Solvent-Assisted Rearrangements between Tautomers of Protonated Peptides

Abstract

The presence of an interacting water or methanol molecule has been shown to catalyze the 1,3-proton shift in a peptide linkage between the tautomers of protonated formamide and glycylglycylglycine. Density functional theory calculations at the B3LYP/6-31++G(d,p) level of theory show that, for glycylglycylglycine, the forward barrier of this shift decreases from a free energy at 298 K of 39.6 kcal/mol in the absence of solvent to 26.7 kcal/mol in the presence of water and to 22.0 kcal/mol in the presence of methanol. Protonation at the amide nitrogen of the second residue results in a large increase in the C−N bond distance from 1.336 to 1.519 Å, whereas protonation at the carbonyl oxygen leads to a decrease in the C−N bond distance from 1.336 to 1.321 Å. Solvent-catalyzed tautomerism may play an important role in the fragmentation of electrosprayed, protonated peptides in the gas phase.

Computational Investigation of 1,3‐H and 1,5‐H Shifts in Isomerization of Enol Acetate of 2‐Aceto‐1,3‐cyclohexanedione

“…As a matter of fact, the experimental isomerization was carried out in the field of solvent, so the reaction could take place as a catalyzed process where the H-shift energy barrier could be lowered by the interaction of the shift H with strong proton-acceptor trimethylamine molecule. 17 To assess the catalytic effect of trimethylamine on the two H-shifts, the energy barriers for 1,3-H shift in model, aldehyde and 1,5-H shift in (Z)-but-2-enal were first calculated both in vacuum and trimethylamine for simplicity. Meanwhile, the catalytic effect of water was also calculated for comparison.…”

Atypical solitary fibrous tumor of the nose and maxillary sinus

“…This methodology was also used to compute reaction and activation energies for some keto‐enol tautomeric equilibria in the gas phase. Unfortunately most of the reference values used for the comparison came from previous theoretical studies 8–20. Experimental results for tautomeric equilibria and enolate anion reactions are usually available for chemical processes in solution.…”

Experimental and theoretical investigation of 5-para-nitro-benzylidene-thiazolidine-2-thione-4-one molecule

“…However, a solvent assisted proton transfer reaction [58] is also plausible due to the fact that NH donor and C@S acceptor H bonding sites are available for stable complexes formation which are predicted to greatly decrease the energy barrier height [48].…”

“…When such a proton migrates to the nearby sulfur atom via an intramolecular transfer, it becomes much more shielded and its chemical shift falls in a significantly lower range [64]. On the other hand, when hydrogen atoms are involved in hydrogen bonds, depending on the particular type of interactions, their protons are significantly more deshielded [58,65,66]. Thus, in order to make a reliable assignment for H(7) proton, we shall consider the coexistence of two tautomers, the thione tautomer with the proton bonded to nitrogen N(3) atom and involved in a hydrogen bond with the oxygen atom from a DMSO solvent molecule, and the thiol tautomer.…”