The toxicity of cadmium ion (Cd²⁺) to phycocyanin: an in vitro spectroscopic study

The pollution of heavy metals is a severer problem for the ecosystems in waters. The toxicity of Cd²⁺ on phycocyanin (PC) is studied in molecular level in this work. The fluorescence quenching of PC is observed by the adding Cd²⁺ from 0 to 500 × 10⁻⁷ mol L⁻¹. From the theoretical calculation and the time-resolved fluorescence decay profiles, the fluorescence quenching of PC by Cd²⁺ is found to be static. The synchronous fluorescence spectra are used to study the change in amino acid residues of PC molecules, indicating that the effect of Cd²⁺ on the Trp of PC is more significant than the Tyr. The UV-Vis absorbance of tetrapyrrole decreases from 0.26 to 0.23 cps with increasing Cd²⁺ concentration, suggesting that Cd²⁺ affects the light adsorption and the photosynthesis function of PC. The circular dichroism spectra reveal that adding Cd²⁺ also changes the secondary structure (α-helix) of PC.