TRYPTOPHAN PHOSPHORESCENCE IN FLUID SOLUTION

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 as 10(-7) and lifetimes as short as 10 mu s. In liquid solution the total luminescence spectrum is composed of normal phosphorescence and of delayed fluorescence that originates from triplet-triplet anhilation. In water at 20 degrees C, the lifetime (tau) 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 tau 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 H3O+. The viscosity (eta) dependence of tau in propylene glycol/water mixtures was determined between 10(9) and 10(-2) P. At high viscosity (eta > 3 x 10(3) P) the log 1/tau vs log T/eta 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 O-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-methyltryptophan suggest that tau obtained by phosphorescence is little affected by diffusional quenching and therefore represents the truly intrinsic lifetime.