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

N-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 microg NMF/dichloromethane(DCM) under N(2) flow first show NMF monomer, dimers, and trimers, which then form surface-organized NMF oligomers as pseudocrystals (P(n)) of increasing length and intensity to P(12). After 4 s, P(12) decays in 1.5 to 4 s steps via P(11), P(10), P(9), P(8), P(7), P(6), and P(5) to P(4) and P(3). The nature of P(n) (n = 5-12) is explained using a model based on the crystal structure of NMF and consisting of a matrix of 7 x 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(n) and prove the value of the model. P(4) (spectrum 325) fits a 5 x 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.