Helical Ordering of α-l-Polyalanine Molecular Layers by Interdigitation

Abstract

Highly efficient magnetic spin memory devices operating at ambient temperatures were recently realized via self-assembled molecular layers of an enantiomeric phase of polyalanine (PA). However, the repeatability and yield of such molecular devices rely sensitively on the layer quality. In this work, we grew molecular layers of α-L-PA (36 monomers of αamino acid, 5.4 nm) on highly oriented pyrolytic graphite surfaces and investigated the structure by scanning tunneling microscopy under ambient conditions at the liquid−solid interface, complemented by X-ray photoelectron and Raman spectroscopies. Our results suggest that interdigitation of adjacent molecules takes place, which leads to a packing density of 0.8 PA/nm 2 , which is more than a factor of 2 higher than that of the closed-packed structure. The interdigitation not only stabilizes the PA film by additional H bonds but also ensures chiral alignment between adjacent molecules, which may be a prerequisite for a coherent electron transmission.

Heterochiral Dimer Formation of α-l- and α-d-Polyalanine Molecules on Surfaces

“…These findings are in contrast to previous measurements with L-entantiomeric polypeptide sequences of similar length (5.4 nm, lysine-doped) on HOPG and Au(111), where only close-packed structures were found. 40,41 Phase Separation of L-and D-PA on HOPG. The detailed analysis of the hexagonal phase (hcp) reveals unit cell vectors of a 1 = a 2 = 1.2 ± 0.1 nm.…”

“…The higher resolution obtained with STM for L-PA phase shows clearly also the rotational order of these molecules, which facilitates formation of stronger intermolecular bonds. 40 Therefore, we assume a similar interdigitated and upright standing configuration also for the hcp phase of the DL-PA mixture. For steric reasons, the interdigitation process of helical molecules should be most efficient if only one enantiomeric phase is present; thus, the hcp structure seen in the upper part of Figure 2a is mostly likely an enantiopure phase.…”

“…For steric reasons, the interdigitation process of helical molecules should be most efficient if only one enantiomeric phase is present; thus, the hcp structure seen in the upper part of Figure 2a is mostly likely an enantiopure phase. 40 Moreover, the PA racemats used in this study were not containing any lysine substitutions along the helix. Therefore, the formation of the hexagonally close-packed structure depends not on the presence of lysine along the helix of the polypeptides.…”

Filtering spins by scattering from a lattice of point magnets

“…The CISS experiments can be modeled by a single layer of scatterers with a 0 ≠ 0 and a 1 ≠ 0. Here the parameter a 0 should be about the molecular diameter, i.e., 1-2 nm 76,77 , and the parameter a 1 should be small (|a 1 | ≪ |a 0 |) since the spin-orbit coupling is weak for organic molecules. Two important corollaries follow from the analysis of this CISS model.…”

An analytic model of chiral-induced spin selectivity

“…In what follows, we present the results of the spin polarization in z direction for different values of the model parameters. In the current calculation we choose l x = 0.8 nm, l y = 2.0 nm and l z = 10.0 nm [43]. For the magnitudes of the molecular dipole moments we use µ x = 2.4 D, µ y = 5.0 D, µ z = 1.8 D as representative values.…”