A guanine radical cation (G +• ) was site-selectively generated in double stranded DNA and the charge transfer in different oligonucleotide sequences was investigated. The method is based on the competition between a charge transfer from G +• through the DNA and its trapping reaction with H 2 O. We analyzed the hole transfer from this G +• to a GGG unit through one, two, three, and four AT base pairs and found that the rate decreases by about 1 order of magnitude with each intervening AT base pair. This strong distance dependence led to a β-value of 0.7 ( 0.1 Å -1 . Within the time scale of this assay the charge transfer nearly vanished when the G +• was separated by four AT base pairs from the GGG unit. However, if the second or the third of the four intervening AT base pairs was exchanged by a GC base pair, the rate of the hole transfer from the G +• to the GGG unit increased by 2 orders of magnitude. In addition, a long-range charge transfer over 15 base pairs could be observed in a mixed strand that contained AT as well as GC base pairs. Because G +• can oxidize G but not A bases, the long-range charge transport can be explained by a hopping of the positive charge between the intervening G bases. Thus, the overall charge transport in a mixed strand is a multistep hopping process between G bases where the individual steps contribute to the overall rate. The distance dependence is no longer described by the β value of the superexchange mechanism.

Hopping between bases of similar redox potentials is the mechanism by which charge transport occurs through DNA. This was shown by rate measurements performed with double strands 1-3. This mechanism explains why hole transfer displays a strong sequence dependence, and postulates that electron transfer in unperturbed DNA should not be dependent on the sequence.

Unique properties of metal complexes, such as structural diversity, adjustable ligand exchange kinetics, fine-tuned redox activities, and distinct spectroscopic signatures, make them exciting scaffolds not only for binding to nucleic acids but increasingly also to proteins as non-traditional targets. This feature article discusses recent trends in this field. These include the use of chemically inert metal complexes as structural scaffolds for the design of enzyme inhibitors, new strategies for inducing selective coordination chemistry at the protein binding site, recent advances in the development of catalytic enzyme inhibitors, and the design of metal complexes that can inject electrons or holes into redox enzymes. A common theme in many of the discussed examples is that binding selectivity is at least in part achieved through weak interactions between the ligand sphere and the protein binding site. These examples hint to an exciting future in which "organic-like" molecular recognition principles are combined with properties that are unique to metals and thus promise to yield compounds with novel and unprecedented properties.

Asymmetric catalysis is seen as one of the most economical strategies to satisfy the growing demand for enantiomerically pure small molecules in the fine chemical and pharmaceutical industries. And visible light has been recognized as an environmentally friendly and sustainable form of energy for triggering chemical transformations and catalytic chemical processes. For these reasons, visible-light-driven catalytic asymmetric chemistry is a subject of enormous current interest. Photoredox catalysis provides the opportunity to generate highly reactive radical ion intermediates with often unusual or unconventional reactivities under surprisingly mild reaction conditions. In such systems, photoactivated sensitizers initiate a single electron transfer from (or to) a closed-shell organic molecule to produce radical cations or radical anions whose reactivities are then exploited for interesting or unusual chemical transformations. However, the high reactivity of photoexcited substrates, intermediate radical ions or radicals, and the low activation barriers for follow-up reactions provide significant hurdles for the development of efficient catalytic photochemical processes that work under stereochemical control and provide chiral molecules in an asymmetric fashion. Here we report a highly efficient asymmetric catalyst that uses visible light for the necessary molecular activation, thereby combining asymmetric catalysis and photocatalysis. We show that a chiral iridium complex can serve as a sensitizer for photoredox catalysis and at the same time provide very effective asymmetric induction for the enantioselective alkylation of 2-acyl imidazoles. This new asymmetric photoredox catalyst, in which the metal centre simultaneously serves as the exclusive source of chirality, the catalytically active Lewis acid centre, and the photoredox centre, offers new opportunities for the 'green' synthesis of non-racemic chiral molecules.