Diphenylalanine (FF) peptide nanotubes (PNTs) present a unique class of selfassembled functional biomaterials owing to a wide range of useful properties including nanostructural variability, mechanical rigidity, and chemical stability. In addition, a strong piezoelectric activity has been recently observed paving the way to their use as nanoscale sensors and actuators. In this work, we fabricated both horizontal and vertical FF PNTs and examined their optical second harmonic generation and local piezoresponse as a function of temperature. The measurements show gradual decrease of polarization with increasing temperature accompanied by irreversible phase transition into another crystalline phase at about 140-150 ºC. The results are corroborated by the molecular dynamic simulations predicting order-disorder phase transition into centrosymmetric (possibly, orthorhombic) phase with antiparallel polarization orientation in neighboring FF rings. Partial piezoresponse hysteresis indicates incomplete polarization switching due to high coercive field in FF PNTs.
The
difference in the crystal structure and growth kinetics of
microtubes formed from l- and d-enantiomers of diphenylalanine
dipeptide is investigated both experimentally and theoretically by
computer simulation. The microtubes of l- and d-enantiomers
grown simultaneously and under identical experimental conditions possess
different crystallographic space groups, have essential difference
in sizes, and demonstrate different growth kinetics. Computer simulation
by molecular mechanics methods revealed a fundamental difference in
the interaction between structural units of microtubes of different
chiralities. A model describing chirality-dependent growth of microtubes
is proposed.
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