Metals Can Change the Colors of Eggshells but How Is This Related to Oxidative Stress and Antibacterial Capacity?

Two main substances are responsible for the pigmentation of the eggshells of birds and reptiles: the bluish-green tone comes from biliverdin (BV), and protoporphyrin IX (PP) gives brown hues. BV and PP can form complexes with metal cations. The main objective of this investigation is to carry out a theoretical study that analyzes the interaction of metal cations (Cu²⁺, Ag²⁺, Au²⁺, Cd²⁺, Zn²⁺, and Hg²⁺) with BV and PP. The divalent metal ions of Cu and Ag are selected to have compounds with the same charge. Density functional theory (DFT) calculations were used to investigate the antiradical capacity of these systems and to obtain ultraviolet-visible (UV-vis) spectra to analyze color modifications. Antiradical capacity is one of the mechanisms that prevents oxidative stress. The antibacterial capacity was investigated through the formation of triplet states. From our results, we can conclude that metal cations interacting with BV and PP affect the electron donor-acceptor properties of the systems, modify coloration of the eggshell, and increase the photoactivating capacity of pigments, which is related to their antimicrobial action. Electron transfer is an important mechanism of antioxidant defense. These results provide useful information on both the influence of contaminants such as heavy metals on the antimicrobial capacity of natural pigments and the signaling value of eggshell coloration.