Effect of pH on fluorescence of tyrosine, tryptophan and related compounds

Abstract

The influence of pH on the fluorescence of tyrosine, tryptophan and related compounds was investigated. The pH of acidic half-quenching of fluorescence (pHf) of tyrosine and tryptophan and their derivatives with an intact carboxyl group corresponds approximately to the pK of the carboxyl and falls to a much lower value in those derivatives where the carboxyl group is substituted or replaced. Tyramine, tyrosine and phenol are converted into non-fluorescent species on dissociation of the phenolic hydroxyl group, the quenching of the fluorescence appearing as a sensitive test of this dissociation as shown by the effect of substituents, hydrogen-bonding and chemical substitution of the hydrogen. The existence of a peak in the alkaline pH range is described for tyrosine and tryptophan derivatives. A free amino group is apparently a necessary but not a sufficient condition for this maximum to be present. The relative quantum yield in neutral aqueous solution and at a chosen wavelength was measured for the compounds mentioned. The fluorescence-excitation spectra of the majority of the derivatives were determined in neutral and alkaline solutions and found to correspond with the fractional absorption spectra at neutrality.