TIME-RESOLVED PHOTOACOUSTIC CALORIMETRY OF TRYPTOPHAN IN WATER AND ORGANIC-SOLVENTS

Very little has been done in the direct study of nonradiative decays o f Trp, indole and derivatives, in spite of their relatively low fluore scence quantum yields. Time-resolved photoacoustic calorimetry is an i deal technique for this kind of study: it is very sensitive in the det ection of small amounts of heat released and, in principle, allows a b road band of temporal resolution. The photoacoustic apparatus used for our measurements offers a temporal window between 10 ns and 10 mu s. The analysis of the waveforms, based on a particular deconvolution met hod, simultaneously gives the fraction of energy released and the asso ciated lifetime. In a broad sense, time-resolved photoacoustic spectro scopy can be seen as a complementary method to traditional radiative t echniques such as static, time-resolved fluorescence and hash photolys is. The present work presents studies of Trp in different solvents in order to acquire new information about the interaction of this chromop hore with solvents. From our measurements the high sensitivity of Trp to solvents is confirmed. The formation of complexes is evident at the excited state (exciplexes), between the chromophore and one or more m olecules of solvent. These exciplexes are characterized by having an e nergy different from that of the singlet and triplet of Trp. Moreover, photoacoustic measurements detect, in water, the presence of another electronic state, which seems to have characteristics similar to a tri plet-like state reported in other work.