Long-range electron transfer (ET) through proteins is a fundamental reaction in living organisms, playing a role in energy-conversion processes like photosynthesis [1a] and respiration [1b] as well as in enzymatic reactions. [1c] The mechanism of these ET processes is still a matter of controversy. Two cases are discussed, a bridge-mediated superexchange (single-step reaction) and a hopping mechanism (stepwise reaction). [2] Today, the hopping mechanism, where each step follows the Marcus rule, [3] is favored for long-range ET through peptides. [4] Nevertheless, for special conformations like a helices with their intramolecular hydrogen bonds, experiments on long-range ET have been discussed as single-step reactions. [5] However, quantum chemical calculations favor a multistep reaction also in a helices. [6] For the polyproline II helix (PPII helix), which does not have hydrogen bonds, a mechanistic transition between a singlestep and a stepwise reaction was observed when the number of the proline units between the electron donor and the electron acceptor is larger than 4. [7] Quantum chemical calculations explain the effect of peptide conformations on the distance influence of ET by a pathway model. [8] Recently it was shown that the rate also depends upon the direction of the ET process, demonstrating the important impact of dipole moments. [9] Another parameter influencing ET is the charge of ions bound as cofactors to the systems. [10] Such a Coulomb effect should also be of importance in peptides with unprotected amino or carboxylate groups that exist as ions under biological conditions. Herein we present our new results, which demonstrate that ET in peptides is indeed influenced by their charged termini.The investigations were carried out with our recently developed model peptides 1 in which ET occurs between the radical cation of a dialkoxyphenyl group (electron acceptor at the C-terminal end) and tyrosine as the electron donor at the N-terminal end. Halfway between the donor and the acceptor we introduced an amino acid with a side chain X. [11] These three amino acids were separated by sequences of three prolines (Scheme 1). If X is a side chain that can be oxidized by the electron acceptor, this central amino acid acts as a stepping stone for a hopping process (relay amino acid). [11] The structure of peptides 1 (in CH 3 CN/H 2 O 3:1) was characterized by their CD spectra as PPII helices. [12] Interestingly, the CD spectra remained unchanged when the temperature was varied between 20 8C and 80 8C, which supports the conformational stability of the peptides. These findings are in agreement with experimental and theoretical data from other polyproline systems. [13,14] In a PPII helix each of the triproline spacers corresponds to a distance of about 10 . [13,14] The triproline units thus separate the donor, the acceptor, and the relay amino acids, and act as a medium for the ET process. The 13 C NMR spectra [15,16] indicate that approximately 80 % of the proline peptide bonds adopt a trans conformat...

Generation 3 PAMAM dendrimers functionalized with 2,3-naphthalimide chromophoric groups on the end branches were synthesized, and the formation of Eu3+ polymetallic complexes was investigated. The luminescence properties of these complexes upon binding were fully characterized. On addition of Eu3+ to the dendrimer solution, lanthanide luminescence appears. The formation of a luminescent species corresponding to a dendrimer:lanthanide ratio of 1:8 was determined by luminescence batch titration and indicated by the maximum of Eu3+ emission. This indicates an overall average coordination number of 7.5 around each lanthanide metal cation. This is the first report of such characterization in the literature. Luminescence lifetimes indicate that the metal cation is well protected from nonradiative deactivation by the dendritic structure. Despite the limited efficiency of the sensitization of Eu3+, the absolute quantum yield being 0.0006, the good protection of the eight lanthanide cations bound in the dendrimer structure and the high absorptivity leads to the red emission from Eu3+ that is easily observed in solution under irradiation with 354 nm UV light.

The life time of aromatic radical cations is limited by reactions like b-elimination, dimerization, and addition to the solvent. Here we show that the attachment of such a radical cation to the C-terminal end of an a-/3 10 -helical peptide further reduces its life time by two orders of magnitude. For PPII-helical peptides, such an effect is only observed if the peptide contains an adjacent electron donor like tyrosine, which enables electron transfer (ET) through the peptide. In order to explain the special role of a-/3 10 -helical peptides, it is assumed that the aromatic radical cation injects a positive charge into an adjacent amide group. This is in accord with quantum chemical calculations and electrochemical experiments in the literature showing a decrease in the amide redox potentials caused by the dipole moments of long a-/3 10 -helical peptides. Rate measurements are in accord with a mechanism for a multi-step ET through a-/3 10 -helical peptides that uses the amide groups or H-bonds as stepping stones.

Electron transfer (ET) is a basic chemical reaction, which involves radicals and radical ions. In all living organisms, ETs through peptides and proteins are vital for energy conversion and metabolic processes. In several cases, distances are so long that ET cannot occur as a single‐step reaction, and the process is a result of a multistep reaction, where the charge hops between relay stations. Each of the hopping steps obeys the Marcus theory with its strong distance dependence, but the overall process is described by the weak distance dependency of diffusion. Aromatic and sulfur‐containing aliphatic amino acids are relay amino acids, and for special peptide conformations also the amide bonds might act as stepping stones. ET rates depend on peptide conformations, redox potentials, dipole moments, charges, and proton transfer processes. In addition to its fundamental biological importance, ET also starts to play a role as electronic material. In this article, the basic principles, ET in biological systems, and the possible applications in materials science are presented and discussed.

Some microorganisms perform anaerobic mineral respiration by reducing metal ions to metal nanoparticles, using peptide aggregates as medium for electron transfer (ET). Such a reaction type is investigated here with model peptides and silver as the metal. Surprisingly, Ag+ ions bound by peptides with histidine as the Ag+‐binding amino acid and tyrosine as photoinducible electron donor cannot be reduced to Ag nanoparticles (AgNPs) under ET conditions because the peptide prevents the aggregation of Ag atoms to form AgNPs. Only in the presence of chloride ions, which generate AgCl microcrystals in the peptide matrix, does the synthesis of AgNPs occur. The reaction starts with the formation of 100 nm Ag@AgCl/peptide nanocomposites which are cleaved into 15 nm AgNPs. This defined transformation from large nanoparticles into small ones is in contrast to the usually observed Ostwald ripening processes and can be followed in detail by studying time‐resolved UV/Vis spectra which exhibit an isosbestic point.