PHOTOPHYSICAL PROPERTIES AND PHOTOBIOLOGICAL ACTIVITY OF THE FURANOCHROMONES VISNAGIN AND KHELLIN

The larger photobiological activity of visnagin (VI) versus khellin (K H) toward several living organisms, including fungi, viruses, yeasts a nd bacteria, induced a detailed investigation of the photophysical pro perties of these naturally-occurring furanochromones, using laser-flas h-photolysis, photoacoustic calorimetry skid fluorescence (steady-stat e and time-resolved) techniques in solvents with different polarity an d content of water, including micelles and vesicles. The results have shown that the magnitude of all the three rate constants oat of S-1 (r adiative, k(f); internal conversion, k(ic) and intersystem crossing, k (isc) for VI and KH strongly depend on the solvent, namely on its hydr ogen bonding ability and polarity. The changes of k(f) and k(isc) are due to the solvent-assisted mixing and/or inversion of the two first s inglet excited states ((1)n,pi and (1) pi,pi*), while k(ic), increase s with a decrease of the S-0-S-1 energy gap, As a consequence, the qua ntum yield of triplet formation (phi(T)) strongly decreases horn value s of similar to 0.8 in dioxane to (0.05 in water for both compounds. T he magnitude of solvent polarity/hydrogen bonding ability required, at which the state order is inverted and phi(T) starts to decrease, is g reater for VI than for KH and consequently phi(T) (VI) >> phi(T), ((KH ) over a broad range of water content including that appropriate to th e environment of the compounds in a living system, These bets account for the larger photobiological activity of VI with respect to KH, rega rding both the fungus Fusarium culmorum L. and the wild strain of Esch erichia coli, studied by us.