Structure and Intramolecular Proton Transfer of Alanine Radical Cations

Abstract

The structures of the four lowest alanine conformers, along with their radical cations and the effect of ionization on the intramolecular proton transfer process, are studied using the density functional theory and MP2 method. The energy order of the radical cations of alanine differs from that of the corresponding neutral conformers due to changes in the basicity of the NH2 group upon ionization. Ionization favors the intramolecular proton transfer process, leading to a proton-transferred radical-cation structure, [NH3 + -CHCH3-COO•], which contrasts with the fact that a proton-transferred zwitterionic conformer is not stable for a neutral alanine in the gas phase. The energy barrier during the proton transfer process is calculated to be about 6 kcal/mol.

A theoretical and experimental1H NMR spectroscopy study of the stereoelectronic interactions that rule the conformational energies of alanine and valine methyl ester

“…Recently, the intramolecular effects that determine amino acid conformational preferences have been replaced under discussion . Traditionally, intramolecular hydrogen bonding (IHB) has been considered sufficient to explain the conformational behavior of amino acids . However, it has been shown that the combination of steric and hyperconjugative effects, and not IHB, determines glycine, alanine, sarcosine and dimethylglycineconformational energies .…”

“…The characterization of the responsible forces that determine amino acid conformational isomerism is not a simple procedure and requires special attention. However, the peer‐reviewed literature of conformational analysis of amino acid compounds arbitrarily continues to assume that IHB dictates conformational energies . How could arbitrary assumptions concerning the contribution of the intramolecular forces actuating in these conformational complex compounds be justified?…”

DFT Study of the Effects of Halogen Anions on the Stability of Alanine Zwitterion

“…He also examined the utility of self-stabilized anionic species to stabilize amino acid zwitterionic structures. On the basis of this finding, and as continuation of our studies on organic radicals 22,23 and alanine radical cations, 24 we report the effects of halogen anions on the stability of the alanine zwitterion in this paper.…”

Reactions between neutral molecules and cation-radicals in the gas-phase: Can protonation occur without proton transfer?

“…Another question that requires explanation is whether self-protonation processes occur via hydrogen or proton abstraction. Several theoretical questions regarding the fragmentation of amino acids' cation-radicals [13][14][15][16] and intramolecular proton transfer in amino acids [17][18][19][20] have been examined in the literature. One should note, however, that while mass spectrometrically observed protonated amino acids might be products of intramolecular reactions, these reactions cannot occur in a single amino acid, but rather proceed within their higher mass charged complexes or as an intermolecular particle transfer during two-body collisions between charged and neutral amino acids [5].…”