title: Hydrophobic clustering in acid-denatured IL-2 and fluorescence of a Trp NH-pi H-bond.
abstract: The single tryptophan at position 121 of human interleukin-2 (IL-2) can form an NH-pi 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-pi hydrogen bond. In the denatured state, longer lifetimes have greater fractional amplitudes, indicating a smaller population of hydrogen-bonded species. Electrostatic-dipolar relaxation of the tryptophan microenvironment upon excitation is greater in the native-state of IL-2 than the acid-denatured state. This suggests that acid-denaturation 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.
authors.affiliation: Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: V
authors.initials: V
authors.lastname: Nanda
authors.forename: S M
authors.initials: SM
authors.lastname: Liang
authors.forename: L
authors.initials: L
authors.lastname: Brand
mesh_terms: D000143:Acids / Q000737:chemistry; D006801:Humans; D006860:Hydrogen Bonding; D007376:Interleukin-2 / Q000737:chemistry*; D008958:Models, Molecular; D010649:Phenylalanine / Q000737:chemistry; D011489:Protein Denaturation; D013050:Spectrometry, Fluorescence; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D000143:Acids; D007376:Interleukin-2; D010649:Phenylalanine; D014364:Tryptophan