Theoretical study of the electronic spectroscopy of peptides .1. The peptidic bond: Primary, secondary, and tertiary amides

With the general aim to characterize the origin of electronic spectra of proteins, the present paper discusses the absorption spectra of a series of simple amides. Excited states were studied by means of ab initio quantum chemical methods-the Complete Active Space (GAS) SCF method and multiconfigurational second order perturbation theory (CASPT2). We calculated the vertical absorption spectra of three primary amides (formamide, acetamide, and propanamide), two secondary amides (N-methylformamide and N-methylacetamide), and one tertiary amide (N,N-dimethylformamide). The calculations comprise a large number of singlet and triplet valence and Rydberg excited states. The results support a basic structure of the spectra: In gas phase one intense valence pi-->pi* transition is placed between 6.5 and 7.4 eV and a second weak valence pi-->pi* transition occurs at 9.6-10.5 eV. Alkyl substitutions on the nitrogen have a major effect on the valence transitions: The energy drops when going from primary to secondary and tertiary amides. In contrast, only minor blue shifts of the excitation energies are observed when alkyl groups of different length are attached to the carboxyl group. For all molecules studied in this paper, n-->pi* transitions from the oxygen lone pair are found at about 5.5 eV and reveal only small sensitivity on the length of the alkyl substituent.