N-methyl-trimethylacetamide in thin films displays infrared spectra of π-helices, with visible static and dynamic growth phases, and then a β-sheet

The simplest (minimal) peptide model is HCONHCH₃. An increase in the π-helix content with increased substitution in the acyl portion suggested the examination of N-methyl-trimethy-lacetamide) (NMT). NMT displays spectra, in which there is evolution of a set of helices defined by their amide I maxima near 1686 (3₁₀), 1655 (first p), and, most importantly, at 1637 cm^-1 (π). Expanded thin-film infrared spectroscopy (XTFIS) shows pauses or slow stages, which are identified as static phases followed by dynamic phases with the incremental gain or loss of a helix turn. In addition, absorbance at 1637 cmr¹ suddenly increases at 82.1 s (30% over 0.3 s), indicating a phase change and crystallization of the p-helix, along with a coincidental decrease in the absorbance for the first π-helix. A sharp peak occurs at the maximum of the phase change at 82.5 s, representing a pure NMT π-helix. The spectra then undergo a decreasing general absorption loss over 150 s, with the p-helix evolving further to an antiparallel β-sheet fragment. The spectral quality arises from the immobilization of polar molecules on polar surfaces. The crystal structure is that of an antiparallel β-sheet.