In vitro assessment of the toxicity of lead (Pb²⁺) to phycocyanin

This work reports the influence of lead (Pb²⁺) on fluorescence characteristics and protein structure of phycocyanin molecules experimentally in vitro. The fluorescence intensity decreases with the increasing concentration of Pb²⁺ from 0 to 5 × 10^-5 mol L^-1, showing the fluorescence quenching of phycocyanin by Pb²⁺. The quenching process is suggested to be static regarding the calculation results and the experimental results of time-resolved fluorescence decay profiles. The synchronous fluorescence spectra show that the effect of Pb²⁺ on the Tyr residues of phycocyanin is more significant than the Trp residues. The forming of aggregation by the interaction of Pb²⁺ with phycocyanin molecules is suggested from the results of resonance light scattering spectra. The UV-Vis spectra of the protein skeleton of phycocyanin have a red-shift of about 10 nm with increasing the Pb²⁺ concentration from 0 to 5 × 10^-5 mol L^-1, indicating a change in the protein skeleton and its secondary structure. With the increasing Pb²⁺ concentration, the two negative peaks (209 nm and 218 nm) on circular dichroism spectra become smaller, showing a decrease of the α-helix structure. These results may give people a deeper understanding of that how the heavy metal (Pb²⁺) can affect the chemo-physical properties of phycocyanin.