Fluorescence Characteristics of Kynurenine and N′-Formylkynurenine, Their Use as Reporters of the Environment of Tryptophan 62 in Hen Egg-White Lysozyme1

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Abstract
Several kynurenine derivatives including N′ -formylkynurenine were prepared in high purity by the ozonization of the corresponding indole compounds. The fluorescence characteristics of those derivatives were examined in connection with the use of their fluorophores as reporters for the local environment of tryptophan in proteins. Kynurenine is a weak emitter of fluorescence, with an emission maximum at 480 nm on excitation at 365 nm. With decreasing solvent polarity, the fluorescence intensity increases logarithmically and the emission maximum shifts to blue. A linear relation between these fluorescence characteristics and solvent polarity exists when the polarity is shown in terms of dielectric constant. N′ -Formylkynurenine is a somewhat stronger emitter of fluorescence than kynurenine. The emission maximum is 434 nm on excitation at 325 nm and it shifts to blue in solvents of low polarity. This blue shift is also linear with respect to the dielectric constant of the solvent. Other factors influencing kynurenine fluorescence and N′ -formylkynurenine fluorescence examined were neighboring groups, ionic strength, temperature, and protein denaturants. Based on the results of the present investigation, the local environment of tryptophan 62 in hen egg-white lysozyme was examined using Kyn 62-lysozyme.

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