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We have carried out structural determination of capped Cα,α-diethylglycine (Deg) homopeptides with different chain lengths, Ac-(Deg)n-OtBu (n= 2−5), solvated in CDCl3, and investigated vibrational properties of the amide I and II modes by linear and 2D IR spectroscopy, ONIOM calculations, and molecular dynamics simulations. 2D IR experiments were performed in the amide I region using the rephasing pulse sequence under the double-crossed polarization and the nonrephasing sequence under a new polarization configuration to measure cross-peak patterns in the off-diagonal regions. The 2D IR spectra measured in the amide I and II regions reveal complex couplings between these modes. Model spectral calculations finely reproduced the measured spectral profiles by using vibrational parameters that were very close to the values predicted by the ONIOM method. The agreement led to a conclusion that peptide backbones are fully extended with the dihedral angles (ϕ,ψ) ≈ (±180°,±180°) and that a sequence of intramolecular C5hydrogen bonds forms along the entire chain regardless of the chain length. This conclusion was endorsed by analysis of the molecular dynamics trajectories for n= 3 and 5 that showed an exclusive population of the C5conformation. The conformationally well-restrained Deg homopeptides serve as an ideal linear exciton chain, which is scarcely obtainable by protein amino acids. We investigated excitonic properties of the linear chain through analytic modeling and compared the measurement and calculation results of the amide I and II modes. The integrated intensity of the amide II band is larger than that of the amide I for the C5structure, untypical behavior in contrast with other secondary structures. This comprehensive study characterized the amide I and II spectral signatures of the fully extended conformation, which will facilitate the conformational analysis of artificial oligopeptides that contain such structural motifs.

The intrinsically blue-colored Ullman imidazolinyl nitronyl nitroxide (NN) mono-radicals have found various applications, in particular as spin probes and organic magnetic materials. Here, we present the solution-phase synthesis, extensive characterization, and conformational analysis of the first peptidomimetics with two pendant, chiral nitronyl nitroxide free radical units. Two (R)-Aic(NN) residues, where Aic(NN) is 2-amino-5-nitronylnitroxide-indan-2-carboxylic acid, have been inserted at positions iand i+3 of the pentapeptide Boc-(R)-Aic(NN)-(Ala)2-(R)-Aic(NN)-Ala-OMe and the hexapeptide Boc-[Ala-(R)-Aic(NN)-Ala]2-OMe as well. The two compounds were obtained in good yields and high purities. Thanks to a combination of several spectroscopic techniques (IR absorption, NMR, VCD, and EPR) we gained clear evidence that both compounds adopt a right-handed 310-helical conformation with both nitronyl nitroxide pendants positioned on the same side of the helix. This peptidomimetic/free radical system is a potentially excellent template for the preparation of a set of appropriate analogs with exciting applications in the area of host–guest organic chemistry, or to spectroscopically evaluate in-depth the intramolecular exchange interactions in this type of probe.