IR-study on stacking manner of peptide nanorings in peptide nanotubes

We here report our theoretical as well as experimental studies on the stacking manner of peptide nanorings (PNRs) in peptide nanotubes (PNTs). We focus on the molecular vibrations of N-H and C=O stretching modes and discuss this subject via their infrared (IR) spectroscopy, because PNTs are formed by the inter-ring H bonds between the adjacent PNRs via -N-H⋯O=C-. Symmetry analysis based on group theory reveals that parallel stacking causes two IR active modes in these molecular vibrations while three modes are active in the antiparallel stacking. This difference in the number of IR active modes is determined only by the stacking manner and not by the number of amino acid residues composing the PNRs. By using two typical PNRs of cyclo[-(L-Gln-D-Ala) ₃] and cyclo[-(L-Gln-D-Ala)₄], we further studied the favorable stacking manners of PNRs via IR observation. Our IR experiments as well as the ab initio energetics show that the former PNRs create a PNT by stacking themselves in parallel while the latter PNRs do so by stacking themselves in an antiparallel manner.