The effect of the antioxidant spermine on the photophysical processes and photodegradation of tryptophan in aqueous solution at 293 K

Abstract

The influence of the antioxidant spermine on the photophysical processes and photodegradation of tryptophan at neutral pH was studied by fluorescence and absorption spectroscopy. Spermine, which is a polyamine, binds cooperatively to tryptophan with a binding constant of 1.2 × 10 −3 mM −3.7 at 20 °C. It is suggested that, when the charged spermine molecules bind to tryptophan, the rotation of the zwitterion around the C αC β bond is blocked and only one rotamer form of tryptophan is present. The hindered rotation gives rise to the following spectroscopic changes. (i) Minor changes are observed in the absorption spectrum, since the two most frequent rotamers have different spectra. (ii) The quantum yield of the S 1 band increases by a factor of 2.6, as is also the case when the amino group is deprotonated. ( iii) The yield of photodegradation increases from zero to 0.08 for excitation at 280 nm, which is equal to the yield of photoionization under similar conditions.