Competition between Solvation and Intramolecular Hydrogen-Bonding in Microsolvated Protonated Glycine and β-Alanine

Abstract

Infrared predissociation (IRPD) spectroscopy is used to reveal and compare the microsolvation motifs of GlyH + (H 2 O) n and β-AlaH + (H 2 O) n . The chemical structure of these amino acids differ only in the length of the carbon chain connecting the amine and carboxyl terminals, which nonetheless leads to a significant difference in the strength of the intramolecular CH−N hydrogen bond in the unsolvated ions. This difference makes them useful in our studies of the competition between solvation and internal hydrogen bonding interactions. Analysis of the IRPD results reveals that the sequential addition of water molecules leads to similar effects on the intramolecular interaction in both GlyH + (H 2 O) n and β-AlaH + (H 2 O) n . Solvation of the −NH 3 + group leads to a weakening of the CO•••H−N hydrogen bond, while solvation of the carboxyl −OH leads to a strengthening of this bond. Additionally, we have found that for β-AlaH + , the addition of a H 2 O to the second solvation shell can still influence the strength of the CO•••H−N hydrogen bonding interaction.Finally, because the CO•••H−N interaction in β-AlaH + is stronger than that in GlyH + , more solvent molecules are needed to sufficiently weaken the intramolecular hydrogen bond such that isomers without this bond begin to be energetically competitive; this occurs at n = 5 for β-AlaH + and n = 1 for GlyH + .

Setting up the HyDRA blind challenge for the microhydration of organic molecules

“…After publication of the results of the HyDRA blind challenge, the training and test sets can be extended to more strongly and even more weakly bound monohydrates, to charged systems (where more powerful experimental techniques are available [106][107][108] ), to dihydrates, to isotope effects, to other vibrations, or to further observables such as structures and relative energies. These extensions may be targeted by different groups and will hopefully lead to a continuous refinement of the theoretical modelling of the microhydration of organic matter, identifying the most powerful and universal models in the field.…”

Conformational Changes Induced by Methyl Side-Chains in Protonated Tripeptides Containing Glycine and Alanine Residues

“…The frequencies of the N–H, O–H and CO modes are particularly sensitive to the presence and strength of H-bond interactions, and thus serve as excellent in situ indicators of the overall 3-D structure. Peptide structures in the gas-phase have been widely studied using IR multiple photon dissociation (IRMPD), − IR-UV hole burning − and IR predissociation (IRPD) of messenger-tagged ions. − IRPD can be further enhanced via conformer-specific IR–IR double-resonance approaches, − which provide the vibrational signatures of individual isomers or conformers contributing to the overall spectrum. This is particularly useful in the case of flexible peptides which can adopt a wide range of conformations.…”

Setting up the HyDRA blind challenge for the microhydration of organic molecules

“…After publication of the results of the HyDRA blind challenge, the training and test sets can be extended to more strongly and even more weakly bound monohydrates, to charged systems (where more powerful experimental techniques are available [101][102][103] ), to dihydrates, to isotope effects, to other vibrations, or to further observables such as structures and relative energies.…”