Hydrophobic clustering in acid-denatured IL-2 and fluorescence of a Trp NH...pi h-bond

The single tryptophan at position 121 of human interleukin-2 (IL-2) can for m an NH...pi hydrogen bond with Phe 117 involving the indole nitrogen and t he benzene aromatic ring. At pH 5.5, this type of aromatic interaction resu lts in a fluorescence quantum yield threefold lower than that of a fully so lvent exposed tryptophan. At pH 2.1, IL-2 forms a compact denatured state w ith 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 componen ts. The fractional amplitudes of the shortest sub-nanosecond lifetimes are higher in the native state, suggesting rapid quenching due to the NH...pi h ydrogen bond. in the denatured state, longer lifetimes have greater fractio nal amplitudes, indicating a smaller population of hydrogen-bonded species. Electrostatic-dipolar relaxation of the tryptophan microenvironment upon e xcitation is greater in the native-state of IL-2 than the acid-denatured st ate. This suggests that acid-denaturation sequesters Trp 121 from polar res idues, while maintaining an interaction with Phe 117. This is consistent wi th the model of secondary structure preservation and hydrophobic clustering in molten-globule intermediates. (C) 2000 Academic Press.