Evaluation and analysis of the toxicity of mercury (Hg²⁺) to allophycocyanin from Spirulina platensis in vitro

As a global environmental pollution problem, heavy metal pollution has brought great harm to human beings. In this work, we studied the toxicity of Hg²⁺ on allophycocyanin (APC) at the molecular level. Firstly, APC was extracted and purified from Spirulina platensis mud and its purity (A₆₅₀/A₂₈₀) reached 3.75. In addition, the fluorescence intensity of APC decreased with increasing Hg²⁺ concentration from 0 to 5 × 10^-6 mol L⁻¹. The theoretical calculation and experimental results showed that the fluorescence quenching of APC by Hg²⁺ was static and had a good linear relationship. Moreover, the UV-Vis spectra of APC showed a significant decrease at 200 nm and 650 nm with the increase of Hg²⁺ concentration from 0 to 5×10⁻⁶ mol L⁻¹, and a red-shift at 200 nm, which indicated that Hg²⁺ not only affected the structure of APC but also affected the light absorption and photosynthetic function of APC. Furthermore, the results of molecular simulation demonstrate that Hg²⁺ combinations with the Met77, Cys81 in the α chain and the Arg77, Cys81 in the β chain, which interact between the peptide chain and the chromophore, and Hg²⁺ forms a Hg-S bond with -SH. This study provides new insights into the structure and how Hg²⁺ effect the optical properties of APC.