Hydrophobic clustering in acid-denatured IL-2 and fluorescence of a Trp NH···π H-bond

Vikas Nanda, Shu Mei Liang, Ludwig Brand

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The single tryptophan at position 121 of human interleukin-2 (IL-2) can form an NH···π hydrogen bond with Phe 117 involving the indole nitrogen and the benzene aromatic ring. At pH 5.5, this type of aromatic interaction results in a fluorescence quantum yield three-fold lower than that of a fully solvent exposed tryptophan. At pH 2.1, IL-2 forms a compact denatured state with twice the emission intensity of the native protein. Global analysis of time-resolved fluorescence emission at multiple emission wavelengths shows that native and acid-denatured IL-2 can be described by four decay components. The fractional amplitudes of the shortest sub-nanosecond lifetimes are higher in the native state, suggesting rapid quenching due to the NH···π hydrogen bond. In the denatured state, longer lifetimes have greater fractional amplitudes, indicating a smaller population of hydrogen-bonded species. Electrostaticdipolar relaxation of the tryptophan microenvironment upon excitation is greater in the native-state of IL-2 than the acid-denatured state. This suggests that aciddenaturation sequesters Trp 121 from polar residues, while maintaining an interaction with Phe 117. This is consistent with the model of secondary structure preservation and hydrophobic clustering in molten-globule intermediates.

Original languageEnglish (US)
Pages (from-to)770-778
Number of pages9
JournalBiochemical and Biophysical Research Communications
Issue number3
Publication statusPublished - Dec 29 2000
Externally publishedYes


All Science Journal Classification (ASJC) codes

  • Biophysics
  • Biochemistry
  • Molecular Biology
  • Cell Biology


  • Dielectric relaxation
  • Heterogeneity
  • Molten globule
  • Time-resolved fluorescence
  • Tryptophan

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