Abstract

Highly organized CdS growth by DNA templating is used to produce either linear arrangements of Q-particles along the DNA strands or continuous conducting nanowires (see figure). By optimizing the reaction conditions, the potential of biopolymers in general and DNA in particular for producing nanometer-scale electronics is exemplified.

The synthesis of supramolecular conducting nanowires can be achieved by using DNA and pyrrole. Oxidation of pyrrole in DNA-containing solutions yields a material that contains both the cationic polypyrrole (PPy) and the anionic DNA polymers. Intimate interaction of the two polymer chains in the self-assembled nanowires is indicated by FTIR spectroscopy. AFM imaging shows individual nanowires to be continuous, ≈5 nm high and conformationally flexible. This feature allows them to be aligned by molecular combing in a similar manner to bare DNA and provides a convenient method for fabricating a simple electrical device by stretching DNA/PPy strands across an electrode gap. Current–voltage measurements confirm that the nanowires are conducting, with values typical for a polypyrrole-based material. In contrast to polymerisation of pyrrole on a DNA template in bulk solution, attempts to form similar wires by polymerisation at surface-immobilised DNA do not give a continuous coverage; instead, a beads-on-a-string appearance is observed suggesting that immobilisation inhibits the assembly process.

The ferrocenyl-nucleoside, 5-ethynylferrocenyl-2′-deoxycytidine (1) has been prepared by Pd-catalyzed cross-coupling between ethynylferrocene and 5-iodo-2′-deoxycytidine and incorporated into oligonucleotides by using automated solid-phase synthesis at both silica supports (CPG) and modified single-crystal silicon electrodes. Analysis of DNA oligonucleotides prepared and cleaved from conventional solid supports confirms that the ferrocenyl-nucleoside remains intact during synthesis and deprotection and that the resulting strands may be oxidised and reduced in a chemically reversible manner. Melting curve data show that the ferrocenyl-modified oligonucleotides form duplex structures with native complementary strands. The redox potential of fully solvated ferrocenyl 12-mers, 350 mV versus SCE, was shifted by +40 mV to a more positive potential upon treatment with the complement contrary to the anticipated negative shift based on a simple electrostatic basis. Automated solid-phase methods were also used to synthesise 12-mer ferrocenyl-containing oligonucleotides directly at chemically modified silicon <111> electrodes. Hybridisation to the surface-bound ferrocenyl-DNA caused a shift in the reduction potential of +34 mV to more positive values, indicating that, even when a ferrocenyl nucleoside is contained in a film, the increased density of anions from the phosphate backbone of the complement is still dominated by other factors, for example, the hydrophobic environment of the ferrocene moiety in the duplex or changes in the ferrocene–phosphate distances. The reduction potential is shifted >100 mV after hybridisation when the aqueous electrolyte is replaced by THF/LiClO₄, a solvent of much lower dielectric constant; this is consistent with an explanation based on conformation-induced changes in ferrocene–phosphate distances.

Komplementäre Stränge und Redox-Intercalatoren können sich an Siliciumhalbleiteroberflächen, die durch kovalente Anknüpfung von DNA modifiziert sind, zu nanometergroßen Gebilden anordnen (siehe schematische Darstellung); an der Elektrodenoberfläche kann Ladungstransfer erfolgen.

Complementary strands and redox intercalators can be self-assembled into nanoscale structures capable of charge transfer at the electrode surface from DNA oligomers synthesized directly at covalently modified semiconductor silicon surfaces (see schematic representation).

Hoogsteen base pairing between coordinated adenine (A) and pendant thymine (T) residues of the Pt complex of a dithioether ligand bearing A and T groups results in the formation of infinite chains in the solid state (see picture). The chains are further involved in π–π stacking interactions.

Hoogsteen-Paarbildung zwischen den Thymin(T)- und koordinierten Adenin(A)-Resten des Pt-Komplexes eines Dithioether-Liganden, an den diese Reste gebunden sind, führt im festen Zustand zur Bildung unendlich ausgedehnter Ketten (siehe Bild). Bei den Ketten wurden auch π-π-Stapel-Wechselwirkungen festgestellt.

Ferrocenylthymidine derivatives have been prepared by Pd-catalysed cross-coupling between ethynylferrocene or vinylferrocene and 5-iodo-2′-deoxyuridine. In the latter case a mixture of trans (2 a) and gem (2 b) isomers was obtained. The cis-vinylferrocenyl (2 c), and ethylferrocenyl (3) derivatives were obtained by catalytic hydrogenation of ethynylferrocenyl-dT (1 a), and 2 c respectively. Single-crystal X-ray data for 1 a, the ferrocenyl-2′furano-pyrimidone 1 b, and 2 a show that the nucleobase is essentially co-planar with the substituted Cp ring of the metallocene. The selective reduction of the linkage between the ferrocenyl and thymidine moieties, from –CC- to -CH₂CH₂-, causes a shift in the reduction potential of −124 mV. DFT calculations for the one-electron oxidised species indicate that the diminished conjugation reduces the spin transfer onto the bridging C₂ group, but has less effect on the extent transferred to the nucleobase from the ferrocenyl group. Compound 1 a was incorporated site-specifically into DNA oligonucleotides by using automated solid-phase methods. However, some interconversion of 1 a1 b occurs, even under rapid mild conditions of deprotection.

The reactions of Pd^II ions with a series of chelate-tethered derivatives of adenine and guanine have been studied and reveal a difference in the reactivity of the purine bases. Reactions of [PdCl₂(MeCN)₂] and A-alkyl-enH⋅Cl (alkyl=propyl or ethyl, A=adenine, en=ethylenediamine) yield the monocationic species [PdCl(A-N3-Et-en)]⁺ (1) and [PdCl(A-N3-Pr-en)]⁺ (2). Both involve co-ordination at the minor groove site N3 of the nucleobase as confirmed by single-crystal X-ray analysis. Reactions with the analogous G-alkyl-enH⋅Cl derivatives (G=guanine, alkyl=ethyl or propyl) were more complex with a mixture of species being observed. For G-Et-en⋅HCl a product was isolated which was identified as [PdCl(G-C8-Et-en)]⁺ (3). This compound contains a biomolecular metal–carbon bond involving C8 of the purine base. Crystallography of a product obtained from reaction of G-Pr-enH⋅Cl and [Pd(MeCN)₄][NO₃]₂ reveals an octacationic tetrameric complex (4), in which each ligand acts to bridge two metal ions through a combination of a tridentate binding mode involving the diamine and N3 and monodentate coordination at N7.

The cover picture shows an inverted G-tetrad (top left) formed through metal–ligand bond formation between chelate-tethered guanine nucleobases and CdSO₄. Individual tetramer units assembled into columnar aggregates as a result of sulfate anions bridging adjacent Cd centres (bottom). Further association of the cylindrical polymers occurs through stacking interactions involving the six-membered rings of the nucleobase (right). This work is described in more detail by A. Houlton et al. on pp. 4371 ff.

A series of Zn^II and Cd^II complexes of adenine and guanine derivatives containing a diamine tether have been isolated from aqueous solutions and characterised by single crystal X-ray analysis. These studies reveal a wide range of structural types including monomeric, dimeric, tetrameric and polymeric architectures. The extended structures arise from the ability of the ligands to bridge metal ions using the chelating tether in conjunction with N7 of the nucleobase. Additional metal–nucleobase co-ordination is generally observed at the N3-site of the adenine derivatives. With Cd^II, ethylenediamine-N9-ethylguanine forms an inverted G-tetrad type structure.

The hydrogen-terminated surface of porous silicon (PS) is sufficiently reactive for the uncatalyzed hydrosilation of alkenes and alkynes. These modifications produce dramatic changes to both the physical and chemical properties of the PS.

Wasserstoff-funktionalisiertes poröses Silicium (PS) ist gegenüber Alkenen und Alkinen für eine unkatalysierte Hydrosilylierung ausreichend reaktiv. Die dabei erhaltenen alkylmodifizierten PS-Oberflächen weisen erheblich veränderte physikalische wie auch chemische Eigenschaften auf.