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

Unlike the absorption spectrum, the fluorescence spectrum of proteins is not the simple sum of the contributions of the aromatic amino acids in neutral water solutions A study of the behaviour of substituted aromatic amino acids and simple peptides was undertaken in an attempt to elucidate phenomena contributing to the fluorescence of proteins. Ultraviolet excitation was used to reach the lowest excited singlet state of the compounds and interest was concentrated in energy loss by processes competing with fluor- escence, i.e. a reduction in the quantum yield of fluorescence. The changes in fluorescence yield with hydrogen-ion concentration were studied.