Piezoelectric and ferroelectric properties of various amino acids and dipeptides tubular nanostructures: Molecular modeling

Piezoelectric and ferroelectric properties of  dipeptide nanotubes (PNT) based on phenylalanine (F), alanine (A) and branched-chain amino acids (BCAAs) isoleucine (I), leucine (L) are investigated. Homodipeptides, such as di-isoleucine (II), di-leucine (LL), and heterodipeptides alanine-isoleucine (AI) dipeptides, and di-phenylalanine (FF), are studied by molecular modeling using a quantum-mechanical (QM) semi-empirical PM3 method in a restricted Hartree-Fock (RHF) approximation based on the HyperChem package. After optimization of the models by the Polak-Ribiere conjugate gradient method, the total dipole moment and polarization of PNTs are calculated, with orientation along c-axis of a PNT with L-chiral isomer and alpha-helix conformation. The values obtained are: Pz (LL) ∼ 3.6 mC/cm2, Pz (II) ∼ 6 mC/cm2, Pz(AI) ∼ 8.02 mC/cm2 Pz(FF) ∼ 2.3 mC/cm2, which are comparable with the known data on the phenylalanine (FF) PNT polarization (for L-chiral in beta-conformation), which are of the order of Pz ∼ 4 mC/cm2. These polarizations allow us to calculate the piezoelectric coefficients d33 along the c-axis (in accordance with known electromechanical coupling relationship): 1) d33(LL) ∼ 8 pm/V, d33(II) ∼ 10 pm/V, d33(AI) ∼ 26 pm/V, d33(FF) ∼ 35 pm/V (for e = 4); 2) d33(LL) ∼ 12 pm/V, d33(II) ∼ 15 pm/V, d33(AI) ∼ 39 pm/V, d33(FF) ∼  52 pm/V (for e = 6). These results are comparable with earlier data for FF PNT d33(FF) ∼ 50 pm/V (in beta-sheet conformation, L-chirality). The data obtained are confirmed by corresponding experimental AFM/PFM observations and measurements.