Qin-Hua Song

Ratiometric two-photon fluorescent probes for mitochondrial hydrogen sulfide in living cells.

Hydrogen sulfide (H2S) is an important signaling molecule with diverse biological roles. Various fluorescent probes for H2S with biological application have been developed. However, two-photon ratiometric imaging of mitochondrial H2S is scarce. In this paper, we report two ratiometric two-photon probes, AcHS-1 and AcHS-2, which employ 4-amino-1,8-naphthalimide as the fluorophore and 4-azidobenzyl carbamate as the H2S response site. These probes exhibit high selectivity toward H2S over biothiols and other reactive species, low detection limits of 50-85 nM, low cytotoxicity, and high stability under physiological conditions. Furthermore, through cell imaging with one-photon and two-photon microscopy, MCF-7 cells incubated with two probes show a marked change in emission color from blue to green in response to H2S. Cell images costraining with a mitochondrial dye reveal that AcHS-2 is a mitochondria-specific two-photon probe for H2S. These results show that AcHS-2 may find useful applications in biological research such as tracking mitochondrial H2S in living biological specimens.

New 2,6-modified BODIPY sensitizers for dye-sensitized solar cells

Abstract A series of new metal-free organic dyes with either a boron dipyrromethene (BODIPY)-phenylene or -thiophene as a π-conjugated bridge have been synthesized for application in dye-sensitized solar cells. The photophysical and electrochemical properties of these dyes were investigated and their performance as sensitizers in dye-sensitized solar cells has been measured. The structure–property relationship shows that the introduction of a methoxy group as the donor and a BODIPY-thiophene unit as the π-conjugated bridge are favorable to improve the efficiency of DSSCs. A combination of a methoxy modified donor and BODIPY-thiophene bridge possesses a stronger electron-donating ability and longer wavelength absorption band, and as a sensitizer reveals the best properties of DSSCs, whose conversion efficiency was 2.26%.

Quinolinium-Based Fluorescent Probes for the Detection of Thiophenols in Environmental Samples and Living Cells

A new type of fluorescent probes for thiophenols, 6HQM-DNP and 7HQM-DNP, containing 6- or 7-hydroxy quinonlinium as fluorophore and 2,4-dinitrophenoxy (DNP) as nucleophilic recognition unit were constructed. As ethers, these non-fluorescent probe molecules can release the corresponding fluorescent quinolinium (6HQM and 7HQM) through aromatic nucleophilic substitution (S(N)Ar) by thiolate anions from thiophenols. The sensing reaction is highly sensitive (detection limit of 8 nM for 7HQM-DNP) and highly selective to thiophenols over aliphatic thiols and other nucleophiles under neutral conditions (pH 7.3). The probes respond rapidly to thiophenols, with second-order rate constants k=45 M(-1)  s(-1) for 7HQM-DNP and 24 M(-1)  s(-1) for 6HQM-DNP. Furthermore, the selective detection of thiophenols in living cells by 7HQM-DNP was demonstrated by confocal fluorescence imaging. In addition, these quinolinium salts show excellent chemical and thermal stability. In conclusion, this type of probes may find use in the detection of thiophenols in environmental samples and biosystems.

Thermodynamics and Conformations in the Formation of Excited States and Their Interconversions for Twisted Donor-Substituted Tridurylboranes

We synthesized a series of donor-substituted tridurylboranes containing different types and number of chromophores including 1-pyrene (PB1-3), 3-carbazole (CBC1-3), or substituted p-carbazol-N-phenyl (CBN3a-c) as various donor-acceptor (D-A) molecules. The photophysical and electrochemical properties of these twisted D-A molecules were investigated by means of UV/Vis absorption and fluorescence spectroscopy as well as cyclic voltammetry (CV). Solvent polarity, viscosity, and temperature effects on the fluorescence emission reveal the existence of three types of excited states, and their equilibria and interconversions between three excited states. In increasing order of the charge-separated extent and the conformational change, three excited states are the locally excited (LE) state, the more planar intramolecular charge-transfer (ICT) state, and the more twisted ICT (TICT) state as compared to the ground state. The TICT state undergoes a conformational change with a higher energy barrier over the ICT state. The solvent polarity effect on the state conversion is opposite to the viscosity effect, and temperature effects derive from its resulting changes of polarity and viscosity. For example, the increase of the polarity of the solvent results in excited-state conversions from the LE state to the ICT state, and/or from the ICT to the TICT state, and an increased viscosity leads to the opposite conversions. On the basis of electrochemical and spectral data, thermodynamics of a possible ICT process were estimated, and correlated with the excited-state character. Finally, three excited states have been characterized by the conformation, the photophysical properties, and the thermodynamics of the ICT processes.

Active Site of Escherichia coli DNA Photolyase: Asn378 Is Crucial both for Stabilizing the Neutral Flavin Radical Cofactor and for DNA Repair †

Escherichia coli DNA photolyase repairs cyclobutane pyrimidine dimer (CPD) in UV-damaged DNA through a photoinduced electron transfer mechanism. The catalytic activity of the enzyme requires fully reduced FAD (FADH (-)). After purification in vitro, the cofactor FADH (-) in photolyase is oxidized into the neutral radical form FADH (*) under aerobic conditions and the enzyme loses its repair function. We have constructed a mutant photolyase in which asparagine 378 (N378) is replaced with serine (S). In comparison with wild-type photolyase, we found N378S mutant photolyase containing oxidized FAD (FAD ox) but not FADH (*) after routine purification procedures, but evidence shows that the mutant protein contains FADH (-) in vivo as the wild type. Although N378S mutant photolyase is photoreducable and capable of binding CPD in DNA, the activity assays indicate the mutant protein is catalytically inert. We conclude that the Asn378 residue of E. coli photolyase is crucial both for stabilizing the neutral flavin radical cofactor and for catalysis.

A novel fluorescent probe for selective detection of thiols in acidic solutions and labeling of acidic organelles in live cells

A novel fluorescent probe, quinoline α,β-unsaturated diacid (QMA), as compared to its ester QME, was constructed and can selectively detect thiols in acidic solutions (pH < 7) via a H-bond activated Michael addition. Furthermore, labeling of lysosomes and cell imaging were achieved by the detection of biothiols in different domains of live cells.

The equilibria and conversions between three excited states: the LE state and two charge transfer states, in twisted pyrene-substituted tridurylboranes

Excited-state conversions were observed from a series of twisted pyrene-substituted tridurylboranes, corresponding to a locally excited (LE) state, a more planar charge transfer (CT) state with a higher fluorescence quantum efficiency, and a more twisted CT state.

Model studies of the (6-4) photoproduct photoreactivation: efficient photosensitized splitting of thymine oxetane units by covalently linked tryptophan in high polarity solvents.

Three covalently linked tryptophan-thymine oxetane compounds used as a model of the (6-4) photolyase-substrate complex have been prepared. Under 290 nm light, efficient splitting of the thymine oxetane with aromatic carbonyl compounds gives the thymine monomer and the corresponding carbonyl compounds by the covalently linked tryptophan via an intramolecular electron transfer, and exhibits a strong solvent dependence: the quantum yield (Phi) is ca. 0.1 in dioxane, and near 0.3 in water. Electron transfer from the excited tryptophan residue to the oxetane unit is the origin of fluorescence quenching of the tryptophan residue, and is more efficient in strong polar solvents. The splitting efficiency of the oxetane radical anion within the tryptophan.+-oxetane.- species is also solvent-dependent, ranging from ca. 0.2 in dioxane to near 0.35 in water. Thus, the back electron transfer reaction in the charge-separated species would be suppressed in water, but is still a main factor causing low splitting efficiencies in the tryptophan-oxetane systems. In contrast to the tryptophan-oxetane system, fast nonradiation processes are the main causes of low efficiency in the flavin-oxetane system. Hence, nonradiative processes of the excited FADH-, rather than electron transfer to oxetane, may be an important factor for the low repair efficiency of (6-4) photolyase.

Efficient photosensitized splitting of the thymine dimer/oxetane unit on its modifying β-cyclodextrin by a binding electron donor

Two modified beta-cyclodextrins (beta-CDs) with a thymine dimer and a thymine oxetane adduct respectively, TD-CD and Ox-CD, have been prepared, and utilized to bind an electron-rich chromophore, indole or N,N-dimethylaniline (DMA), to form a supramolecular complex. We have examined the photosensitized splitting of the dimer/oxetane unit in TD-CD/Ox-CD by indole or DMA via an electron-transfer pathway, and observed high splitting efficiencies of the dimer/oxetane unit. On the basis of measurements of fluorescence spectra and splitting quantum yields, it is suggested that the splitting reaction occurs in a supramolecular complex by an inclusion interaction between the modified beta-CDs and DMA or indole. The back electron transfer, which leads low splitting efficiencies for the covalently-linked chromophore-dimer/oxetane compounds, is suppressed in the non-covalently-bound complex, and the mechanism has been discussed.

Photo-Ritter Reaction of Arylmethyl Bromides in Acetonitrile

The photo-Ritter reaction of five arylmethyl bromides can occur in acetonitrile to give acetamides. The intermediates, carbocations, which are formed from subsequent electron transfer between the radical pairs generated from initial homolytic cleavage of the C–Br bond, are trapped by acetonitrile, and subsequent hydrolysis generates the corresponding acetamides.