Proceedings Article | 25 October 2005
KEYWORDS: Luminescence, Hydrogen, Absorbance, Quantum efficiency, Refractive index, Sodium, Dielectrics, Ions, Molecules, Electrodes
L-Histidine is a naturally occurring essential amino acid. In addition it is biologically important in the dismutation of superoxide radical (O2-.) 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, λmax, was observed at 272 nm and a molar absorptivity, ε, of 1.50 x 103/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, kq, of 1.68 x 108/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 108/M-s and an electron transfer rate constant of 3.65 x 108/s. A quantum yield, Φf, of 0.09 and a fluorescence lifetime of 12.2 ns, respectively, were determined. The observed radiative and non-radiative rate constants, kr and knr, of 7.38 x 106/s and 7.46 x 107/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, λs, 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-H2O2 reaction in SBF medium is through a solvent separated mechanism.
