Photoinduced Electron Transfer on β-Sheet Cyclic Peptides

Peptides as Bio-inspired Molecular Electronic Materials

Understanding the electronic properties of single peptides is not only of fundamental importance to biology, but it is also pivotal to the realization of bio-inspired molecular electronic materials. Natural proteins have evolved to promote electron transfer in many crucial biological processes. However, their complex conformational nature inhibits a thorough investigation, so in order to study electron transfer in proteins, simple peptide models containing redox active moieties present as ideal candidates. Here we highlight the importance of secondary structure characteristic to proteins/peptides, and its relevance to electron transfer. The proposed mechanisms responsible for such transfer are discussed, as are details of the electrochemical techniques used to investigate their electronic properties. Several factors that have been shown to influence electron transfer in peptides are also considered. Finally, a comprehensive experimental and theoretical study demonstrates that the electron transfer kinetics of peptides can be successfully fine tuned through manipulation of chemical composition and backbone rigidity. The methods used to characterize the conformation of all peptides synthesized throughout the study are outlined, along with the various approaches used to further constrain the peptides into their geometric conformations. The aforementioned sheds light on the potential of peptides to one day play an important role in the fledgling field of molecular electronics.

A controllable mechanistic transition of charge transfer in helical peptides: from hopping to superexchange

A controllable mechanistic transition of charge transfer in helical peptides is demonstrated as a direct result of side-bridge gating.

Conductance of α-Helical Peptides Trapped within Molecular Junctions

Self-assembled monolayers of alpha-helical peptides on a gold surface were employed as model systems for the investigation of mediated electron transfer. The peptides contained 14, 15, 16, and 17 amino acid residues. The measurements of electron transmission through single molecules of helical peptides were performed using scanning tunneling spectroscopy (STS). The molecules were trapped between the gold tip and the substrate. Electrical contact between the molecule and the gold probe was achieved by the use of peptides containing thiol groups present at each end of the helix. The conductance behavior of the peptides was examined as a function of tip-substrate distance at fixed bias voltage. Measurements performed with peptides containing different numbers of amino acid residues indicate that the distance dependence of electron transmission through an alpha-helix is weaker than that through simple n-alkyl bridges.

Understanding Electron Transfer across Negatively-Charged Aib Oligopeptides

The physicochemical effects modulating the conformational behavior and the rate of intramolecular dissociative electron transfer in phthalimide-Aibn-peroxide peptides (n = 0-3) have been studied by an integrated density functional/continuum solvent model. We found that three different orientations of the phthalimide ring are possible, labeled Phihel, PhiC7, and PhipII. In the condensed phase, they are very close in energy when the system is neutral and short. When the peptide chain length increases and the system is negatively charged, Phihel becomes instead the most stable conformer. Our calculations confirm that the 3(10)-helix is the most stable secondary structure for the peptide bridge. However, upon charge injection in the phthalimide end of the phthalimide-Aib3-peroxide, the peptide bridge can adopt an alpha-helix conformation as well. The study of the dependence of the frontier orbitals on the length and on the conformation of the peptide bridge (in agreement with experimental indications) suggests that for n = 3 the process could be influenced by a 3(10) --> alpha-helix conformational transition of the peptide chain.

Anomalous distance dependence of electron transfer across peptide bridges

The first investigation on the distance dependence of a dissociative electron transfer process across peptide bridges is reported. This study was carried out by using a series of donor-peptide-acceptor systems in which the donor is a phthalimido moiety, the peptide bridges are provided by alpha-aminoisobutyric acid (Aib) homooligomers, and the acceptor is a peroxide functional group. The intramolecular electron transfer from the electrogenerated phthalimido radical anion to the peroxide was studied in comparison with the thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular rate constants were determined in N,N-dimethylformamide by taking into account the corresponding intermolecular values. The experimental results point to an unusual non-exponential dependence of the intramolecular electron transfer rate on the number of bridge units. The same trend could be verified also by taking into account the actual donor-acceptor edge-to-edge distance. The peculiar distance dependence that was observed for the intramolecular electron transfer rate is attributed to the mediating effect of the intramolecular C=O...H-N hydrogen bonds.

Position-specific incorporation of a fluorophore-quencher pair into a single streptavidin through orthogonal four-base codon/anticodon pairs

Four-base codon strategy was applied to incorporate a fluorophore-quencher pair into specific positions on a single protein; beta-anthraniloyl-L-alpha,beta-diaminopropionic acid (atnDap) was employed as a fluorophore and p-nitrophenylalanine (ntrPhe) as a quencher. Their positions were directed by the CGGG/CCCG and GGGC/CCCG four-base codon/anticodon pairs and two doubly mutated streptavidins, i.e., ((52)atnDap, (84)ntrPhe) and ((54)ntrPhe, (84)atnDap) mutants were synthesized through Escherichia coli in vitro protein synthesizing systems. Intramolecular photoinduced electron transfer (ET) was observed as the decrease of intensity in steady-state fluorescence spectroscopy and as the shortening of fluorescence decaytimes. The quenching data indicated that the ET rate reflects the detailed structure of the protein.