The characterization of the fluorescence of l-histidine in simulated body fluid

In addition to being an essential natural amino acid, l-histidine is biologically important in the dismutation of superoxide radical (O2−) by superoxide dismutase (SOD). In this work, fluorescence and absorptiometric techniques were used to characterize the photo-phenomenon and optical properties of this compound in a simulated body fluid (SBF). l-Histidine fluoresces at 360 nm when excited at 220 nm. Its molar absorptivity, ɛ, is 4.8 × 103 M−1 cm−1. The observed bimolecular quenching rate constant, kq, of 7.5 × 108 M−1 s−1, by hydrogen peroxide, suggests a non-diffusional activation-controlled mechanism with a rate constant, ka, of 8.55 × 108 M−1 s−1 and an electron transfer rate constant, kET of 6.06 × 108 s−1. The determined radiative and non-radiative rate constants, 4.73 × 107 and 2.9 × 108 s−1, respectively, suggests that the deactivation of the thermally excited l-histidine is by non-radiative route rather than by normal fluorescence, which accounts for the low quenching constant, KSV, of 2.22 M−1 that was obtained. The solvent reorganization energy, λs, and the reaction free energy change, ΔG, of 1.48 and −5.62 eV, respectively, suggest that the electron transfer reaction in the l-histidine–H2O2 reaction is through a solvent separated mechanism.