Istituto di Biofisica     
Strambini G. B., Gonnelli M. Tryptophan Phosphorescence in Fluid Solution. In: Journal of the American Chemical Society, vol. 117 (29) pp. 7646 - 7651. American Chemical Society, 1995.
The weak delayed luminescence of some indole derivatives was monitored for the first time in aqueous solutions at room temperature by means of an apparatus capable of detecting emissions with quantum yields as low and lifetimes as short as 10 ps. In liquid solution the total luminescence spectrum is composed of normal asphosphorescence and of delayed fluorescence that originates from triplet-triplet anhilation. In water at 20 "C, the lifetime (z) of the triplet state of indole, tryptophan, and N-acetyltryptophanamide was found to be 1.2 ms while for 1-methyltryptophan it was 0.6 ms. Compared with published data, obtained by triplet-triplet absorption measurements,these values of z are almost two orders of magnitude larger. Discrepancies with absorption data were also found for the second-order rate constant that describes the quenching by molecules in the ground state and by H30f. The dependence of z in propylene glycoYwater mixtures was determined between lo9 and viscosity (7) P. At high viscosity (7 > 3 x lo3 P) the log l/z vs log T/q plot had a slope of 0.29 while at lower viscosity the slope increases to 0.7 and in either case the slope is smaller than 1, the value expected for diffusion-controlled quenching reactions. By comparison phosphorescence quenching experiments conducted in the same viscosity range with 0 2 , Cu2+, and NO3- yielded a log-log plot with a gradient of 1. The difference in slope together with a distinct lifetime for 1-methyltryptophansuggest that z obtained by phosphorescence is little affected by diffusional quenching and therefore represents the truly intrinsic lifetime.
DOI: 10.1021/ja00134a008
Subject Tryptophan
Room temperature phosphorescence
Viscosity dependence

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