Oxygen fluorescence quenching studies with single tryptophan-containing proteins

The work of Lakowicz and Weber [Biochemistry 12, 4161 (1973)] demonstrated that molecular oxygen is a powerful quencher of tryptophan fluorescence in proteins. Here we report studies of the oxygen quenching of several proteins that have a single, internal tryptophan residue. Among these are apoazurin (Pseudomonas aeruginosa), asparaginase (Escherichia coli), ribonuclease T1 (Aspergillus oryzae), and cod parvalbumin. Both fluorescence intensity and phase lifetime quenching data are reported. By comparison of these data we find that there is a significant degree of apparent static quenching in these proteins. The dynamic quenching rate constants,k q, that we find are low compared to those for tryptophan residues in other proteins. For example, for apoazurin we find an apparentk q of 0.59×10(9) M (-1) s(-1) at 25°C. This value is the lowest that has been reported for the oxygen quenching of tryptophan fluorescence.