Valence anions of N-acetylproline in the gas phase: Computational and anion photoelectron spectroscopic studies

Abstract

We report the photoelectron spectrum of anionic N-acetylproline, (N-AcPro) − , measured with 3.49 eV photons. This spectrum, which consists of a band centered at an electron binding energy of 1.4 eV and a higher energy spectral tail, confirms that N-acetylproline forms a valence anion in the gas phase. The neutrals and anions of N-AcPro were also studied computationally at the B3LYP/6-31++G(d,p) level. Based on the calculations, we conclude that the photoelectron spectrum is due to anions which originated from proton transfer induced by electron attachment to the π * orbital localized at the acetyl group of N-AcPro. We also characterized the energetics of reaction paths leading to pyrrolidine ring opening in the anionic N-AcPro. These data suggest that electron induced decomposition of peptides/proteins comprising proline strongly depends on the presence of proton donors in the close vicinity to the proline residue.

Interstellar Anions: The Role of Quantum Chemistry

“…The valence excitation of an additional electron, however, makes the photophysics of any anion substantially richer . In a valence arrangement, the additional electron is bound to the molecule independent of the dipolar strength and fills an exceptionally stabilized MO.…”

“…In a valence arrangement, the additional electron is bound to the molecule independent of the dipolar strength and fills an exceptionally stabilized MO. Valence excited states have been discovered or computed in medium-sized molecules with strong electron-withdrawing groups like 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and 7,7,8,8-tetracyanoquinodimethane, , as well as a few other systems, ,,,, but this phenomenon has not routinely appeared with small anions. Some exceptional experimental evidence from Maier and co-workers ,, clearly shows valence excited states in the analysis of pure carbon chains and those that are hydrogen- or nitrogen-terminated.…”

“…73−76 The valence excitation of an additional electron, however, makes the photophysics of any anion substantially richer. 119 In a valence arrangement, the additional electron is bound to the molecule independent of the dipolar strength and fills an exceptionally stabilized MO. Valence excited states have been discovered or computed in medium-sized molecules with strong electron-withdrawing groups like 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and 7,7,8,8-tetracyanoquinodimethane, 120,121 as well as a few other systems, 31,32,119,122,123 but this phenomenon has not routinely appeared with small anions.…”

Presolvated Low Energy Electron Attachment to Peptide Methyl Esters in Aqueous Solution: C–O Bond Cleavage at 77 K

“…Our recent efforts also have shown the cyclic peptide, N-acetylproline is susceptible to N-C bond cleavage; however in this case the cyclic nature of proline prevents the chain from full cleavage. 10,14 …”

“…18 Thus it is of interest to investigate the possibility of this cleavage in a peptide which can provide several other functional groups for electron attachment. 14,18,19 …”