Simultaneous Interaction of Hydrophilic and Hydrophobic Solvents with Ethylamino Neurotransmitter Radical Cations: Infrared Spectra of Tryptamine(+)-(H2O)m-(N2)n Clusters (m,n ≤ 3).
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.
Institut für Optik und Atomare Physik, Technische Universität Berlin , D-10623 Berlin, Germany.
Markus
M
Schütz
Department of Chemistry, Faculty of Sciences, and Department of Molecular Chemistry, Graduate School of Science, Kyushu University , 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
Kenji
K
Sakota
Institut für Optik und Atomare Physik, Technische Universität Berlin , D-10623 Berlin, Germany.
Raphael
R
Moritz
Institut für Optik und Atomare Physik, Technische Universität Berlin , D-10623 Berlin, Germany.
Matthias
M
Schmies
Department of Chemistry, Faculty of Sciences, and Department of Molecular Chemistry, Graduate School of Science, Kyushu University , 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
Takamasa
T
Ikeda
Department of Chemistry, Faculty of Sciences, and Department of Molecular Chemistry, Graduate School of Science, Kyushu University , 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
Hiroshi
H
Sekiya
Institut für Optik und Atomare Physik, Technische Universität Berlin , D-10623 Berlin, Germany.
Otto
O
Dopfer
D002412:Cations; D005021:Ethylamines / Q000737:chemistry*; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D018377:Neurotransmitter Agents / Q000737:chemistry*; D009584:Nitrogen / Q000737:chemistry*; D012995:Solubility; D012997:Solvents / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D014363:Tryptamines / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
D002412:Cations; D005021:Ethylamines; D018377:Neurotransmitter Agents; D012997:Solvents; D014363:Tryptamines; D014867:Water; D009584:Nitrogen; C041564:ethylamine