Tyrosyl fluorescence in hemocyanin from the scorpion Leirus quinquestriatus

The fluorescence properties of hemocyanin from the scorpion Leirus quinquestriatus were studied. Emission and excitation spectra were determined for the protein in both its oxygenated and deoxygenated forms. Oxygenation was found to bring about a large blue shift in the position of the fluorescence maximum, in addition to a marked quenching of the fluorescence intensity as noted before for another hemocyanin. An appreciable tyrosyl contribution to the fluorescence of oxyhemocyanin. was inferred from the wavelength dependence of its emission and excitation spectra. No tyrosyl fluorescence could be observed in either apo- or deoxyhemocyanin. It was concluded that the inability to observe tyrosyl emission in deoxyhemocyanin is due to the dominating emission of tryptophan. The implication of the findings to the often noted failure to detect tyrosyl emission in proteins containing both tyrosine and tryptophan is discussed.