Some Factors Affecting the Phosphorescence/Fluorescence Ratio in Tyrosine Following Excitation to Higher-Lying Excited Levels

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Abstract
With respect to its phosphoresence/fluorescence (P/F) ratio at 77°K, the behavior of tyrosine can be divided into four categories: (a) Above pH 8 there are large increases in P/F which parallel the OH⇌O− equilibrium changes. Since identical changes are observed in both (P/F)225 and (P/F)275 (i.e., excitation to either the 225- or 275-nm band), the effect of the acid—base shift must be only on the lowest excited state. (b) Also, in the range pH 5–8 (P/F)225 = (P/F)275, indicating that essentially all of the intersystem crossing still occurs in the lowest exicted state. (c) Below pH4, however, (P/F)225 increases rapidly until it becomes ∼1.3 (P/F)275: This ``acid effect'' has a pK of about 3.5 whether the acid is formic or HCl. The corresponding increase in phosphorescence yield suggests that either the acidification enhances intersystem crossing in the second excited level at the expense of radiationless transitions which are most pronounced at neutral pH or else protonation changes lead to excited molecules with changed emission probabilities—since the yields, energies, and lifetimes of phosphorescence remain constant in this region, the acidification must not perturb the lowest lying triplet of those molecules which do emit. (d) Throughout the acid and neutral region, increasing the ionic concentration increases (P/F)225 and (P/F)275 equally and causes a decrease in the phosphorescence lifetime but not energy: 0.07M NaCl is sufficient to produce one-half the maximum effect. This ``salt effect'' is apparently on the lowest excited triplet and presumably reflects perturbations depending upon changes in the frozen matrix surrounding the solute molecules. These effects cannot be accounted for on the basis that appreciable crystallization of tyrosine occurs during freezing.