Self-assembled monolayers of single-stranded (ss) peptide nucleic acids (PNAs) containing seven nucleotides (TTTXTTT), a C-terminus cysteine, and an N-terminus ferrocene redox group were formed on gold electrodes. The PNA monomer group (X) was selected to be either cytosine (C), thymine (T), adenine (A), guanine (G), or a methyl group (Bk). The charge transfer rate through the oligonucleotides was found to correlate with the oxidation potential of X. Kinetic measurements and computational studies of the ss-PNA fragments show that a nucleobase mediated charge transport mechanism in the deep tunneling superexchange regime can explain the observed dependence of the kinetics of charge transfer on the PNA sequence. Theoretical analysis suggests that the charge transport is dominantly hole-mediated and takes place through the filled bridge orbitals. The strongest contribution to conductance comes from the highest filled orbitals (HOMO, HOMO-1, and HOMO-2) of individual bases, with a rapid drop off in contributions from lower lying filled orbitals. Our studies further suggest that the linear correlation observed between the experimental charge transfer rates and the oxidation potential of base X arises from weak average interbase couplings and similar stacking geometries for the four TTTXTTT systems.
This
work demonstrates the chiral-induced spin selectivity effect for inorganic
copper oxide films and exploits it to enhance the chemical selectivity
in electrocatalytic water splitting. Chiral CuO films are electrodeposited
on a polycrystalline Au substrate, and their spin filtering effect
on electrons is demonstrated using Mott polarimetry analysis of photoelectrons.
CuO is known to act as an electrocatalyst for the oxygen evolution
reaction; however, it also generates side products such as H2O2. We show that chiral CuO is selective for O2; H2O2 generation is strongly suppressed on
chiral CuO but is present with achiral CuO. The selectivity is rationalized
in terms of the electron spin-filtering properties of the chiral CuO
and the spin constraints for the generation of triplet oxygen. These
findings represent an important step toward the development of all-inorganic
chiral materials for electron spin filtering and the creation of efficient,
spin-selective (photo)electrocatalysts for water splitting.
Charge transfer studies have been performed for self-assembled monolayers of single-stranded and double-stranded peptide nucleic acids (PNAs) having a C-terminus cysteine and an N-terminus ferrocene group as a redox reporter. The decay of the charge transfer rate with distance was strong for short single-stranded PNA molecules and weak for long single-stranded and double-stranded PNAs. Possible mechanisms for this "softening" of the distance dependence are discussed. The nature of the mechanism change can be explained by a transition of the charge transport mechanism from superexchange-mediated tunneling for short PNAs to a "hopping" mechanism for long PNAs.
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