Tryptophan photooxidation promoted by new hybrid materials prepared by condensation of naphthalene imides with silicate by the sol–gel process

Three novel hybrid organic/inorganic materials were synthesized from 4-substituted (NO₂, Br, H) 1,8-naphthalene imide-N-propyltriethoxysilane by the sol–gel process. These materials were obtained as a xerogel and partially characterized. The ability to photosensitize the oxidation and degradation of tryptophan indole ring by these materials was studied through photophysical and photochemical techniques. Although the derivatives containing Br and NO₂ as substituent do not cause efficient tryptophan photodamage, the hybrid material obtained from 1,8-naphthalic anhydride is very efficient to promote tryptophan photooxidation. By using laser flash photolysis it was possible to verify the presence of naphthalene imide transient radical species. The presence of oxygen causes an increase of the yield of radical formation. These results suggest that the mechanism of photodegradation of tryptophan occurs by type I, i.e. the transient radical (TrpH⁺) formed by the direct reaction of the triplet state of the naphthalene imide moiety with tryptophan. Thus a inorganic–organic hybrid material that can be used to promote the oxidation of biomolecules was obtained.