Time-resolved and steady-state fluorescence studies of excited-state proton-transfer reactions of proflavine

Time-resolved and steady-state fluorescence studies of proflavine in aqueous solution are presented. The observation of a monoexponential fluorescence decay with a time constant decreasing with increasing pH and the presence of an anomalous red-shift in the fluorescence spectrum as a function of pH indicate the existence of a complex proton-transfer mechanism in the excited state. A reaction scheme is proposed and the corresponding proton-transfer rates are evaluated. An excited-state pK value of 12.85 is obtained for the equilibrium between the cationic form of proflavine and the same form dissociated at an amino group.     

Time-resolved and steady-state fluorescence spectroscopy of eumelanin and indolic polymers

Eumelanin plays a variety of important physiological roles in human skin. However, its structure and fundamental properties still remain poorly understood. Although the absorbance of eumelanin is broad and reveals little about its structure, a variety of techniques have revealed the presence of a disordered array of chromophores within the melanin compound. In order to examine the fluorescence decay dynamics of these chromophores, time-resolved spectroscopy was applied to solutions of synthetic eumelanin and a melanin-like polymer of N-methyl,5-hydroxy,6-methoxyindole (N-Me-5H6MI). Solutions were excited with 80 fs laser pulses at 355, 370, 390 and 400 nm, and decay time courses were acquired at 20 nm intervals between 400 and 600 nm for each excitation wavelength. Decay profiles for both eumelanin and the polymer exhibited a characteristic multiexponential behavior with decay times between 0.5 and 15 ns, although steady-state spectra for the polymer exhibited only two peaks. The long-decay component in the polymer showed a significant decrease in both amplitude (30-5%) and decay time (14-6 ns) with increasing emission wavelength. In contrast, the amplitude and decay time in melanin increased slightly (10-15% and 7-10 ns, respectively) from 400 to 520 nm emission, at which point they leveled off. These trends were consistent for all excitation wavelengths. These results suggest that the multiexponential behavior of melanin fluorescence is characteristic of each oligomer within the eumelanin compound, and is consistent with the assertion that the diversity of constituents within eumelanin provides it with a robustness in spectral properties.     

Time-resolved fluorescence spectroscopic and scanning near-field optical microscopic studies of rhodamine dye adsorbed in cationic Langmuir-Blodgett films

The fluorescence lifetimes decays and picosecond time-resolved fluorescence spectra were measured to investigate the dynamics of the excited state of sulforhodamine B (SRB) molecules adsorbed in the mono- and multilayered Langmuir-Blodgett (LB) films of octadecylamine. Steady-state and time-resolved fluorescence spectroscopy reveals that the fluorescence lifetimes and contents of the monomer and dimers in the molecular organizates depend upon the concentration of the dye in the solution and the adsorption process. SRB dye molecules adsorbed in LB films have been imaged with scanning near-field optical microscopy (SNOM). This information is exploited to map the distribution with molecular spatial resolution. SNOM provide the visual evidence of the monomers and dimers of SRB in cationic LB films.     

Time-resolved fluorescence and light scattering studies of water-soluble block-copolymer micelles with polyelectrolyte shells

Fluorescently-tagged diblock polystyrene-block-poly(methacrylic acid) copolymers were prepared by anionic polymerization and subsequent hydrolysis. The copolymers dissolve in 1, 4-dioxane-rich/water solvents and form micelles which may be transferred into water and buffers by a stepwise dialysis. Fluorescence techniques and quasielastic light scattering (QELS) were used to study the properties of those micelles.     

Time-resolved resonance Raman spectroscopic studies on the triplet excited state of thioxanthone

Thioxanthone has been investigated extensively owing to its unique photochemical and photophysical applications and its solvatochromic behavior. Here, we report the time-resolved resonance Raman studies on the structure of the lowest triplet excited state of thioxanthone in carbon tetrachloride. In addition, FT-IR and FT-Raman techniques have been used to study the vibrational structure in the ground state. To corroborate the experimental findings, density functional theory calculations have been carried out. Isotopic calculations and normal coordinate analysis have been used to help in assigning the observed bands to Raman vibrational modes. Structural information derived from this study is expected to help in better understanding the triplet state photochemistry of thioxanthone.     

Gas-phase spectroscopy of protonated 3-OH kynurenine and argpyrimidine. comparison of experimental results to theoretical modeling

The aging process of the human lens is associated with accumulation of chromophores and fluorophores that impair visual function. In the present study, we examined the photodissociation of 3-OH-kynurenine and argpyrimidine. Furthermore, absorption spectra obtained in gas phase using an electrostatic ion storage ring were studied as gas phase absorption have been shown to be more similar to the in vivo condition than absorption spectra obtained in the liquid phase. Experimental results were compared to theoretical modeling using the multistate, multireference perturbation theory approach combined with advanced molecular modeling tools to account for the solvent effects and to provide direct support for band assignments. Absorption maxima were determined both experimentally and theoretically and significant differences between the two chromophores were found. In particular, 3-OH-kynurenine demonstrated a blue-shift of more than 130 nm in the aqueous phase compared to the gas-phase due to the existence of different 3-OH-kynurenine conformers, which are stable under different conditions and originate from the interplay between intra- and intermolecular interactions. Photodissociation of argpyrimidine and 3-OH-kynurenine was observed in vacuum thus confirming the results previously obtained in liquid phase demonstrating that the photodestruction takes place in both media.     

Interaction of coumarin derivatives with human serum albumin: investigation by fluorescence spectroscopic technique and modeling studies

Interactions of several 7-aminocoumarins with human serum albumin (HSA) were studied by using fluorescence spectroscopic technique and modeling studies. There is a large change in fluorescence spectral parameters like intensity, emission maxima and anisotropy for all aminocoumarins. There were two binding sites for cou-1, 311 and a single binding site for other coumarins. The binding constant(s) are large for all coumarins reflective of a strong binding. These spectral studies show that structural variants at the third, fourth and seventh position affects binding. The probable location of these coumarins in domain II has been predicted based on modeling. The effect of structural modification on the efficiency of binding was obtained for various other coumarins, using modeling.     

Preparative, analytical and fluorescence spectroscopic studies of sulphonated aluminium phthalocyanine photosensitizers

Fluorescence spectroscopic studies were carried out on aluminium phthalocyanine with defined numbers (mono, di, tri and tetra) of sulphonate groups. Selective sulphonation was achieved using one of two synthetic methods to prepare a mixture of components which were separated using reverse-phase liquid chromatography. Fluorescence lifetimes were measured in methanol and buffer solution using time-correlated single-photon counting with picosecond laser excitation; the lifetime shows little variation with the number of sulphonate groups. Using steady state excitation, fluorescence quantum yields were determined for the tetrasulphonated component (phi F = 0.51) and, for comparison, unsulphonated aluminium phthalocyanine.     

Time-resolved spectroscopic studies of B(12) coenzymes: a comparison of the primary photolysis mechanism in methyl-, ethyl-, n-propyl-, and 5'-deoxyadenosylcobalamin.

An ultrafast transient absorption study of the primary photolysis of ethyl- and n-propylcobalamin in water is presented. Data have been obtained for two distinct excitation wavelengths, 400 nm at the edge of the UV gamma-band absorption, and 520 nm in the strong visible alphabeta-band absorption. These data are compared with results reported earlier for the B(12) coenzymes, methyl- and adenosylcobalamin. The data obtained for ethylcobalamin and n-propylcobalamin following excitation at 400 nm demonstrate the formation of one major photoproduct on a picosecond time scale. This photoproduct is spectroscopically identifiable as a cob(II)alamin species. Excitation of methyl-, ethyl-, and n-propylcobalamin at 520 nm in the low-lying alphabeta absorption band results in bond homolysis proceeding via a bound cob(III)alamin MLCT state. For all of the cobalamins studied here competition between geminate recombination of caged radical pairs and cage escape occurs on a time scale of 500 to 700 ps. The rate constants for geminate recombination in aqueous solution fall within a factor of 2 between 0.76 and 1.4 ns(-1). Intrinsic cage escape occurs on time scales ranging from 相似文献   

Time-resolved EPR, fluorescence, and transient absorption studies on phthalocyaninatosilicon covalently linked to one or two TEMPO radicals.

The photophysical properties of tetra-tert-butylphthalocyaninatosilicon (SiPc) covalently linked to one or two 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radicals (R1, R2) have been studied by fluorescence, transient absorption, and time-resolved electron paramagnetic resonance (TREPR) spectroscopies. It is found that the fluorescence quantum yields and lifetimes of R1 and R2 decrease compared with those of (dihydroxy)SiPc ((dihydroxy)SiPc = 6.8 ns, R1 = 4.7 ns and 42 ps, and R2 = 4.7 ns and <30 ps). Transient absorption measurements indicate that the lifetime of the excited triplet SiPc is markedly dependent on the number of linking TEMPO radicals ((dihydroxy)SiPc = 500 micros, R1 = 7.6 micros, and R2 = 3.7 micros). These short lifetimes of R1 and R2 in the excited states are explained as a result of the interaction with TEMPO changing the ISC between the singlet and triplet states to spin-allowed transitions. Quantitative TREPR investigations have been carried out for the radical-quartet pair mechanism of R1 and the photoinduced population transfer of R2. It is determined that the rise and decay times of these electron spin polarizations denote the spin-lattice relaxation time of the ground state and the lifetime of the excited multiplet state, respectively. This study contributes not only to an elucidation of radical-chromophore interactions but also to a novel approach for controlling magnetic properties by photoexcitation.     

Time-resolved spectroscopic studies of B12 coenzymes: comparison of the influence of solvent on the primary photolysis mechanism and geminate recombination of methyl-, ethyl-, n-propyl-, and 5'-deoxyadenosylcobalamin

A transient absorption study of the photolysis of methylcobalamin (MeCbl), ethylcobalamin (EtCbl), and n-propylcobalamin (PrCbl) in ethylene glycol spanning six decades in time, from 10 fs to 10 ns, is reported. These measurements probe the influence of solvent on the formation and decay of the metal-to-ligand charge transfer (MLCT) intermediate observed following excitation of MeCbl, the photolysis mechanism in EtCbl and PrCbl, and the rate constants for geminate recombination of the alkyl radicals with cob(II)alamin and for the escape of the alkyl radicals from the initial solvent cage. Earlier investigations probed the dynamics of 5'-dexoyadenosylcobalamin (coenzyme B(12)) in water and ethylene glycol (Yoder, L. M.; Cole, A. G.; Walker, L. A., II; Sension, R. J. J. Phys. Chem. B 2001, 105, 12180-12188) and alkylcobalamins in water (Cole, A. G.; Yoder, L. M.; Shiang, J. J.; Anderson, N. A.; Walker, L. A., II; Banaszak Holl, M. M.; Sension, R. J. J. Am. Chem. Soc. 2002, 124, 434-441). The results of these investigations are discussed in the context of the literature on the frictional influence of solvent on chemical reaction dynamics. The measurements allow a separation of the influence of the solvent on the intrinsic rate constant for geminate recombination and the rate constant for escape from the initial solvent cage. The rate constant for the intrinsic geminate recombination of cob(II)alamin with the alkyl radical is weakly dependent on the solvent and on the nature of the alkyl radical (Me, Et, Pr, or Ado). The Et, Pr, and Ado radicals exhibit the behavior expected for diffusion-controlled escape from the initial solvent cage. In contrast, the magnitude of cage escape for the Me radical is much larger than anticipated on the basis of hydrodynamic arguments.     

Time-resolved near-infrared and two-dimensional near-infrared correlation spectroscopic studies on polymerization of the silane coupling agent perfluorooctyltrimethoxysilane

 The polymerization behavior of perfluorooctyltrimethoxysilane (PFOS) in ethanol, which is acid-catalyzed by 0.25 M HCl, has been examined using time-resolved near-IR and 2D near-IR correlation spectra. In the time-resolved near-IR spectra, the bands at 5164 and 4825 cm⁻¹ have been assigned to the combination bands of water and ethanol OH groups, respectively. It has been found that the absorbance variation of the two near-IR bands occurs in a two-step process. The absorbance of the 5164 cm⁻¹ band rapidly decreases in the initial step but increases exponentially in the second step, while that of the 4825 cm⁻¹ band rapidly increases both in the initial step and, exponentially, in the second step. These results indicate that the time-dependent absorbance variation of the two near-IR bands reflects the polymerization process of PFOS, in which consumption and release of water molecules and release of methanol in the two-step process occur as a consequence of the acid-catalyzed hydrolysis of methoxy groups and the formation of silanols (SiOH) to form a siloxane bond. It has also been found that this polymerization process is distinctly reflected in the 2D near-IR correlation spectra. Received: 23 November 1999/Received in Revised form: 23 February 2000/Accepted 4 March 2000     

Time-resolved fluorescence studies on the formation and decomposition of the aromatic hydrocarbon-aliphatic amine exciplex in solution

Fluorescence rise and decay processes were measured and rate parameters were determined for the pyrene-tri-n-butylamine and pyrene-N,N-diethylaniline exciplex system in various solvents. An additional activation energy over that for the diffusional motion appears necessary for both exciplex formation and the deactivating quenching processes in the case of the pyrene-tri-n-butylamine system. The rate-determining step for these processes is electron transfer in the encounter collision leading to the nonrelaxed electron transfer state.     

Molecular modeling and spectroscopic studies on the binding of guaiacol to human immunoglobulin

The fluorogenic property of guaiacol was exploited for the first time to analyze the interaction with target protein as a probe by molecular modeling, fluorescence, circular dichroism (CD) and Fourier transform infrared (FT-IR) spectroscopy. Molecular docking was performed to reveal the possible binding mode or mechanism and suggested that guaiacol can strongly bind to human immu- noglobulin (HIgG). It is considered that guaiacol binds to HIgG mainly by a hydrophobic interaction and there are two hydrogen bond interactions between the drug and the residues LEU 80 and ASP 65, which is in good agreement with the results from the experimental thermodynamic parameters (the enthalpy change △H0 and the entropy change △S0 were calculated to be 65.55 kJ·mol-1 and 132.95 J·mol-1·K-1 according to the Vant’ Hoff equation). Data obtained by the fluorescence spectroscopy indicated that binding of guaiacol with HIgG leads to dramatic enhancement in the fluorescence emission intensity along with significant occurrence of efficient Frster resonance energy transfer (FRET) from the residue of HIgG to the protein bound guaiacol. From the low value of fluorescence anisotropy (r = 0.06), it is argued that the probe molecule is located in the motionally unrestricted environment of the protein. The alterations of protein’s secondary structure in the presence of guaiacol in aqueous solution were quantitatively calculated by the evidences from FT-IR and CD spectroscopes.     

Interactions of the plant flavonoid fisetin with macromolecular targets: insights from fluorescence spectroscopic studies

Fisetin (3,7,3',4'-tetrahydroxyflavone) is a bioactive plant flavonoid of immense importance as a potentially useful therapeutic drug, for various free radical mediated as well as other diseases. In a recent paper, we demonstrated the novel uses of the exquisitely sensitive intrinsic fluorescence of this compound to explore its binding characteristics in liposomal membranes [B. Sengupta, A. Banerjee, P.K. Sengupta, Investigations on the binding and antioxidant properties of the plant flavonoid fisetin in model biomembranes, FEBS Lett. 570 (2004) 77-81]. Here, we have exploited this technique to examine its interactions with relevant macromolecular targets, namely double stranded DNA (from calf thymus), and the physiologically important circulatory protein, Human Serum Albumin (HSA). In the presence of DNA dramatic changes are observed in the intrinsic fluorescence behaviour of fisetin. These, along with other relevant supporting spectroscopic data, suggest that fisetin binds intercalatively between the base pairs of DNA. From the studies on fisetin-HSA interaction, the existence of two distinct binding sites are inferred. Furthermore we present evidence for the occurrence of efficient F?rster type fluorescence resonance energy transfer from tryptophan to fisetin, indicating that both binding sites of fisetin in HSA are proximal to the unique tryptophan - 214 residue present in the interdomain (between IIA and IIIA domains) loop region of the protein.     

Time-resolved absorption spectroscopic studies on the reaction of conformationally fixed o-(9-fluorenyl)phenylnitrenes

Laser-flash photolysis studies have been carried out on a solution of ap- and sp-9-(2-azido-4,6-dimethylphenyl)fluorenes (3) in methanol-ether at 25°C. The rates of formation of azanorcaradiene (5) from ap-nitrene (4) ap, and of 9-methoxyfluorene (7) from o-quinoid intermediate (6) were determined. The deuterium isotope effect on the migration of the hydrogen is discussed.     

Interaction of acrylodan with human serum albumin. A fluorescence spectroscopic study

The binding of the fluorescent probe acrylodan (AC) to human serum albumin (HSA) was studied by fluorescence spectroscopy. The binding isotherms could be fitted to two types of sites. Competition experiments using iodoacetamide suggested that AC binds tightly on HSA by the cysteine-34. Attempts were made to find the location of the second site using high concentrations of warfarin, phenylbutazone, diazepam, indomethacin, palmitic acid or bilirubin in order to displace the bound AC to the HSA. Bilirubin was the only ligand able to displace the bound AC. This result suggests that AC, which is a very hydrophobic molecule also capable of labeling lysine residues, should also bind the human albumin in the primary site of bilirubin, but with less affinity than to the cysteine-34.     

Time-resolved spectroscopic behavior of Fe2O3 and ZnFe2O4 nanocrystals

Using nanosecond (ns) and femtosecond (fs) time-resolved absorption spectroscopies (pump-probe technique), the carrier dynamics in transition metal oxide nanocrystals of alpha-Fe2O3 and ZnFe2O4 was studied during the photolysis process. For Fe2O3 and ZnFe2O4 nanocrystals, the fs measurements detect similar profiles of a positive nonlinear absorption in their capped nanocrystals, whereas much weak signals in the naked particles. In the nanosecond measurements Fe2O3 and ZnFe2O4 nanocrystals show obvious excitation-power dependent absorption properties and at the low pump power they show weak photobleaching, but at high pump power they produce positive nonlinear absorptions. For Fe2O3 nanocrystals, the threshold power of negative absorption (bleach) to positive absorption increases with reducing size, whereas for the ZnFe2O4 samples, the threshold powers reach minimum at a critical size of 11 nm, grow for both the bigger and the smaller nanocrystals. These results reflect the influences of their microscopic magnetic couplings and carrier correlation on biexciton absorption in Fe2O3 and ZnFe2O4 nanocrystals. All the results indicate that the time resolved photoabsorption techniques are useful to study the microscopic spin interactions and carrier correlations in transition metal oxide nanocrystals.     

时间分辨表面增强拉曼光谱(TRSERS)研究硫脲衍生物与ClO4^-的共吸附行为

运用时间分辨表面增强拉曼光谱(TRSERS)结合电位双阶跃的方法, 研究了硫脲衍生物甲基硫脲(MTU)和烯丙基硫脲(ATU)在银电极表面与ClO4^-离子的共吸附行为, SERS强度-时间曲线表明它们各自相应的SERS谱峰强度随电位阶跃的响应速率不同, 离电极表面较近的基团的特征谱峰强度的响应速率较快; MTU以S端且垂直吸附在电极表面,ATU也以S端和表面发生化学吸附, 但整个分子斜躺吸附在电极表面上。     

Time-resolved microspectrofluorimetry and fluorescence lifetime imaging of hypericin in human retinal pigment epithelial cells

Hypericin is the active ingredient of the off-the-shelf antidepressant St. John's Wort. It is an effective phototoxic agent and its systemic administration at therapeutic doses could induce particular damage in the eye due to continuous light exposure. Hypercin is strongly fluorescent and its fluorescence properties can be monitored to investigate noninvasively its localization and interactions. To this aim, time-resolved microspectrofluorimetry and fluorescence lifetime imaging were used to assess the spectral and temporal properties as well as the spatial distribution of the fluorescence emitted by retinal pigment epithelium (RPE) cells treated with Hyp at concentrations in the micromolar range (0.5-10 microM). In the presence of hypericin, the emission peaks at 600-605 nm and the fluorescence decay is best fitted with three lifetimes (5.5-7 ns, 1.9-2.5 ns and <0.8 ns). Spectral and temporal differences were observed between high (> or =5 microM) and low hypericin concentrations. In particular, upon increasing concentration, the emission spectrum of the slow component broadens and its lifetime shortens. The latter change is observed also when high concentrations are reached locally, due to more efficient localization within the cell.