Protamine, one of the nucleoproteins, is contained in the sperm of fishes and is a linear polypeptide with many arginine (Arg) residues. The basic polypeptide chain can behave as the proton acceptor site in the acid-base composite material. Therefore, we prepared the composite material by the mixing of the basic protein, protamine, and the acidic molecule, methanediphosphonic acid (MP). The protamine-MP composite materials formed the acid-base complex by the electrostatic interaction between the protonated guanidino group in Arg residue and the deprotonated phosphonic acid group in the MP molecule. In addition, the protamine-MP composite material showed the anhydrous proton conductivity of 9.1 × 10−4 S cm−1 at 160 °C in the flowing dry nitrogen. The proton conducting mechanism in the protamine-MP composite material depended on the proton transfer to the proton defect site in the MP molecule without the need of diffusible molecules, such as the oxonium ion.
