N-Methyl-trimethylacetamide in Thin Films Displays Infrared Spectra of pi-Helices, with Visible Static and Dynamic Growth Phases, and then a beta-Sheet

摘要

The simplest (minimal) peptide model is HCONHCH3. An increase in the pi-helix content with increased substitution in the acyl portion suggested the examination of N-methyl-trimethylacetamide) (NMT). NMT displays spectra, in which there is evolution of a set of helices defined by their amide I maxima near 1686 (3(10)), 1655 (first pi), and, most importantly, at 1637 cm(-1) (pi). 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 cm(-1) suddenly in-creases at 82.1 s (30% over 0.3 s), indicating a phase change and crystallization of the pi-helix, along with a coincidental decrease in the absorbance for the first pi-helix. A sharp peak occurs at the maximum of the phase change at 82.5 s, representing a pure NMT pi-helix. The spectra then undergo a decreasing general absorption loss over 150 s, with the pi-helix evolving further to an antiparallel beta-sheet fragment. The spectral quality arises from the immobilization of polar molecules on polar surfaces. The crystal structure is that of an antiparallel beta-sheet.