Piezoelectric and ferroelectric properties of various amino acids and tubular dipeptide nanostructures: Molecular modelling

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 diphenylalanine (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 alphahelix conformation. The values obtained are: Pz (LL) ~ 3.6 C/cm2, Pz (II) ~ 6 C/cm2, Pz(AI) ~ 8.02 C/cm2 Pz(FF) ~2.3 C/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 C/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  = 4); 2) d33(LL) ~ 12 pm/V, d33(II) ~ 15 pm/V, d33(AI) ~ 39 pm/V, d33(FF) ~ 52 pm/V (for  = 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.