N-Methylformamide, a hyperplectic model for peptides in thin film infrared spectroscopy on planar AgX

Af-Methylformamide (NMF), the simplest model for peptides, exhibits hyperplectic (both simple and complex) behavior as revealed by thin film infrared spectroscopy on planar AgX [AgCl:AgBr] fiber. IR spectra (0.1 s scans) of 10 μg NMF/dichloromethane(DCM) under N₂ flow first show NMF monomer, dimers, and trimers, which then form surface-organized NMF oligomers as pseudocrystals (P(«)) of increasing length and intensity to P(12). After 4 s, P(12) decays in 1.5 to 4 s steps via P(l 1), P(10), P(9), P(8), P(7), P(6), and P(5) to P(4)* and P(3)*. The nature of P(«) (n = 5-12) is explained using a model based on the crystal structure of NMF and consisting of a matrix of 7 × 7 helices, alternating R(ight) and L(eft) with TDC (transition dipole coupling) in groups with 2, 3, or 4 neighbors. The total (10) dipolar couplings are matched to the 10 maxima of P(«) and prove the value of the model. P(4)* (spectrum 325) fits a 5 × 5 matrix without corners. P(3)* is transformed into the very weakly absorbing cyclic hexamer, shown to be very stable and swelling in DCM with increased intensity but without wavelength changes.