Stepwise Microhydration of Aromatic Amide Cations: Formation of Water Solvation Network Revealed by Infrared Spectra of Formanilide⁺–(H₂O)_n Clusters (n ≤ 5)

Abstract

Hydration of peptides and proteins has a strong impact on their structure and function. Infrared photodissociation spectra (IRPD) of size-selected clusters of the formanilide cation, FA(+)-(H2O)n (n = 1-5), are analyzed by density functional theory calculations at the ωB97X-D/aug-cc-pVTZ level to determine the sequential microhydration of this prototypical aromatic amide cation. IRPD spectra are recorded in the hydride stretch and fingerprint ranges to probe the preferred interaction motifs and the cluster growth. IRPD spectra of cold Ar-tagged clusters, FA(+)-(H2O)n-Ar, reveal the important effects of temperature and entropy on the observed hydration motifs. At low temperature, the energetically most stable isomers are prominent, while at higher temperature less stable but more flexible isomers become increasingly populated because of entropy. In the most stable structures, the H2O ligands form a hydrogen-bonded solvent network attached to the acidic NH proton of the amide, which is stabilized by large cooperative effects arising from the excess positive charge. In larger clusters, hydration bridges the gap between the NH and CO groups (n ≥ 4) solvating the amide group rather than the more positively charged phenyl ring. Comparison with neutral FA-(H2O)n clusters reveals the strong impact of ionization on the acidity of the NH proton, the strength and topology of the interaction potential, and the structure of the hydration shell.

Competing Insertion and External Binding Motifs in Hydrated Neurotransmitters: Infrared Spectra of Protonated Phenylethylamine Monohydrate

“…With increasing temperature, the population of isomer (1) decreases and those of (2,3) increase. Such ad rastic effect of cooling by Rg tagging on the relative energy ordering and isomer population hasb een noticed previously for hydrated ChemPhysChem 2016, 17,232 -243 www.chemphyschem.org and solvated aromatic cluster ions, [53,60] and protonatedw ater and methanolc lusters. [61][62][63] It further demonstrates the importance of cooling the clusters for the evaluation of the most stable isomers on the potentiale nergys urface, which may either be achieved by Rg tagging of the ions (to enable singlephoton IR photodissociation of cold clusters) or by cooling the bare ions in ac ryogenic ion trap (whicht hen requires IR multiple-photondissociation for the strongly bound cluster ions).…”

“…[9,44,53,60,[69][70][71][72][73][74][75][76][77][78][79][80][81] It rests on aq uadrupole-octopole-quadrupole tandem mass spectrometer combined with an electron ionization source. PEA was introduced into the gas phase by heating the liquid sample purchased from Sigma Aldrich (> 99.5 %) to 330 K and seeding its vapor into Ne (12 bar) or Ar (5 bar) carrier gas.…”

“…[9] The n(NHp)b and is observed at 3122 cm À1 for H + PEANe 2 and extrapolated to 3115 AE 3cm À1 for H + PEA(G). [9] In the H + PEA-H 2 Os pectrum, three n(NH f )/n(NHp)m axima are measured at 3339 (F), 3231 (Y)a nd 3179cm À1 (H)i nt he corre- [53] single-photonI Ra bsorption in the investigated spectral range does not provide enough energy for dissociation of the cold hydrates. Hence, only hot clusters with an internal energy in excesso fa pproximately 2500 cm À1 are observedi n the IRPD spectrum of bare H + PEA-H 2 O, which causes substantial spectralb roadening and shifts of the observed transitions.…”

“…Indeed, strong Fermic oupling between NÀHs tretchf undamentals n(NH) and NÀHb end overtones 2b(NH) are an obvious possibility. [53] In summary,b oth scenarios have their strong and weak points, and it is currently impossible to exclude one or the other.U nfortunately,t he calculated anharmonic IR spectra could also not clarify this issue. [59] Scenario Ii mplies al arge deviationb etween experimental and theoretical n(NH O )v alues, whereas scenario II requires the assumption of strong Fermi resonances.…”

Diastereo-specific conformational properties of neutral, protonated and radical cation forms of (1R,2S)-cis- and (1R,2R)-trans-amino-indanol by gas phase spectroscopy

“…Recent applications of this IRPD approach of tagged ions in the Berlin laboratory include hydrocarbon ions, [48][49][50][51][52][53][54][55][56][57] silicon-containing ions, [58][59][60] and biological molecules and their hydrates. [61][62][63][64][65][66] 2.3.2. Infrared multiple photon dissociation (IRMPD).…”