Changes in the Tryptophan Fluorescence in Gamma-irradiated Human Haemoglobin

SummaryThe 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.