2.
2DIR spectra for model helical peptides (A) [EK]1, (B) [EK]2, (C) [EK]3, and (D) [EK]4, as well as predominantly α-helical globular proteins (E) HEWL and (F) Myo. The frequency of the amide I′ peak (1635–1655 cm–1) is given on each spectrum, while peak pairs below 1600 cm–1 arise from sequence-dependent IR-active side chains. (G) Correlation between number of helical residues and amide I′ frequency for model [EK] N peptides (blue) and globular proteins (black). The model peptides were fit to a trendline (dashed blue). HEWL and Myo were placed along the x-axis according to the number of residues in their longest α-helix. Each data point is the average frequency with errors bars representing standard deviation over n = 3–6.