L-Histidine is a naturally occurring essential amino acid. In addition it is biologically important in the dismutation of superoxide radical (O<sub>2</sub><sup>-.</sup>) by superoxide dismutase (SOD). In this work, fluorescence and absorptiometric techniques are used to characterize the photo-phenomena of this compound in a simulated body fluid (SBF). In this medium L-histidine fluoresces at 458 nm when excited at 390 nm. Its wavelength of maximum absorbance, λ<sub>max</sub>, was observed at 272 nm and a molar absorptivity, ε, of 1.50 x 10<sup>3</sup>/M-cm. This absorptiometric data suggest that L-histidine undergoes an n→π* electronic transition reaction in an SBF medium. The observed bimolecular quenching rate constant, k<sub>q</sub>, of 1.68 x 10<sup>8</sup>/M-s, by hydrogen peroxide as quencher, suggests a non-diffusional quenching but rather an activation-controlled mechanism with a rate constant, ka, of 1.68 x 10<sup>8</sup>/M-s and an electron transfer rate constant of 3.65 x 10<sup>8</sup>/s. A quantum yield, Φ<sub>f</sub>, of 0.09 and a fluorescence lifetime of 12.2 ns, respectively, were determined. The observed radiative and non-radiative rate constants, k<sub>r</sub> and k<sub>nr</sub>, of 7.38 x 10<sup>6</sup>/s and 7.46 x 10<sup>7</sup>/s, respectively, suggest that the deactivation of the thermally excited L-histidine is mainly through a non-radiative route rather than by normal fluorescence, which can account for the low quenching constant, KSV, of 2.13/M that was obtained. The solvent reorganization energy, λ<sub>s</sub>, and the reaction free energy change, ΔG, of 1.48 eV and -2.47 eV, respectively, suggest that the electron transfer reaction in the L-histidine-H<sub>2</sub>O<sub>2</sub> reaction in SBF medium is through a solvent separated mechanism.
Due to the importance of fluoride in clinical treatment of osteoporosis and its toxicity from over accumulation in bones there is an increased interest in developing selective optical methods for the detection of fluoride anion. Anion recognition and sensing are of interest because of their importance in biological environmental assays and efforts are paid for developing sensitive methods. We synthesized salicylidene furfurylamine 1 and studied spectral properties. Compound 1 fluoresced strongly and the fluorescence was strongly enhanced in the presence of anions as fluoride at low concentrations. A substantially red-shifted emission in acetonitrile was observed. The excitation at 390 nm and the emission was observed at 469nm. Fluoride showed strong absorption and fluorescence enhancement with a significant Stokes shift. Acetate, dihydrogen phosphate, showed small effect, while chloride, bromide had no significant effect.