Electrochemical study of the interaction between Eu3+ and ciliate Euplotes octocarinatus centrin

A new species was formed when protein P23 (one segment of ciliate Euplotes octocarinatus centrin) was added to a solution of Eu3+. The interaction between P23 and Eu3+ was investigated by cyclic voltammetry, pulse voltammetry and electrochemical impedance spectroscopy in 10 mM N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid (HEPES) buffer (pH 7.4) using a pyrolytic graphite electrode. The formal potential (Eo′) of Eu3+ shifted from −0.61 to −0.84 V (versus saturated calomel electrode) after P23 was added to the Eu3+ solution. The diffusion coefficient (D), the charge-transfer coefficient (α) and the electron transfer standard rate constant (ks) were obtained in the absence and the presence of P23. The affinity constant of Eu3+ and P23 was determined to be (1.89 ± 0.51) × 104 M−1. The electrochemical investigation of europium bound to the protein provided useful data for the studies of calcium-binding proteins.