Assessment of density functionals for predicting the infrared spectrum of sodiated octa-glycine

The sodiated peptide GGGGGGGG-Na⁺ or G₈-Na⁺ has a remarkable structure with a highly coordinated sodium ion and an acidic OH that is strongly hydrogen-bound with the N-terminus. The presence of the sodium ion makes this hydrogen bond unusually strong and makes proton transfer easy, leading to an equally stable, salt bridge isomer. The performances of a variety of density functionals in describing the geometries, energetics and infrared spectra of these two isomers were investigated. Usual density functionals were tested and moreover, more recent functionals such as dispersion-corrected ones and Truhlar's M06 series were also considered. The computed infrared spectra are compared with ab initio results and InfraRed Multiple Photon Dissociation (IRMPD) experiments. Two functionals in the M06 series have been proved to be quite efficient. A large number of functionals seems to be inadequate to compute infrared spectra for peptide in the amide N-H stretching region. In addition, a detailed analysis of the sodium-peptide interaction and of the hydrogen bond between the two peptide terminations points out a distinct electronic structure for the two isomers.