Issue 48, 2020

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

Abstract

Hydration of biomolecules and pharmaceutical compounds has a strong impact on their structure, reactivity, and function. Herein, we explore the microhydration structure around the radical cation of the widespread pharmaceutical drug amantadine (C16H15NH2, Ama) by infrared photodissociation (IRPD) spectroscopy of mass-selected Ama+Wn = 1–3 clusters (W = H2O) recorded in the NH, CH, and OH stretch range of the cation ground electronic state. Analysis of the size-dependent frequency shifts by dispersion-corrected density functional theory calculations (B3LYP-D3/cc-pVTZ) provides detailed information about the acidity of the protons of the NH2 group of Ama+ and the structure and strength of the NH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. The preferred sequential cluster growth begins with hydration of the two acidic NH protons of the NH2 group (n = 1–2) and continues with an extension of the H-bonded hydration network by forming an OH⋯O H-bond of the third W to one ligand in the first hydration subshell (n = 3), like in the W2 dimer. For n = 2, a minor population corresponds to Ama+W2 structures with a W2 unit attached to Ama+via a NH⋯W2 H-bond. Although the N–H proton donor bonds are progressively destabilized by gradual microhydration, no proton transfer to the Wn solvent cluster is observed in the investigated size range (n ≤ 3). Besides the microhydration structure, we also obtain a first impression of the structure and IR spectrum of bare Ama+, as well as the effects of both ionization and hydration on the structure of the adamantyl cage. Comparison of Ama+ with aliphatic and aromatic primary amine radical cations reveals differences in the acidity of the NH2 group and the resulting interaction with W caused by substitution of the cycloalkyl cage.

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

Supplementary files

Article information

Article type
Paper
Submitted
08 Oct 2020
Accepted
18 Nov 2020
First published
18 Nov 2020
This article is Open Access
Creative Commons BY license

Phys. Chem. Chem. Phys., 2020,22, 28123-28139

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

M. A. R. George, F. Buttenberg, M. Förstel and O. Dopfer, Phys. Chem. Chem. Phys., 2020, 22, 28123 DOI: 10.1039/D0CP05299J

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