Shana O. Kelley

Long‐Range Electron Transfer through DNA Films

Regardless of its position within the DNA film, cross-linked daunomycin (DM) is efficiently reduced electrochemically, indicating that the electron transfer exhibits a shallow distance dependence. Upon the introduction of an intervening cytosine-adenine (CA) mismatch, the electrochemical response is dramatically attenuated (shown schematically). Therefore, the DNA double helix can facilitate long-range electron transfer, but only in the presence of a well-stacked pathway.

DNA-mediated electron transfer from a modified base to ethidium: π-stacking as a modulator of reactivity

BACKGROUNDnThe DNA double helix is composed of an array of aromatic heterocyclic base pairs and, as a molecular pi-stack, represents a novel system for studying long-range electron transfer. Because many base damage and repair processes result from electron-transfer reactions, the ability of DNA to mediate charge transport holds important biological implications. Seemingly contradictory conclusions have been drawn about electron transfer in DNA from the many different studies that have been carried out. These studies must be reconciled so that this phenomenon can be understood both at a fundamental level and in the context of biological systems.nnnRESULTSnThe photoinduced oxidation of a modified base, 7-deazaguanine, has been examined as a function of distance, sequence, and base stacking in DNA duplexes covalently modified with ethidium. Over ethidium/deazaguanine separations of 6-27 A, the photooxidation reaction proceeded on a subnanosecond time scale, and the quenching yield exhibited a shallow distance dependence. The efficiency of the reaction was highly sensitive to small changes in base composition. Moreover, the overall distance-dependence of the reaction is sensitive to sequence, despite the constancy of photoexcited ethidium as acceptor.nnnCONCLUSIONSnThe remarkable efficiency of deazaguanine photooxidation by intercalated ethidium over long distances provides new evidence for fast electron-transfer pathways through DNA. By varying sequence as well as reactant separation, this work provides the first experimental demonstration of the importance of reactant stacking in the modulation of long-range DNA mediated electron transfer.

Dimerization of a pathogenic human mitochondrial tRNA

Mutations of human mitochondrial transfer RNA (tRNA) are implicated in a variety of multisystemic diseases. The most prevalent pathogenic mitochondrial mutation is the A3243G substitution within the gene for tRNALeu(UUR). Here we describe the pronounced structural change promoted by this mutation. The A3243G mutation induces the formation of a tRNA dimer that strongly self-associates under physiological conditions. The dimerization interface in the mutant tRNA is a self-complementary hexanucleotide in the D-stem, a particularly weak structural element within tRNALeu(UUR). Aminoacylation of the A3243G mutant is significantly attenuated, and mutational studies indicate that dimerization is partially responsible for the observed loss of function. The disruption of a conserved tertiary structural contact also contributes to the functional defect. The pathogenic mutation is proposed to interfere with the cellular function of human mitochondrial tRNALeu(UUR) by destabilizing the native structure and facilitating the formation of a dimeric complex with low biological activity.

Heterogeneous deposition of noble metals on semiconductor nanoparticles in organic or aqueous solvents

We describe a versatile approach for synthesizing heterogeneous semiconductor/noble metal nanostructures both in organic and in aqueous solvent at room temperature. The deposition is based on the preferential nucleation and growth of noble metals (Au, Ag) at a single nucleation site on semiconductor (PbS) nanocrystals. The synthesis of multifunctional heterogeneous nanostructures of PbS–Au and PbS–Ag was demonstrated. The self-assembly of the heterogeneous structures into ordered arrays was observed by transmission electron microscopy (TEM), which may lead to semiconductor–metal nanostructures with potential for self-assembly, geometric complexity, and multifunctionality. Also, the successful deposition of Au on PbS in the aqueous phase may find use in biological applications.

Microfluidic Three-Electrode Cell Array for Low-Current Electrochemical Detection

This paper reports the implementation and calibration of a microscopic three-electrode electrochemical sensor integrated with a polydimethylsiloxane (PDMS) microchannel to form a rapid prototype chip technology that is used to develop sensing modules for biomolecular signals. The microfluidic/microelectronic fabrication process yields identical, highly uniform, and geometrically well-defined microelectrodes embedded in a microchannel network. Each three-microelectrode system consists of a Au working electrode with a nominal surface area of 9 mum2, a Cl2 plasma-treated Ag/AgCl reference electrode, and a Au counter electrode. The patterned electrodes on the glass substrate are aligned and irreversibly bonded with a PDMS microchannel network giving a channel volume of 72 nL. The electrokinetic properties and the diffusion profile of the microchannels are investigated under electrokinetic flow and pressure-driven flow conditions. Cyclic voltammetry of 10 mM K3 Fe(CN)6 in 1 M KNO3 demonstrates that the electrode responses in the cell are characterized by linear diffusion. The voltammograms show that the system is a quasi-reversible redox process, with heterogeneous rate constants ranging from 3.11 to 4.94times10-3 cm/s for scan rates of 0.1-1 V/s. The current response in the cell is affected by the adsorption of the electroactive species on the electrode surface. In a low-current DNA hybridization detection experiment, the electrode cell is modified with single-stranded thiolated DNA. The electrocatalytic reduction of 27 muM Ru(NH3)6 3+ in a solution containing 2 mM Fe(CN)6 3- is measured before and after the exposure of the electrode cell to a 500-nM target DNA sample. The preliminary result showing an increase in the peak current response demonstrates the hybridization-based detection of a complementary target DNA sequence

Nucleotide-stabilized cadmium sulfide nanoparticles

Cadmium sulfide nanoparticles stabilized by both natural and unnatural 5′-nucleotide triphosphates were investigated to elucidate the specific chemical functionalities involved in the synthesis of emissive materials. The roles of the nucleobase functionalities in semiconductor nanocrystal synthesis are deconvoluted using photoluminescence spectroscopy, transmission electron microscopy and agarose gel electrophoresis. Through a survey of all the natural nucleosides, it was discovered that 5′-guanosine triphosphate most effectively stabilizes emissive nanoparticles, while adenosine, inosine, cytidine, uracil and 7-methylguanosine nucleobases do not facilitate successful synthesis of emissive product unless used under basic conditions. The work presented systematically explores and identifies important functionalities within polynucleic acids that can be used to produce aqueous soluble semiconductor nanocrystals.

WEITREICHENDER ELEKTRONENTRANSFER DURCH DNA-FILME

Unabhangig von der Position im DNA-Film wird kovalent an die DNA angeknupftes Daunomycin (DM) elektrochemisch effizient reduziert – ein Hinweis auf eine nur geringe Abhangigkeit des Elektronentransfers von der Entfernung. Nach Einfuhrung einer C-A-Fehlpaarung geht das elektrochemische Ansprechen drastisch zuruck (schematisch dargestellt). Die DNA-Doppelhelix kann demnach weitreichenden Elektronentransfer erleichtern, aber nur bei Vorliegen eines Ubertragungsweges durch einen geordneten π-Basenstapel.

DNA-directed synthesis of zinc oxide nanowires on carbon nanotube tips

This paper describes a class of three component hybrid nanowires templated by DNA directed self-assembly. Through the modification of carbon nanotube (CNT) termini with synthetic DNA oligonucleotides, gold nanoparticles are delivered, via DNA hybridization, to CNT tips that then serve as growth sites for zinc oxide (ZnO) nanowires. The structures we have generated using DNA templating represent an advance toward building higher order sequenced one dimensional nanostructures with rational control.

Photosensitized DNA cleavage promoted by amino acids

A novel class of DNA cleavage agents are reported that derive activity from amino acids tethered to a photoactive intercalator.

Tunable DNA cleavage by intercalating peptidoconjugates.

The properties of a novel family of peptide‐based DNA‐cleavage agents are described. Examination of the DNA‐cleavage activities of a systematic series of peptide–intercalator conjugates revealed trends that show a strong dependence on peptide sequence. Conjugates differing by a single residue displayed reactivities that varied over a wide range. The cleavage activity was modulated by the electrostatic or steric qualities of individual amino acids. Isomeric conjugates that differed in the position of the tether also exhibited different reactivities. The mechanism of DNA cleavage for these compounds was also probed and was determined to involve hydrogen‐atom abstraction from the DNA backbone. Previous studies of these compounds indicated that amino acid peroxides were the active agents in the cleavage reaction; in this report, the chemistry underlying the reaction is characterized. The results reported provide insight into how peptide sequences can be manipulated to produce biomimetic compounds.