Guoqing Jia

Trap states and carrier dynamics of TiO2 studied by photoluminescence spectroscopy under weak excitation condition

Anatase and rutile TiO(2) were investigated with photoluminescence techniques under the weak excitation condition, where trap states play a vital role in carrier dynamics. The visible emission of anatase and near-infrared (NIR) emission of rutile both exhibit extremely long lifetimes up to milliseconds. The decay processes can be well described by the power-law decay which corresponds to the trapping-detrapping effect. These results indicate that the luminescence processes in both anatase and rutile TiO(2) have a close relationship with trap states. The visible emission band was assigned to the donor-acceptor recombination. Oxygen vacancies and hydroxyl groups mainly serve as the donor and acceptor sites, respectively. The NIR luminescence is originated from the recombination of trapped electrons with free holes, while the trapped electrons were formed through two paths, direct trapping or trap-to-trap hopping. The trap states in anatase and rutile TiO(2) may largely influence the photocatalysis process of TiO(2) and determine the photocatalytic activity under stationary illumination.

Enantioselective Diels-Alder Reactions with G-Quadruplex DNA-Based Catalysts

Deoxyribonucleic acid (DNA) is the genetic material of living organisms. In the past, double-stranded DNA (dsDNA) with its ubiquitous architecture has not been regarded as a catalytic species, since the duplex structure precludes the formation of catalytically competent tertiary structures. To date, although no naturally occurring catalytic DNA has been reported, DNA for nonbiological applications has aroused much interest in chemists for applications in areas such as catalysis, encoding, and stereocontrol. Among these applications, a series of DNA-based asymmetric catalysis have been developed, which use a hybrid catalyst composed of dsDNA and a copper(II) complex. This same strategy was later applied to G-quadruplex DNA (G4DNA) and modest enantioselectivities in the Diels–Alder (D–A) reaction were obtained. Very recently, a G4DNA metalloenzyme composed of G4DNA and copper(II) ions has been reported to be able to catalyze an enantioselective Friedel–Crafts reaction. This biology/chemistry interface is an attractive area of research and awaits further extensive exploration. Herein, we report an enantioselective D–A reaction achieved through the use of human telomeric G4DNA-based catalysts. We show that the absolute configuration of the products can be reversed when the conformation of the G4DNA is switched from antiparallel to parallel. Furthermore, both the reaction rate and the enantioselectivity of the reaction were found to be dependent on the DNA sequence. The D–A reaction is an important carbon–carbon bond forming reaction in organic synthesis. In the past few decades, it has received much attention in the development of innovative catalytic strategies to control the creation of the new carbon–carbon bonds and stereocenters. Among those strategies, biological molecules have been viewed as interesting and promising catalysts. Herein, human telomeric G4DNA (ODN-1, 5’-G3(T2AG3)3-3’) was selected owing to its tunable conformation. As an initial attempt, a model D–A reaction between aza-chalcone (1a) and cyclopentadiene (2) was chosen to probe the catalytic performance of ODN-1. We found that ODN-1 alone in its antiparallel conformation could promote the D–A reaction and the enantiomeric excess of the endo isomer of product 3a is 17 % (Table 1, entry 2).

Transfer hydrogenation of aldehydes on amphiphilic catalyst assembled at the interface of emulsion droplets

An amphiphilic polymer-based iridium catalyst assembled at the interface of emulsion droplets shows a remarkable rate acceleration for the transfer hydrogenation of aldehydes in water, which may result from the high surface area of the emulsion droplets and the high local concentrations of reactants around the active sites.

The structural transition and compaction of human telomeric G-quadruplex induced by excluded volume effect under cation-deficient conditions.

The structure polymorphism of human telomeric G-quadruplex (ht-quadruplex) is currently an important topic but remains controversy. Here, we present study of the ht-quadruplex under the cation-deficient but molecular crowding conditions by circular dichroism (CD), microchip electrophoresis (MCE) and UV-melting experiments. Our results show that with concentration increasing of poly(ethylene glycol) (PEG), the structural transition of ht-quadruplex occurs accompanied by structural compaction and enhanced stabilization, which may be caused by excluded volume effect. This work also demonstrates that ht-quadruplex can be well assembled without cations and the structure of ht-quadruplex is actually very complex in vivo.

Dynamic Insight into the Interaction between Porphyrin and G-quadruplex DNAs: Time-Resolved Fluorescence Anisotropy Study

Understanding the nature of the interaction between small molecules and G-quadruplex DNA is crucial for the development of novel anticancer drugs. In this paper, we present the first data on time-resolved fluorescence anisotropy study on the interaction between a water-soluble cationic porphyrin H(2)TMPyP4 and four distinct G-quadruplex DNAs, that is, AG(3)(T(2)AG(3))(3), thrombin-binding aptamer (TBA), (G(4)T(4)G(4))2, and (TG(4)T)4. The anisotropy decay curves show the monoexponential for free H(2)TMPyP4 and the biexponential upon binding to the excess amount of G-quadruplex DNAs. The biexponential anisotropy decay can be well interpreted using a wobbling-in-the-cone model. The orientational diffusion of the bound H(2)TMPyP4 is initially restricted to a limited cone angle within the G-quadruplex DNAs, and then an overall orientational relaxation of the G-quadruplex DNA-H(2)TMPyP4 complexes occurs in a longer time scale. It was found that the dynamics of the restricted internal rotation of bound H(2)TMPyP4 strongly depends on the ending structures of the G-quadruplex DNAs. According to the order parameter (Q) calculated from the wobbling-in-the-cone model, we deduce that the degree of restriction around the bound H(2)TMPyP4 follows the order of TBA > (TG(4)T)4 > AG(3)(T(2)AG(3))(3) > (G(4)T(4)G(4))2. Especially, based on the maximum order parameter (Q) of bound H(2)TMPyP4 within TBA, a new sandwich-type binding mode for TBA-H(2)TMPyP4 complex was proposed in which both terminal G-quartet and T*T base pair stack on the porphyrin ring through pi-pi interaction. This study thus provides a new insight into the interaction between G-quadruplex DNAs and H(2)TMPyP4.

A direct imaging of amphiphilic catalysts assembled at the interface of emulsion droplets using fluorescence microscopy

An amphiphilic fluorescent catalyst Q9[EuW10O36] (Q = [(C18H37)2N+ (CH3)2]), assembled in the interface of emulsion systems, was directly imaged by fluorescence microscopy; the catalyst shows high selectivity and activity in the oxidation of alcohols using H2O2 as oxidant and the catalyst can be easily separated and recycled by demulsifying.

The binding mode of porphyrins with cation side arms to (TG4T)4 G-quadruplex: Spectroscopic evidence

Interactions of 5,10,15,20-Tetrakis(N-methylpyridinium-4-yl)-21H,23H-porphyrin (TMPyP4) and 5,10,15,20-Tetrakis(N-propylpyridinium-4-yl)-21H,23H-porphyrin (TPrPyP4) with the parallel four-stranded (TG(4)T)4 G-quadruplex DNA in 100 mM K(+)-containing buffer were studied using circular dichroism (CD) spectroscopy, visible absorption titration, and steady and time-resolved fluorescence spectroscopies. The results show that the binding stoichiometric ratios of both TMPyP4 and TPrPyP4 to (TG(4)T)4 are 3:1. Two types of independent and nonequivalent binding sites with the higher and lower binding affinities are confirmed, and the stronger and weaker binding constants are 9.44x10(7) and 6.94x10(5) M(-1) for (TG(4)T)4-TMPyP4 complex, 7.86x10(7) and 6.35x10(5) M(-1) for (TG(4)T)4-TPrPyP4 complex, respectively. For both TMPyP4-(TG(4)T)4 and TPrPyP4-(TG(4)T)4 complexes, one porphyrin molecule stacks on the one end of G-quadruplex with the higher binding affinity, another two porphyrins bind weakly to the two external grooves. The size of cation side arms around porphyrin core almost fails to affect the binding mode, stoichiometry and affinity of porphyrin to (TG(4)T)4 G-quadruplex in 100 mM K(+)-containing buffer.

Na+/K+ switch of enantioselectivity in G-quadruplex DNA-based catalysis

Here we found that the enantioselectivity of G-quadruplex DNA-based Diels-Alder reaction can be switched by just changing Na(+) to K(+), which is ascribed to the structural transformation of the G-quadruplex from antiparallel to hybrid-type. By tuning the ratio of Na(+)/K(+), the enantioselectivity of the Diels-Alder reaction could be switchable and shows much more sensitive to K(+) than to Na(+).

Study on the interaction of porphyrin with G-quadruplex DNAs.

Interactions of 5,10,15,20-Tetrakis(N-propylpyridinium-4-yl)-21H,23H-porphyrin (TPrPyP4) with dimer hairpin (G4T4G4)2 and parallel four-stranded (TG4T)4 G-quadruplex DNAs in Na+-containing buffer were studied. The results show that two TPrPyP4 molecules bind to both G-quadruplexes by a noncooperative and nonequivalent binding mode, and there are one high affinity site and one low affinity site, the respective binding constants are 8.06 x 10(8) and 1.13 x 10(6) M(-1) for (G4T4G4)2-TPrPyP4, 8.04 x 10(7) and 9.08 x 10(5)M(-1) for (TG4T)4-TPrPyP4. TPrPyP4 presents two lifetimes of about 5.8 and 12.0 ns in the complexes of G-quadruplexes-TPrPyP4. The primary results suggest that two TPrPyP4 molecules bind to both G-quadruplexes by terminal stacking and outside binding mode.

Formation and stabilization of G-quadruplex in nanosized water pools

We demonstrate here that G-quadruplex structure can form and exhibits strong stability in nanosized water pools, providing new insight into investigating G-quadruplexes in the cellular environment.