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Effect of pH on the Phosphorescence of Tryptophan, Tyrosine, and Proteins

From the ¹ From the Department of Chemistry, Columbia University, New York, New York 10027

Whereas the fluorescence of proteins has been extensively investigated, in particular by Weber and co-workers, the phosphorescence has been little investigated. This paper reports studies of the phosphorescence of a representative group of proteins and aromatic amino acids.

The previously discovered quenching of the fluorescence of tyrosine and tryptophan in acid and alkaline pH is accompanied by an enhancement of the phosphorescence. The quenching of tyrosine fluorescence in proteins at neutral pH and by formate ion at neutral pH is accompanied by a quenching of the phosphorescence.

Interpretation of these phenomena depends on a series of assumptions concerning the excited states of tyrosine and tyrosinate ion, which are shown in Figs. 6 and 7.

Basically, tyrosine and tyrosinate ion are assumed to have , * states at lower energies than n, * states. In tyrosinate ion the ³n, * state is assumed to lie below the first excited singlet , * state. This facilitates singlet to triplet conversion (intersystem crossing) and accounts for the quenching of the fluorescence and the enhancement of the phosphorescence of tyrosinate ion as compared to tyrosine. An even more speculative assumption concerning the ³n, * state is made to explain the fact that in tyrosine hydrogen-bonded to bases both the fluorescence and phosphorescence are quenched.

In the heme proteins myoglobin and hemoglobin, neither fluorescence nor phosphorescence is seen at neutral or alkaline pH. When the heme is separated, the residual protein exhibits normal emission. In neutral solution the emission is quenched by energy transfer from the excited singlet states of tyrosine and tryptophan to the excited states of the heme. In alkaline solution, on the other hand, reasons are given for believing that the transfer is either from the excited triplet or from singlet states of the acids to the excited singlets of the heme.

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