QTAIM-Based Characteristic Group Infrared Intensities of Amino Acids and Their Transference to Peptides

Dynamic atomic contributions (DACs) to the infrared intensities of 14 amino acids have been transferred to three peptide molecules, glycylglycine, trialanine, and the melanocyte-inhibiting factor MIF-1, to estimate the infrared intensities of the most strategic peptide bands. The DACs of the amino acids and infrared intensities of the peptides were determined at the DFT B3LYP/6-311+(d,p) level. The Quantum Theory of Atoms In Molecules (QTAIM) Charge-Charge Transfer-Dipolar Polarization (CCTDP) model at this Density Functional Theory (DFT) level was used to classify the O-H, NH₂, N-H, and C═O stretching as well as the NH₂ bending characteristic groups for use in the transference procedure. Contrary to the frequencies, the intensities within these groups can have very diverse values, although their electronic structure changes upon vibration have predictable QTAIM behaviors for each group. Compared to the DFT calculated values, the two transferred O-H stretching intensities of the peptides are estimated with a root-mean-square (rms) error of 19.1 km mol^-1. Six NH₂ symmetric and antisymmetric stretching intensities were determined with a 9.9 km mol^-1 error. The eight estimated C═O stretching bands have a rms error of 78.0 km mol^-1 or 23.6% of the average DFT peptide C═O intensity of 328.4 km mol^-1. The proposed procedure is applicable to experimental infrared intensities if a calibration set of molecules with known atomic polar tensors and normal coordinate transformations is available.