Peptide chains might be linked with one another using various external bifunctional molecules. We postulate an existence of a novel molecule termed 'double amino acid' containing four functional groups connected to one C atom ((NH2)(2)C(COOH)(2)). Using correlated ab initio approach (QCISD and MP2 methods) we provide its structure, simulated IR spectrum and verify its stability in gas and aqueous phases. The proposed double amino acid is predicted to enable the design of a novel family of interpenetrating peptides in which it is expected to serve as a built-in amino acid residue that might be shared by two independent peptide chains. (C) 2014 Elsevier B. V. All rights reserved.