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

• Published in: Chemphyschem 2014-11, Vol.15 (16), p.3592-3597
• Main Authors: Kosower, Edward M., Borz, Galina, Goldberg, Israel, Ermakov, Natalya
• Format: Article
• Language: English
• Subjects: Acetamides - chemistry; Chemistry; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; General and physical chemistry; helical structures; Hydrogen Bonding; hydrogen-bonded peptides; IR spectroscopy; N-methyl-trimethylacetamide; Peptides - chemistry; Physics; planar fibers; Protein Structure, Secondary; Spectrophotometry, Infrared; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201402326

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‐trimethylacetamide) (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 π), 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 cm−1 suddenly increases at 82.1 s (30 % over 0.3 s), indicating a phase change and crystallization of the π‐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 π‐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. Growth spurt: Increasing the size of the acyl group in a simple peptide model (N‐methyl‐acylamide) favors the formation of more open helices such as the π‐helix. Sudden changes in spectroscopic data reveal that a crystalline π‐helix can be formed, and a succession of further changes leads to a quasiplanar π‐form. Independently, a planar β‐sheet form is also obtained.