Vibrational absorption spectra, DFT and SCC-DFTB conformational study and analysis of [Leu]enkephalin

The endogenous morphine-like pentapeptide, [Leu]enkephalin, which binds to the opiate receptor in the brain, spinal core and gut, is the subject of this study. Vibrational absorption (VA) measurements were carried out on [Leu] enkephalin in non-polar solvent, DMSO-D6 to stabilize the environment. Ab initio (DFT at the B3LYP/6-31G* level of theory) and semi-empirical (SCC-DFTB) with and without dispersion correction were applied to simulate the VA spectra of [Leu] enkephalin. In these calculations structures taken from X-ray measurements for different conformers of the molecule were used as initial structures for SCC-DFTB geometry optimizations, while the optimized SCC-DFTB geometries were used as initial structures for the DFT geometry optimizations. The experimental VA spectrum and the VA spectra calculated for the low energy conformers at each level of theory are then compared for different conformers of this peptide. This comparison allowed structural study of this molecule as it is here presented.