PROTEIN SECONDARY STRUCTURES IN WATER FROM 2ND-DERIVATIVE AMIDE-I INFRARED-SPECTRA

Infrared spectra have been obtained for 12 globular proteins in aqueous solution at 20 °C. The proteins studied, which vary widely in the relative amounts of different secondary structures present, include myoglobin, hemoglobin, immunoglobulin G, concanavalin A, lysozyme, cytochrome c, α-chymotrypsin, trypsin, ribonuclease A, alcohol dehydrogenase, β2-microglobulin, and human class I major histocompatibility complex antigen A2. Criteria for evaluating how successfully the spectra due to liquid and gaseous water are subtracted from the observed spectrum in the amide I region were developed. Comparisons of second-derivative amide I spectra with available crystal structure data provide both qualitative and quantitative support for assignments of infrared bands to secondary structures. Band frequency assignments assigned to α-helix, β-sheet, unordered, and turn structures are highly consistent among all proteins and agree closely with predictions from theory. α-Helix and unordered structures can each be assigned to only one band whereas multiple bands are associated with β-sheets and turns. These findings demonstrate a method of analysis of second-derivative amide I spectra whereby the frequencies of bands due to different secondary structures can be obtained. Furthermore, the band intensities obtained provide a useful method for estimating the relative amounts of different structures. © 1990, American Chemical Society. All rights reserved.