Changes in the tryptophan fluorescence in gamma-irradiated human haemoglobin

The effect of gamma-radiation on human haemoglobin was studied by means of fluorescence spectroscopy. Irradiation with doses in the range of 0-2.4 Mrad led to an increase in fluorescence intensity and a red shift of the fluorescence maximum. Such spectral changes should be attributed mainly to a more polar environment of tryptophan residues in irradiation haemoglobin caused by protein unfolding and to a concomitant increase in separation between tryptophan residues and haem groups. Differences in fluorescence intensity but not spectral distribution between control and irradiated samples denatured in guanidinium hydrochloride indicate a radiation-induced decrease in tryptophan content. Fluorescence quenching by caesium ions was observed in irradiated haemoglobin but not in native haemoglobin. The Stern-Volmer constant calculated for caesium quenching indicates that the fraction of tryptophan residues accessible to the quencher increases after irradiation.