ABSTRACT
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 100â nm Ag@AgCl/peptide nanocomposites which are cleaved into 15â 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.
Subject(s)
Electron Transport , Metal Nanoparticles , Peptides/chemistry , Silver/chemistry , Microscopy, Electron, TransmissionABSTRACT
This short historical review describes the work of the Giese group on radicals and radical ions that has led to the award of the Paracelsus prize in 2012.
Subject(s)
Amino Acids/chemistry , Electron Transport , Electrons , Free Radicals/chemistry , Peptide Fragments/chemistry , StereoisomerismABSTRACT
Artificial implants and biomaterials lack the natural defense system of our body and, thus, have to be protected from bacterial adhesion and biofilm formation. In addition to the increasing number of implanted objects, the resistance of bacteria is also an important problem. Silver ions are well-known for their antimicrobial properties, yet not a lot is known about their mode of action. Silver is expected to interact on many levels, thus the development of silver resistance is very difficult. Nevertheless, some bacteria are able to resist silver, even at higher concentrations. One such defense mechanism of bacteria against heavy-metal intoxication includes an efflux system. SilE, a periplasmic silver-binding protein that is involved in this defense mechanism, has been shown to possess numerous histidine functions, which strongly bind to silver atoms, as demonstrated by ourselves previously. Herein, we address the question of how histidine binds to silver ions as a function of pH value. This property is important because the local proton concentration in cells varies. Thus, we solved the crystal structures of histidine-silver complexes at different pH values and also investigated the influence of the amino-acid configuration. These results were completed by DFT calculations on the binding strength and packing effects and led to the development of a model for the mode of action of SilE.
Subject(s)
Anti-Bacterial Agents/chemistry , Histidine/chemistry , Silver Compounds/chemistry , Silver/chemistry , Histidine/metabolism , Hydrogen-Ion Concentration , Models, Theoretical , Quantum Theory , Silver Compounds/metabolism , X-Ray DiffractionABSTRACT
Electron transfer (ET) through peptides and proteins is a key biochemical process, which involves radicals and radical ions as reactive intermediates. We have developed an assay that allows us to study this fundamental chemical reaction.
Subject(s)
Free Radicals/chemistry , Peptides/chemistry , Amino Acids/chemistry , Electron Transport , Ions/chemistry , Molecular StructureABSTRACT
The life time of aromatic radical cations is limited by reactions like ß-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 α-/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 α-/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 α-/3(10)-helical peptides. Rate measurements are in accord with a mechanism for a multi-step ET through α-/3(10)-helical peptides that uses the amide groups or H-bonds as stepping stones.
