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 s eries of simple amides. Excited states were studied by means of ab ini tio quantum chemical methods-the Complete Active Space (GAS) SCF metho d 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-methy lformamide and N-methylacetamide), and one tertiary amide (N,N-dimethy lformamide). The calculations comprise a large number of singlet and t riplet valence and Rydberg excited states. The results support a basic structure of the spectra: In gas phase one intense valence pi-->pi t ransition is placed between 6.5 and 7.4 eV and a second weak valence p i-->pi transition occurs at 9.6-10.5 eV. Alkyl substitutions on the n itrogen have a major effect on the valence transitions: The energy dro ps when going from primary to secondary and tertiary amides. In contra st, only minor blue shifts of the excitation energies are observed whe n 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 sens itivity on the length of the alkyl substituent.