ELECTRONIC ABSORPTION AND FLUORESCENCE SPECTROSCOPIC STUDIES OF DIPYRIDAMOLE - EFFECTS OF SOLUTION COMPOSITION

Electronic absorption and fluorescence spectra of dipyridamole, a coronary vasodilator drug, were monitored in aqueous solutions as a function of pH in the range between 13 and 2. The spectra were decomposed into Gaussian components and the parameters were obtained and compared both in aqueous solution at different pHs and in several other solvents. The extinction coefficients were also obtained as well as the quantum yields and fluorescence lifetimes. At pH 7.0 epsilon-415 = (5.4 +/-0.2) x 10(3), phi(fluor) = 0.95 +/- 0.05 and tau(fluor) = 17.8 +/-0.4 ns, while at pH 5.0 epsilon-415 = (4.8 +/- 0.2) x 10(3), phi(fluor) = 0.16 +/- 0.01 and tau(fluor) = 5.7 +/- 0.2 ns. The decrease of pH is accompanied by sharp fluorescence changes in two different regions: in the range of pH 13-12 an increase of fluorescence is observed (pK1 is-approximately-equal-to 12.5) while in the range of pH 6.5-5.0 a dramatic decrease of fluorescence (pK2 is-approximately-equal-to 5.7) is apparent. Changes in the position and shape of the absorption bands are also observed in this pH range (around pK2). A Forster cycle calculation using absorption and fluorescence data shows that the pKa of the excited state is around 1 unit below pK2. Similar changes are also observed in ethanolic solution of DIP with addition of acid. They could be explained as probably due to the protonation of two types of nitrogen atoms which are part of the structure of the DIP molecule. Changing the ionic strength of the solution in the range 0.01M gives only a slight (is-approximately-equal-to 10%) decrease of the fluorescence intensity and does not affect the absorption spectrum of DIP. This suggests that ionic association is not very effective in changing the spectral properties of DIP.