We investigated the photoinduced one-electron oxidation of a series of DNA oligomers having a covalently linked anthraquinone group (AQ) and containing [(A)(n)GG](m) or [(T)(n)GG](m) segments. These oligomers have m GG steps, where m = 4 or 6, separated by (A)(n) or (T)(n) segments, where n = 1-7 for the (A)(n) set and 1-5 for the (T)(n) set. Irradiation with UV light that is absorbed by the AQ causes injection of a radical cation into the DNA. The radical cation migrates through the DNA, causing chemical reaction, primarily at GG steps, that leads to strand cleavage after piperidine treatment. The uniform, systematic structure of the DNA oligonucleotides investigated permits the numerical solution of a kinetic scheme that models these reactions. This analysis yields two rate constants, k(hop), for hopping of the radical cation from one site to adjacent sites, and k(trap), for irreversible reaction of the radical cation with H(2)O or O(2). Analysis of these findings indicates that radical cation hopping in these duplex DNA oligomers is a process that occurs on a microsecond time scale. The value of k(hop) depends on the number of base pairs in the (A)(n) and (T)(n) segments in a systematic way. We interpret these results in terms of a thermally activated adiabatic mechanism for radical cation hopping that we identify as phonon-assisted polaron hopping.
A series of anthraquinone-linked (AQ) duplex DNA oligomers were prepared and investigated. Irradiation of the AQ injects a radical cation into the DNA. The radical cation migrates through the DNA and reacts selectively at GG steps, which leads to strand cleavage after treatment with piperidine. The oligomers investigated in this work were selected to assess the effect on long-distance charge transport of placing a T base (or bases) in a strand of repeating purine bases. With notable exceptions, the amount of strand scission decreases with the distance between the AQ and the GG step. The results are consistent only with models for long-distance transport, such as thermally activated polaron-like hopping, that incorporate radical cation delocalization over two or more adjacent bases.
Herein, we developed
a natural surface-enhanced Raman scattering
(SERS) substrate based on size-tunable Au@Ag nanoparticle-coated mussel
shell to form large-scale three-dimensional (3D) supercrystals (up
to 10 cm2) that exhibit surface-laminated structures and
crossed nanoplates and nanochannels. The high content of CaCO3 in the mussel shell results in superior hydrophobicity for
analyte enrichment, and the crossed nanoplates and nanochannels provided
rich SERS hot spots, which together lead to high sensitivity. Finite-difference
time-domain simulations showed that nanoparticles in the channels
exhibit apparently a higher electromagnetic field enhancement than
nanoparticles on the platelets. Thus, under optimized conditions (using
Au@AgNPs with 5 nm shell thickness), highly sensitive SERS detection
with a detection limit as low as 10–9 M for rhodamine
6G was obtained. Moreover, the maximum electromagnetic field enhancement
of different types of 3D supercrystals shows no apparent difference,
and Au@AgNPs were uniformly distributed such that reproducible SERS
measurements with a 6.5% variation (613 cm–1 peak)
over 20 spectra were achieved. More importantly, the as-prepared SERS
substrates can be utilized for the fast discrimination of Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa by discriminant
analysis. This novel Au@Ag self-assembled mussel shell template holds
considerable promise as low-cost, durable, sensitive, and reproducible
substrates for future SERS-based biosensors.
Catalyzed by a nitrile hydratase/amidase-containing microbial Rhodococcus sp. AJ270 whole-cell catalyst, a number of racemic trans-2,3-epoxy-3-arylpropanenitriles 1 underwent rapid and efficient hydrolysis under very mild conditions to afford 2R,3S-2-arylglycidamides 2 in excellent yield with enantiomeric excess higher than 99.5%. The overall enantioselectivity of the biotransformations originated from the combined effects of a dominantly high 2S-enantioselective amidase and low 2S-enantioselective nitrile hydratase involved in the cell. The influence of the substrates on both reaction efficiency and enantioselectivity was also discussed in terms of steric and electronic effects.
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