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Fig. 2. Amide-I region of the 2D IR spectra acquired for Ac-(Deg) n -OtBu, with n=2-5,showing the characteristic pattern of the fully-extended conformation.The Deg homo-peptides exhibit unusual spectroscopic features: the amide-II band is moreintense than the amide-I band and it red shifts with increasing main-chain length. We haveperformed detailed analysis of the peptide conformation through stringent comparisons ofmeasured and simulated amide-I and -II 2D IR spectra (Figure 2). The results indicate thatthe backbones of these homo-peptides are fully extended regardless of their main-chainlength. This conclusion is corroborated by molecular dynamics simulations and densityfunctional theory calculations. The complete characterization of the vibrational propertiesof the amide-I and II modes in the fully-extended structure will facilitate theconformational analysis of other peptides involving C 5 motifs.AcknowledgmentsThis research was supported by grants from the American Chemical Society Petroleum Research Fund(39148-G6) and the National Science Foundation (CHE-0450045, CHE-0802913, and DMS-0835863)to N.-H. G.References1. Toniolo, C., Benedetti, E. Trends Biochem. Sci. 16, 350-353 (1991).2. Toniolo, C., Crisma, M., Formaggio, F., Peggion, C., Broxterman, Q.B., Kaptein, B. Biopolymers(Pept. Sci.) 76, 162-176 (2004).603