Dissociation and degradation of thiol-modified DNA on gold nanoparticles in aqueous and organic solvents

Gold nanoparticles functionalized with thiol-modified DNA have been widely used in making various nanostructures, colorimetric biosensors, and drug delivery vehicles. Over the past 15 years, significant progress has been made to improve the stability of such functionalized nanoparticles. The stability of the gold-thiol bond in this system, however, has not been studied in a systematic manner. Most information on the gold-thiol bond was obtained from the study of self-assembled monolayers (SAMs). In this study, we employed two fluorophore-labeled and thiol-modified DNAs. The long-term stability of the thiol-gold bond as a function of time, salt, temperature, pH, and organic solvent has been studied. We found that the bond spontaneously dissociated under all tested conditions. The dissociation was favored at high salt, high pH, and high temperature, and little DNA degradation was observed in our system. Most organic solvents showed a moderate protection effect on the gold-thiol bond. The stability of the gold-thiol bond in the DNA system was also compared with that in SAMs. While there are many similarities, we also observed opposite trends for the salt and ethanol effect. This study suggests that the purified DNA-functionalized gold nanoparticles should be freshly prepared and used in a day or two. Long-term storage should be carried out at relatively low temperature in low salt and slightly acidic buffers.     

Electrochemical stability of self-assembled alkylphosphate monolayers on conducting metal oxides

Alkylphosphate self-assembled monolayers (SAMs) were prepared on Nb-doped SrTiO(3) (Nb-STO) conducting metal oxide substrates. Unlike thiols on gold, the alkylphosphate SAMs on Nb-STO exhibited an electrochemical stability over a wide voltage range from -2 to 2 V. Cyclic voltammetry showed that the SAM modification inhibited the electrochemical activity of the underlying conducting substrate with an efficiency dependent on the chain length. Impedance spectroscopy showed that SAM-modified Nb-STO substrates have a significantly higher resistance than bare substrates.     

A review on synthetic account of 1,2,4-oxadiazoles as anti-infective agents

Molecular Diversity - Most of the currently marketed drugs consist of heterocyclic scaffolds containing nitrogen and or oxygen as heteroatoms in their structures. Several research groups have...     

f-block MOFs: A Pathway to Heterometallic Transuranics

A novel series of heterometallic f-block-frameworks including the first examples of transuranic heterometallic ²³⁸U/²³⁹Pu-metal–organic frameworks (MOFs) and a novel monometallic ²³⁹Pu-analog are reported. In combination with theoretical calculations, we probed the kinetics and thermodynamics of heterometallic actinide(An)-MOF formation and reported the first value of a U-to-Th transmetallation rate. We concluded that formation of uranyl species could be a driving force for solid-state metathesis. Density of states near the Fermi edge, enthalpy of formation, band gap, proton affinity, and thermal/chemical stability were probed as a function of metal ratios. Furthermore, we achieved 97 % of the theoretical maximum capacity for An-integration. These studies shed light on fundamental aspects of actinide chemistry and also foreshadow avenues for the development of emerging classes of An-containing materials, including radioisotope thermoelectric generators or metalloradiopharmaceuticals.     

Heteroatoms and substituent effects: The importance of heteroatom hyperconjugation

We have found that the specific rate of α‐sulfonyl carbanion formation in a β‐substituted sulfone shows a sizable dependence on the H C_α C_β X torsion angle. Defining k_N = (k_exch)_X/(k_exch)_model (where the model has X = H or an alkyl group) we observed for a collection of β‐alkoxy sulfones (X = OR) acceptable agreement with the expression log k_N = a + b cos² θ (where a = 1.70 and b = 2.62). Extension to other β‐substituents (X = RS, R₂N, and R₃N⁺) yields the same pattern, with the last showing very large dependence of k_N on the torsion angle (b = 6.3). These observations are ascribed to the presence (in addition to the inductive and field effects) of negative hyperconjugation responsible for accelerations of 1000‐fold and more, deriving from donation of the incipient negative charge on carbon into the σ*_{C X} orbital in the transition state. These observations reflect, and at the same time underline, the importance of the low‐lying antibonding orbitals present in heteroatomic molecules. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:397–405, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10067