Simultaneous Interaction of Hydrophilic and Hydrophobic Solvents with Ethylamino Neurotransmitter Radical Cations: Infrared Spectra of Tryptamine⁺-(H₂O)_m-(N₂)_n Clusters (m,n ≤ 3)

Abstract

Solvation of biomolecules by a hydrophilic and hydrophobic environment strongly affects their structure and function. Here, the structural, vibrational, and energetic properties of size-selected clusters of the microhydrated tryptamine cation with N2 ligands, TRA(+)-(H2O)m-(N2)n (m,n ≤ 3), are characterized by infrared photodissociation spectroscopy in the 2800-3800 cm(-1) range and dispersion-corrected density functional theory calculations at the ωB97X-D/cc-pVTZ level to investigate the simultaneous solvation of this prototypical neurotransmitter by dipolar water and quadrupolar N2 ligands. In the global minimum structure of TRA(+)-H2O generated by electron ionization, H2O is strongly hydrogen-bonded (H-bonded) as proton acceptor to the acidic indolic NH group. In the TRA(+)-H2O-(N2)n clusters, the weakly bonded N2 ligands do not affect the H-bonding motif of TRA(+)-H2O and are preferentially H-bonded to the OH groups of the H2O ligand, whereas stacking to the aromatic π electron system of the pyrrole ring of TRA(+) is less favorable. The natural bond orbital analysis reveals that the H-bond between the N2 ligand and the OH group of H2O cooperatively strengthens the adjacent H-bond between the indolic NH group of TRA(+) and H2O, while π stacking is slightly noncooperative. In the larger TRA(+)-(H2O)m clusters, the H2O ligands form a H-bonded solvent network attached to the indolic NH proton, again stabilized by strong cooperative effects arising from the nearby positive charge. Comparison with the corresponding neutral TRA-(H2O)m clusters illustrates the strong impact of the excess positive charge on the structure of the microhydration network.

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.…”

Infrared spectrum of the cold ortho-fluorinated protonated neurotransmitter 2-phenylethylamine: competition between NH+⋯π and NH+⋯F interactions

“…4c and d characterize xF-H + PEA-(H 2 O) n clusters using the same experimental and computational approach in order to probe the effects of site-specific aromatic fluorination on the structure of the microhydration network around this prototypical protonated neurotransmitter under more realistic physiological conditions. 16,[41][42][43][44][45][46][47][48]…”

Gas-phase conformational preference of the smallest saccharide (glycolaldehyde) and its hydrated complexes with bridged hydrogen bonding

“…The uB97X-D functional contains the empirical treatment of dispersion interactions and gives reliable non-covalent interactions for many systems, i.e., dimethyl ether dimer, tryptamine + -(N 2 ) n and their hydrated complexes with satisfactory vibrational frequencies. [43][44][45][46][47] Thus, the dispersion-corrected density functional theory calculations were performed at the unrestricted uB97X-D/cc-pVTZ level to obtain vibrational energy of the GA and its hydrated complexes. The harmonic frequencies are scaled by a factor of 0.9415.…”

“…The harmonic frequencies are scaled by a factor of 0.9415. 43,44 For the hydrated complexes, natural bond orbital (NBO) analysis was also employed with the uB97X-D functional and cc-pVTZ basis set. 48 All relative energies were zero-point energy (ZPE)-corrected in the Gaussian program.…”