Photophysical properties and photobiological activity of the furanochromones visnagin and khellin

Surface-enhanced Raman scattering detection of dibenzothiophene and its derivatives without π acceptor compound using multilayer Ag NPs modified glass fiber paper

Dibenzothiophene (DBT) and its derivatives are important constituents of organosulfur compound in fossil fuels, which can result in one type of "acid rain" after burning process. Several technologies and methods have been developed to detect DBT, but they can be sophisticated and expensive. We have developed a two-step in-situ reduction method to fabricate Ag NPs modified glass fiber paper for SERS detection of DBT and its derivatives in a convenient and cheap way. Different from previous reports showing DBT cannot be detected by SERS, the substrate fabricated by our method revealed DBT's characteristic Raman peak at 1600 cm^-1. The dense and multilayer Ag NPs on glass fiber provided abundant spatial surface for DBT absorption and chemical interaction with Ag NPs, which led to CHEM enhancement in SERS detection. The mechanism was verified by UV-visible absorption spectrum and calculated Raman spectra. There was a good linear relationship between the SERS intensity at 1600 cm^-1 and the concentration of DBT solution between 1 × 10 ^-5 and 1 × 10^-3 mol/L and the limit of detection was 1 × 10^-6 mol/L. Spiked petrol sample was detected and the recovery rate of DBT is in the range of 94.53%-107.39%. This method provides a convenient and reliable way to detect DBT and its derivatives.

Spectral and Photophysical Behavior of Cytisine in n-Hexane. Experimental Evidence for the S1(n,π*) → S0 Fluorescence

Spectral and photophysical properties of (-)-cytisine (the compound used as a smoking cessation aid and a potential drug in Alzheimer's and Parkinson's diseases) were investigated. The two conformers of cytisine, whose presence in the S₀ state has been earlier proved by the NMR and IR methods as well as in theoretical calculation, in nonpolar n-hexane show a rarely observed prompt fluorescence from the S₁(n,π*) excited state. This observation is unambiguously evidenced by very small radiative rate constants of these two emitting conformers, k_F = 7.4 × 10⁵ and 3.0 × 10⁵ s^-1. Their lifetimes in the S₁(n,π*) state are relatively long, τ_S1 = 1.9 and 6.7 ns; therefore, their fluorescence quantum yield is relatively high ϕ_F ∼ 10^-3. The long-wavelength band in the cytisine absorption originates from the excitation to the S₂(π,π*) state, while the S₁(n,π*) state is not observed in this spectrum. Thus, the excited state S₂(π,π*) is manifested only in the absorption spectrum, while the excited state S₁(n,π*) is seen only in the fluorescence spectrum, so cytisine in n-hexane is characterized by close lying (n,π*) and (π,π*) excited singlet states.

Ultraviolet photoionization of the photosensitizers khellin and visnagin in aqueous solution and in micelles: one-photon ionization is a minor process

One-photon ionization, leading to formation of hydrated electrons and radical cations, has been proposed as a possible mechanism of action of some sensitizers in photobiology. In this contribution, we have investigated this proposal for the compounds khellin and visnagin, used in photomedical applications. Nanosecond transient absorption spectroscopy covering a wide range of laser pulse energies was employed to measure the formation of radical cations and hydrated electrons in aqueous solution and in cationic (CTAB) as well as anionic (SDS) micellar solutions. A model allowing for simultaneous one- and two-photon processes and fully accounting for the nonlinearity of the pulse energy dependence was used to simulate the data. The results did not support the hypothesis of a significant role of one-photon ionization, the upper limits of the quantum yields of radical cation formation being phi < 0.01 for visnagin and phi < 0.004 for khellin.

Protein Stabilization by Osmolytes from Hyperthermophiles

2-O-alpha-Mannosylglycerate, a negatively charged osmolyte widely distributed among (hyper)thermophilic microorganisms, is known to provide notable protection to proteins against thermal denaturation. To study the mechanism responsible for protein stabilization, pico-second time-resolved fluorescence spectroscopy was used to characterize the thermal unfolding of a model protein, Staphylococcus aureus recombinant nuclease A (SNase), in the presence or absence of mannosylglycerate. The fluorescence decay times are signatures of the protein state, and the pre-exponential coefficients are used to evaluate the molar fractions of the folded and unfolded states. Hence, direct determination of equilibrium constants of unfolding from molar fractions was carried out. Van't Hoff plots of the equilibrium constants provided reliable thermodynamic data for SNase unfolding. Differential scanning calorimetry was used to validate this thermodynamic analysis. The presence of 0.5 m potassium mannosylglycerate caused an increase of 7 degrees C in the SNase melting temperature and a 2-fold increase in the unfolding heat capacity. Despite the considerable degree of stabilization rendered by this solute, the nature and population of protein states along unfolding were not altered in the presence of mannosylglycerate, denoting that the unfolding pathway of SNase was unaffected. The stabilization of SNase by mannosylglycerate arises from decreased unfolding entropy up to 65 degrees C and from an enthalpy increase above this temperature. In molecular terms, stabilization is interpreted as resulting from destabilization of the denatured state caused by preferential exclusion of the solute from the protein hydration shell upon unfolding, and stabilization of the native state by specific interactions. The physiological significance of charged solutes in hyperthermophiles is discussed.

Fluorometric determination of khellin in human urine and serum by high-performance liquid chromatography using postcolumn photoirradiation

For the determination of khellin in urine and serum, fluorometry using HPLC-postcolumn photoirradiation has been developed. Khellin and visnagin of similar structure were separated on a column of Capcell Pak C8. The mobile phase consisted of 40%(v/v) ethanol containing 75 mmol l(-1) H2O2. The postcolumn reagent, 70 mmol l(-1) KH2PO4-NaOH buffer (pH 12.7) containing 50%(v/v) ethanol, were mixed with the mobile phase, which was irradiated with ultraviolet light to induce fluorescence. The fluorescence was monitored with excitation at 378 nm and emission at 480 nm. The calibration graph for khellin was linear over the range of 65 - 2620 ng ml(-1) using an injection volume of 20 microl. The pretreatment of the urine or serum samples consisted of diluting steps or deproteinizing steps using perchloric acid, respectively.

Photobiological properties of hydroxy-substituted flavothiones

Flavothione (FT) and a series of 18 hydroxy- and methoxy-substituted flavothiones were screened for photobiological activity. The 5-hydroxy-substituted compounds (group 3) and the methoxy-substituted flavothiones were inactive. FT and the remaining hydroxy-substituted compounds, all displayed photobiological activity. Among these, the 3-hydroxy-substituted compounds (group 2) were the most efficient photosensitizers overall in spite of their concurrent fast photodegradation. FT and all other hydroxyflavothiones, not substituted in the 3- or 5-positions (group 1), were inefficient compared with group 2. Detailed photobiological tests were carried out for four flavothiones of groups 1 and 2. The biological tests included fungi, several strains of Escherichia coli, Salmonella typhimurium and mammalian cells. In addition, the ability of these flavothiones to perform lipid peroxidation was evaluated. FT and 6-hydroxyflavothione (group 1) induce DNA damage via H-atom abstraction from the lowest n, pi* triplet state of the thione (oxygen independent). For 3-hydroxy and 3,6-dihydroxyflavothione (group 2), both DNA and the membrane are targets. The mechanism likely involves both energy transfer and electron transfer from the lowest pi, pi* triplet state to oxygen, to form singlet oxygen and the superoxide anion. Some of these compounds could be considered as models for environmentally safe photopesticides.

Furanochromone radical cations: generation, characterization and interaction with DNA

The radical cations of naturally occurring furanochromones visnagin (VI) and khellin (KH) have been generated and identified for the first time by use of laser flash photolysis and pulse radiolysis techniques. The lifetimes of VI(.+) and KH(.+) are determined as approximately 6 and approximately 35 micros under these conditions, respectively. Direct 308-nm excitation of VI in aqueous buffer at physiological pH results in monophotonic photoionization to generate VI(.+), with a quantum yield of 0.075, which is much higher than that of 8-methoxypsoralen and KH under identical conditions. Though VI(.+) is a more powerful oxidant than KH(.+), both of them react with guanosine mononucleotide (k=1.2x10(9) and 3.8x10(7) dm(3) mol(-1) s(-1), respectively) via electron transfer to give the guanine radical cation. Furthermore, selective oxidation of guanine in single and double strand DNA by VI(.+) was also observed. These novel findings suggest that electron transfer reactions involving furanochromone radical cations may be of considerable importance in furanochromone photochemotherapy.