How do amino acids transport electrons through peptides?

and Outlook
A peptide model was designed, which allows the investigation of amino acid side chain participation in ET through peptides. Aromatic amino acids function as oxidizable spectroscopic sensors for the direct observation of charged intermediates during the ET process. Tyrosine as electron donor, situated at the N-terminus of the peptide, provides driving force for the ET process and is irreversibly oxidized to a long-lived phenoxyl radical with a sharp absorption band. Two methoxysubstituted phenylalanine derivatives were chosen as additional spectroscopic sensors, yielding oxidized transients with different absorption spectra. They were synthesized in their enantiopure form and investigated with respect to their electrochemical and spectroscopical properties. In the peptide model, they function as C-terminal electron acceptor precursor and central relay, separated from each other and the donor by a proline matrix. The electron acceptor can be generated by laser irradiation of an injection unit, containing a tbutyl ketone as chromophore. The half-life time of acceptor generation is estimated to 20 ns and intermolecular ET started to gain importance from approximately 100 ns after irradition. Thus, transient absorption spectra recorded 40 ns after the laser flash were used for the examination of intramolecular ET efficiencies between the redox sites. In a first study, the injection unit was connected to the peptide C-terminus via an ester bond and peptides with a total length of 5-10 residues were synthesized. In this case, ET efficiency was observed to depend on the construction of the separating matrix. Efficient intramolecular ET to the electron acceptor within 40 ns after irradiation was observed in peptides, in which either donor and relay or relay and acceptor were arranged in close proximity (separated by one proline). Signals of oxidized relay were observed in all cases, proving that the occurence of oxidized intermediates in donor to acceptor ET can be detected in this model. Efficient intramolecular ET through the whole chain, i.e. from the N-terminal donor to the electron acceptor, was only observed in peptides containing a total of up to four prolines in the matrix. The synthesis of a new electron acceptor precursor, in which the injection unit was connected directly, via an ether bond, to a phenoxyl function at the aromatic side chain of the C-terminal amino acid, then allowed the observation of intramolecular ET from the N-terminus to the C-terminus of a nonapeptide, which consisted of two triproline spacers and the three spectroscopic sensors. This peptide showed a well-defined polyproline II helical structure, leading to a donor/acceptor separation of 20 Å. The aromatic relay, functioning as spectroscopic sensor for the detection of oxidized intermediates in donor to acceptor ET, could be exchanged by a number of different amino acids, without changing the overall structure of the peptide. Thus, the peptide model allowed us to determine the influence of the central amino acid - separated from both, donor and acceptor, by 10 Å - on ET. Efficient ET was observed in those two cases, where the central amino acid was able to function as an oxidizable relay in ET. The spectrum of the peptide containing the three distinguishable spectroscopic sensors, showed
the simultaneous occurence of electron acceptor, oxidized donor and oxidized relay and thereby
proved that the charge resides on the relay site during ET. In two peptides containing aliphatic central
residues, intramolecular donor to acceptor ET did not take place efficiently within the time frame of
our experiments – a 20 to 30 fold decrease in donor oxidation was observed. A peptide containing an
anisol ring as central aromatic side chain, with an oxidation potential far above the electron acceptor,
did also not render efficient intramolecular ET possible. In conclusion, efficient ET across the 20 Å
distance in our model peptides was only possible within 40 ns, if an oxidizable amino acid side chain
was present inside the matrix. This side chain functioned as a stepping stone in ET, with the charge
residing on it, forming a chemical intermediate. Thus, electron hopping via aromatic amino acid side
chains was proven to allow distal (20 Å) ET transfer in peptides within 40 ns whereas single-step
superexchange ET did not take place within 40 ns in peptides of similar structure. From considerations
regarding the competition of intermolecular and intramolecular ET, the rate of intramolacular ET
between electron acceptor and electron donor by a two-step electron hopping process, with 10 Å
distance for each step, was determined to 5 - 7 ·106 s-1. In the peptide containing three spectroscopic
sensors, the relative intensities of all signals could be used to conclude on the ratio of rate constants in
the consecutive ET reaction, leading to a five times faster rate constant for donor to relay, than for
relay to acceptor ET. This is consistent with the differences in driving force for the two reactions.
Our model provides an experimental approach to determine the suitability of individual amino acids to
function as relays in electron hopping. Future experiments could be carried out with the naturally
occuring aromatic amino acids tryptophane and histidine, as well as with cysteine, as an additional
oxidizable amino acid. The introduction of protection groups, to avoid deprotonation upon oxidation,
which renders the ET irreversible (as in the case of tyrosine) might be necessary. First experiments
lead to the assumption, that phenylalanine plays an unexpected role in the mediation of long-range ET.
Whether this is due to unknown local structural features, not visible in the CD spectrum, or to
electronic effects is not yet clear. This might be further investigated by the introduction of electrondonating
or electron-withdrawing groups to the benzene ring.