Stepwise microhydration of aromatic amide cations: water solvation networks revealed by the infrared spectra of acetanilide⁺–(H₂O)_n clusters (n ≤ 3)

Abstract

The structure and activity of peptides and proteins strongly rely on their charge state and the interaction with their hydration environment. Here, infrared photodissociation (IRPD) spectra of size-selected microhydrated clusters of cationic acetanilide (AA, N-phenylacetamide), AA-(HO) with n ≤ 3, are analysed by dispersion-corrected density functional theory calculations at the ωB97X-D/aug-cc-pVTZ level to determine the stepwise microhydration process of this aromatic peptide model. The IRPD spectra are recorded in the informative X-H stretch (ν, ν, ν, amide A, 2800-3800 cm) and fingerprint (amide I-II, 1000-1900 cm) ranges to probe the preferred hydration motifs and the cluster growth. In the most stable AA-(HO) structures, the HO ligands solvate the acidic NH proton of the amide by forming a hydrogen-bonded solvent network, which strongly benefits from cooperative effects arising from the excess positive charge. Comparison with neutral AA-HO reveals the strong impact of ionization on the acidity of the NH proton and the topology of the interaction potential. Comparison with related hydrated formanilide clusters demonstrates the influence of methylation of the amide group (H → CH) on the shape of the intermolecular potential and the structure of the hydration shell.

Microhydration of protonated 5-hydroxyindole revealed by infrared spectroscopy

“…All 5HIH + protomers exhibit competing H-bonding sites, and most important are the acidic functional OH (2) and NH groups acting as proton donors in OHÁ Á ÁO and NHÁ Á ÁO H-bonds to W. As already observed for related heterocyclic arene cations, 23,36,41,[51][52][53] p-stacking of W is rather unfavorable, e.g., D 0 = 33.3 kJ mol À1 for s5HIH + (C3)-W(p), and thus not considered further. Only one isomer with W attached to the protonated CH 2 group could be located, namely, s5HIH + (C4)-W(CH) with D 0 = 28.6 kJ mol À1 .…”

“…In contrast, the permanent dipole moment of W promotes the formation of water networks as the W-W interaction is rather strong (B1000 cm À1 ). 42,51,52 Upon network formation, strong cooperative effects strengthen preexisting H-bonds, in particular in the presence of a positive charge. Indeed, the formation of W 2 and W 3 chains is indicated by the characteristic triplet of n 3 , n f(W) OH , and n 1 in the IRPD spectra of 5HIH + -W 2/3 .…”

Microhydration of substituted diamondoid radical cations of biological relevance: infrared spectra of amantadine+-(H2O)n = 1–3clusters

“…A similar complex pattern has previously been observed for a variety of monohydrated cationic dimers with a NH…O ionic H-bond such as formanilide, acetanilide, hydroxyindole, or pyrrole. 54,57,[60][61][62] As the NH b band is expected to carry the highest IR oscillator strength, it is assigned to the broad band H at 2992 cm -1 with a width of 66 cm -1 . Its large redshift of 376 cm -1 from NH f is a measure of the strength of the NH…O H-bond.…”

“…Cationic NH2 and NH groups of primary and secondary amines are known to form strong NH…O ionic H-bonds with W ligands and larger Wn clusters. 54,55,57,[69][70][71][72][73] A particular question to be addressed is thus the competition between the formation of the H-bonded solvent network (which is strongly favoured by nonadditive cooperative three-body interactions) and interior cation hydration with individual ligands binding by simple charge-dipole forces (suffering from noncooperative three-body forces). 44 A further issue to be addressed is the question of proton transfer to solvent.…”

Unraveling the protonation site of oxazole and solvation with hydrophobic ligands by infrared photodissociation spectroscopy

“…Indeed, as predicted by the calculations, N-protonation increases the n CH frequencies. A possible assignment of the bands D1 and D2 to the NH bend overtone, which may gain intensity by anharmonic interaction with the intense n NH fundamental, 73,74 can safely be excluded because of the low frequency predicted for the NH bend fundamental (1427 cm À1 for fundamental and 2844 cm À1 for first overtone from anharmonic calculations). For completeness, we also consider an assignment of bands A to combination modes n b NH + n s of the H + Ox-L(H) isomers involving the intermolecular stretch vibration (n s ).…”