Tautomerism in 7-hydroxyquinoline: A combined experimental and theoretical study in water

Najla Al-Lawatia, John Husband, Thomas Steinbrecher, Osama K. Abou-Zied

Research output: Contribution to journalArticle

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Abstract

Tautomerism in the ground and excited states of 7-hydroxyquinoline (7HQ) was studied in different solvents using steady-state and lifetime spectroscopic measurements, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. Equilibrium between the enol and the keto/zwitterion tautomers exists in 7HQ, which is solvent-dependent. Of the solvents used in this study, only in water does the absorbance spectrum of 7HQ show absorption from both the enol and zwitterion tautomers. In addition, in aqueous media, fluorescence is observed from the zwitterion tautomer only, which is attributed to self-quenching of the enol fluorescence by energy transfer to the ground-state zwitterion tautomer and energetically favorable excited-state proton transfer. Solvation of the hydrogen bonding sites of 7HQ was studied in binary mixtures of 1,4-dioxane and water, and three water molecules were estimated to connect the polar sites and induce intermolecular proton transfer. The results are confirmed by DFT calculations showing that three water molecules are the minimum number required to form a stable solvent wire. Mapping the water density around the polar sites using MD simulations shows well-defined hydrogen bonds around the amino and hydroxyl groups of the enol tautomer and slightly less well-defined hydrogen bonds for the zwitterion tautomer. The presence of three-member water wires connecting the polar centers in 7HQ is evident in the MD simulations. The results point to the unique spectral signatures of 7HQ in water, which make this molecule a potential probe to detect the presence of water in nanocavities of macromolecules.

Original languageEnglish
Pages (from-to)4195-4201
Number of pages7
JournalJournal of Physical Chemistry A
Volume115
Issue number17
DOIs
Publication statusPublished - May 5 2011

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tautomers
zwitterions
Water
water
Molecular dynamics
Hydrogen bonds
Proton transfer
molecular dynamics
Excited states
Ground state
Molecules
Density functional theory
Computer simulation
Fluorescence
wire
Wire
hydrogen bonds
density functional theory
molecules
fluorescence

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Tautomerism in 7-hydroxyquinoline : A combined experimental and theoretical study in water. / Al-Lawatia, Najla; Husband, John; Steinbrecher, Thomas; Abou-Zied, Osama K.

In: Journal of Physical Chemistry A, Vol. 115, No. 17, 05.05.2011, p. 4195-4201.

Research output: Contribution to journalArticle

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