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Center for Nonlinear Studies |
Infrared spectroscopy is a powerful tool for analyzing the structure of proteins and peptides. The amide I band is particularly sensitive to the strength and position of the hydrogen bonds which define secondary structure as well as dipole-dipole interactions which are affected by the geometry of the peptide backbone. Introduction of a single 13C labeled carbonyl into a peptide backbone results in a resolvable shoulder to the main amide I band which can be analyzed as a separate peak. Thus, site-specific structural information can be obtained by sequential, systematic labeling of the backbone. Moreover, in beta-sheet forming peptides, the register of the beta-sheet can be determined by isotope edited infrared (IE-IR) spectroscopy; coupling between 13C labeled carbonyls which are aligned within the beta-sheets gives rise to a unique 13C amide I band, distinguishable from the band which arises from labeled carbonyls which are not aligned within the structure [1, 2]. Peptides such as abeta;16-22 and the prion peptide PrP109-122 are known to form beta-sheets which can then aggregate into amyloid-like fibrils; these fibrils are associated with the neurodegenerative diseases Alzheimer's and prion diseases (e.g. CJD, BSE, scrapie) respectively. The molecular-level structure of these aggregates are difficult to determine by NMR spectroscopy. However, the IE-IR spectra give structural detail about the organization of polypeptides within the aggregates, including the alignment and registry of strands within beta-sheets and the structural rearrangements which occur during the aggregation process. Several examples will be discussed, including discrimination between parallel and antiparallel organization in abeta16-22 aggregates; observation of strand reptation within beta-sheets of PrP109-122; and elucidation of structure, stability, and morphology of a series of prion peptide derivatives. [1] Silva, R. A. G. D., Barber-Armstrong, W., Decatur, S. M. (2003) JACS, 125, 13674-13675. [2] Petty, S. A., Adalsteinsson, T., Decatur, S. M. (2005) Biochemistry, 44, 4720-4726.