The role of water in solvating the hydrogen-bonding center of 2-(2′-hydroxyphenyl)benzoxazole

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Abstract

2-(2′-Hydroxyphenyl)benzoxazole (HBO) has been useful as a probe to study tautomerization in DNA. In order to clarify the role of water in solvating the hydrogen-bonding center of HBO, we examine here the effect of water on the ground- and excited-state properties of HBO by steady state absorption and fluorescence spectroscopy, and by ab initio calculations. The study was carried out in solvents of varying polarity and hydrogen-bonding capability and in binary mixtures of 1,4-dioxane/water and methanol/water. HBO exists in three equilibrium structures in its ground state which are solvent-dependent, solvated and closed syn-enols and anti-enol. Only closed syn-enol is able to undergo tautomerization in the excited state to form syn-keto tautomer. Fluorescence due to solvated HBO was observed at 370 nm. The intensity change in the fluorescence peak at 370 nm as a function of water content in the binary solvents was quantitatively analyzed. Two water molecules were found to solvate the hydrogen-bonding center of the syn-enol tautomer in the ground state. The ground-state structures of the syn-enol tautomer with one to four water molecules were calculated by performing ab initio calculations at the MP2/6-31G(d) level and it was found that only two water molecules interact with the hydrogen-bonding center of HBO, confirming the above results. As water content increased in the binary mixtures, the keto tautomer fluorescence shifted to the blue and increased in intensity. This change was attributed to the formation of an anion species in the excited state which is stabilized in high polar binary mixtures. This result was confirmed by measuring the fluorescence in aqueous solution of pH 13. At high water contents, the behavior of the binary mixtures approaches that of pure water in which HBO undergoes water-assisted tautomerization to give the syn-keto tautomer.

Original languageEnglish
Pages (from-to)1-10
Number of pages10
JournalChemical Physics
Volume337
Issue number1-3
DOIs
Publication statusPublished - Aug 16 2007

Fingerprint

Benzoxazoles
Hydrogen bonds
tautomers
Water
hydrogen
water
Binary mixtures
Ground state
binary mixtures
fluorescence
Fluorescence
Excited states
Water content
moisture content
ground state
Molecules
excitation
molecules
Fluorescence spectroscopy
Absorption spectroscopy

Keywords

  • 2-(2′-Hydroxyphenyl)benzoxazole
  • Ab initio calculations
  • Absorption
  • Binary mixtures
  • Fluorescence

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Spectroscopy
  • Atomic and Molecular Physics, and Optics

Cite this

The role of water in solvating the hydrogen-bonding center of 2-(2′-hydroxyphenyl)benzoxazole. / Abou-Zied, Osama K.

In: Chemical Physics, Vol. 337, No. 1-3, 16.08.2007, p. 1-10.

Research output: Contribution to journalArticle

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