study on globular proteins.

Differences in molar absorptivity of the various secondary structures in the amide I region of infrared protein spectra would have a great impact on the interpretation of the data published thus far on protein films studied by attenuated total reflection infrared spectroscopy. In this work, representative values for amide I absorptivities are obtained for 15 different films of globular proteins spread from H2O solutions. The observed intensities are corrected for variations in film thickness and for contributions of hydration water, atmospheric water, and side chains. These absorptivities, together with the reported secondary structure of the proteins investigated, are used to deduce the molar absorptivities of the individual secondary structure types. It is found that the molar absorptivity of beta-strands is 1.4-1.6 times larger than that of alpha-helices, which in turn is 1.3-2.1 times larger than those found for beta-turns or random coiled structures. The implications of our findings for spectral analysis currently used in literature are discussed.