Synthesis and optical properties of ordered-vacancy perovskite cesium bismuth halide nanocrystals.

Perovskite-phase cesium bismuth halide (Cs3Bi2X9; X = Cl, Br, I) nanocrystals were synthesized using a hot-injection approach. These nanocrystals adopted ordered-vacancy perovskite crystal structures and demonstrated composition-tunable optical properties. Growth occurred by initial formation of Bi0 seeds, and morphology was controlled by precursor and seed concentration. The Cs3Bi2I9 nanocrystals demonstrated excellent stability under ambient conditions for several months. Contrary to previous reports, we find that photoluminescence originates from the precursor material as opposed to the Cs3Bi2X9 nanocrystals.

Experimental and theoretical investigations of solvation dynamics of ionic fluids: appropriateness of dielectric theory and the role of DC conductivity.

An analysis is provided of the subnanosecond dynamic solvation of ionic liquids in particular and ionic solutions in general. It is our hypothesis that solvation relaxation in ionic fluids, in the nonglassy and nonsupercooled regimes, can be understood rather simply in terms of the dielectric spectra of the solvent. This idea is suggested by the comparison of imidazolium ionic liquids with their pure organic counterpart, butylimidazole (J. Phys. Chem. B 2004, 108, 10245-10255). It is borne out by a calculation of the solvation correlation time from frequency dependent dielectric data for the ionic liquid, ethylammonium nitrate, and for the electrolyte solution of methanol and sodium perchlorate. Very good agreement is obtained between these theoretically calculated solvation relaxation functions and those obtained from fluorescence upconversion spectroscopy. Our comparisons suggest that translational motion of ions may not be the predominant factor in short-time solvation of ionic fluids and that many tools and ideas about solvation dynamics in polar solvents can be adapted to ionic fluids.

Synthesis of hydroxy and methoxy perylene quinones, their spectroscopic and computational characterization, and their antiviral activity.

Hydroxy and methoxy perylene quinones are synthesized in an attempt to isolate the essential spectroscopic and biological features of light-induced antiviral agents such as hypericin and hypocrellin. Unlike their naturally occurring counterparts, these synthetic quinones bear the carbonyl, hydroxyl, and methoxy groups in the "bay region." The hydroxy and methoxy compounds have rich absorption spectra with broad features in the visible (approximately 450-800 nm) and relatively more intense and narrow features at wavelengths < or = 350 nm. High-level ab initio quantum mechanical calculations assign the features in the absorption spectra to electronic transitions from S0 to S2 and to higher-lying electronic states. The calculations indicate that in the ground state the trans dihydroxy isomer is 12.5 kcal/mol lower in energy than the cis dihydroxy isomer and is thus the only species present. The lowest-energy trans methoxy ground state isomer and the lowest-energy cis methoxy ground state isomer are found to be degenerate. An additional cis methoxy isomer 6.3 kcal/mol higher in energy than the global minimum is assumed to contribute to the spectrum and is also considered. Finally, the synthetic compounds exhibit similar light-induced antiviral activity to each other, but significantly less than that of hypericin.

Interaction of glutathione S-transferase with hypericin: A photophysical study.

The photophysics of hypericin have been studied in its complex with two different isoforms, A1-1 and P1-1, of the protein glutathione S-transferase (GST). One molecule of hypericin binds to each of the two GST subunits. Comparisons are made with our previous results for the hypericin/human serum albumin complex (Photochem. Photobiol. 1999, 69, 633-645). Hypericin binds with high affinity to the GSTs: 0.65 microM for the A1-1 isoform and 0.51 microM for the P1-1 isoform (Biochemistry 2004, 43, 12761-12769). The photophysics and activity of hypericin are strongly modulated by the binding protein. Intramolecular hydrogen-atom transfer is suppressed in both cases. Most importantly, while there is significant singlet oxygen generation from hypericin bound to GST A1-1, binding to GST P1-1 suppresses singlet oxygen generation to almost negligible levels. The data are rationalized in terms of a simple model in which the hypericin photophysics depends entirely upon the decay of the triplet state by two competing processes, quenching by oxygen to yield singlet oxygen and ionization, the latter of these two are proposed to be modulated by A1-1 and P1-1.

The complex of apomyoglobin with the fluorescent dye coumarin 153.

Understanding a protein's dielectric response requires both a theoretical model and a well-defined experimental system. The former has already been proposed by Song (J. Chem. Phys. 116, 9359 [2002]). We suggest that the latter is provided by the complex of coumarin 153 (C153) with apomyoglobin (ApoMb). C153 has been exhaustively studied and has proven to be an excellent probe of the solvation dynamics of polar solvents. Myoglobin is one of the most thoroughly studied proteins. Myoglobins from a wide range of species have been subject to X-ray structural analysis and site-directed mutagenesis. Here, we demonstrate the existence of a robust C153-apomyglobin system by means of molecular dynamics simulations, equilibrium binding studies using a Job's plot and capillary electrophoresis, circular dichroism and time-resolved fluorescence. The reorganization energy of C153 bound to ApoMb is compared with that of C153 in bulk solvent using the method of Jordanides et al. (J. Phys. Chem. B 103, 7995 [1999]).

Does solvent influence the ground-state tautomeric population of hypericin?

Nuclear magnetic resonance measurements indicate that hypericin exists in the same "normal" tautomeric form irrespective of whether the solvent is dimethyl sulfoxide or tetrahydrofuran. This result is discussed in the context of previous experimental and theoretical work. It is concluded that solvent perturbations cannot induce tautomerization in hypericin.

Toward the molecular flashlight: preparation, properties, and photophysics of a hypericin-luciferin tethered molecule.

The synthesis of a molecule containing hypericin and luciferin moieties joined by a tether is reported. The light-induced (in vitro) antiviral activity as well as the photophysical properties of this new compound are measured and compared with those of the parent compounds, hypericin and pseudohypericin. This tethered molecule exhibits excited-state behavior that is very similar to that of its parent compounds and antiviral activity that is identical, within experimental error, to that of its more closely related parent compound, pseudohypericin. The implications for a photodynamic therapy that is independent of external light sources are discussed.

Environment of tryptophan 57 in porcine fructose-1,6-bisphosphatase studied by time-resolved fluorescence and site-directed mutagenesis.

The environment of Trp57, introduced by the mutation of a tyrosine in the dynamic loop of porcine liver fructose-1,6-bisphosphatase (FBPase), was examined using time-resolved fluorescence and directed mutation. The Trp57 enzyme was studied previously by X-ray crystallography and steady-state fluorescence, the latter revealing an unexpected redshift in the wavelength of maximum fluorescence emission for the R-state conformer. The redshift was attributed to the negative charge of Asp127 in contact with the indole side chain of Trp57. Time-resolved fluorescence experiments here reveal an indole side chain less solvent exposed and more rigid in the R-state, than in the T-state of the enzyme, consistent with X-ray crystal structures. Replacement of Asp127 with an asparagine causes a 6 nm blueshift in the wavelength of maximum fluorescence emission for the R-state conformer, with little effect on the emission maximum of the T-state enzyme. The data here support the direct correspondence between X-ray crystal structures of FBPase and conformational states of the enzyme in solution, and provide a clear example of the influence of microenvironment on the fluorescence properties of tryptophan.

Identification of a vibrational frequency corresponding to H-atom translocation in hypericin.

Using time-resolved infrared spectroscopy, ab initio quantum mechanical calculations and synthetic organic chemistry a region in the infrared spectrum of triplet hypericin has been found between 1400 and 1500 cm-1 corresponding to the translocation of the hydrogen atom between the enol and the keto oxygens, O...H...O. This result is discussed in the context of the photophysics of hypericin and of eventual measurements to observe directly the excited-state H-atom transfer.

Tumor cell toxicity of hypericin and related analogs.

A series of hypericin analogs were found to differ in their cytotoxic activity induced by ambient light levels. These analogs vary in their ability to partition into cells, to generate singlet oxygen as well as in other photophysical properties. The data suggest that the biological activity of hypericin is due to a combination of factors whose roles may vary under different circumstances.

Steady-state and time-resolved spectroscopy of F420 extracted from methanogen cells and its utility as a marker for fecal contamination.

Methanogenic bacteria, which are common inhabitants of the animal digestive tract, contain the fluorescent compound F420 (coenzyme 420), a 7,8-didemethyl-8-hydroxy-5-deazariboflavin chromophore. F420 was characterized as an initial step in determining if this compound would be useful as a fluorescent marker for the detection of fecal and ingesta contamination. Using a single anion exchange chromatographic process, F420 was separated from other cell components of a Methanobrevibacter sp. cell culture. The extent of separation was determined spectroscopically. To aid in the development of possible techniques for the detection of fecal contamination using F420 as a marker, further spectroscopic investigation of F420 was conducted using steady-state and time-resolved fluorescence methods. The fluorescence lifetime of F420 in an elution buffer of pH 7.5 was found to be 4.2 ns. At higher pH values, the fluorescence decay, F(t), was best described by a sum of two exponentials: at pH 13, F(t) = 0.31 exp(-t/4.20 ns) + 0.69 exp(-t/1.79 ns). Further investigation using front-faced fluorescence techniques has shown that emission from F420 can be collected efficiently from samples of methanogen cell cultures as well as from fecal material.

Is the excited-state H-atom transfer in hypericin concerted?

The excited-state intramolecular H-atom transfer of hypericin (Hyp) was investigated as a function of pH in monodispersed reverse micelles formed by sodium bis(2-ethylhexyl)sulfosuccinate/heptane/water and in complexes with Tb3+ under conditions in which one of the two carbonyl groups of Hyp is incapable of accepting a hydrogen atom. The results of pump-probe transient absorption experiments provide no evidence for a concerted H-atom transfer mechanism.

Effect of pH on the fluorescence and absorption spectra of hypericin in reverse micelles.

The well-characterized, monodispersed nature of reverse micelles formed by sodium bis(2-ethylhexyl)sulfosuccinate/heptane and their usefulness in approximating a membrane-like environment have been exploited to investigate the effect of pH and water pool size on the photophysical properties of hypericin (Hyp). Our measurements reveal two titratable groups of pKa approximately 1.5 and approximately 12.5. These are assigned to the HypH+/Hyp equilibrium (the deprotonation of a carbonyl group) and the Hyp-/Hyp2- equilibrium (the deprotonation of a peri hydroxyl group). The low-energy absorbance maxima of HypH+, of Hyp and Hyp- and of Hyp2- are 583, 594 and 613 nm, respectively. Neither at pH 13 nor at 1 M HCl is the system entirely in the Hyp2- or the HypH+ forms. Ours is the first study of Hyp in reverse micelles as well as the first time-resolved study of Hyp as a function of pH.

Multidimensional reaction coordinate for the excited-state H-atom transfer in perylene quinones: importance of the 7-membered ring in hypocrellins A and B.

The excited-state intramolecular H-atom transfer reactions of hypocrellins B and A are compared by using time-resolved absorption and fluorescence upconversion techniques. The hypocrellin B photophysics are well described by a simple model involving one ground-state species and excited-state forward and reverse H-atom transfer with a nonfluorescent excited state. We suggest that excited-state conformational changes are coupled to the H-atom transfer in hypocrellin B just as gauche/anti changes are coupled to the H-atom transfer in hypocrellin A.

Fluorescence properties of recombinant tropomyosin containing tryptophan, 5-hydroxytryptophan and 7-azatryptophan.

Tropomyosin mutants containing either tryptophan (122W), 5-hydroxytryptophan (5OH122W) or 7-azatryptophan (7N122W) have been expressed in Escherichia coli and their fluorescence properties studied. The fluorescent amino acids were located at position 122 of the tropomyosin primary sequence, corresponding to a solvent-exposed position c of the coiled-coil heptapeptide repeat. The emission spectrum of the probe in each mutant is blue-shifted slightly with respect to that of the probe in water. The fluorescence anisotropy decays are single exponential, with a time constant of 2-3 ns while the fluorescence lifetimes of the probes incorporated into the proteins, in water, are nonexponential. Because tryptophan in water has an intrinsic nonexponential fluorescence decay, it is not surprising that the fluorescence decay of 122W is well described by a triple exponential. The fluorescence decays in water of the nonnatural amino acids 5-hydroxytryptophan and 7-azatryptophan (when emission is collected from the entire band) are single exponential. Incorporation into tropomyosin induces triple-exponential fluorescence decay in 5-hydroxytryptophan and double-exponential fluorescence decay in 7-azatryptophan. The range of lifetimes observed for 5-hydroxyindole and 5-hydroxytryptophan at high pH and in the nonaqueous solvents were used as a base with which to interpret the lifetimes observed for the 5OH122W and indicate that the chromophore exists in several solvent environments in both its protonated and unprotonated forms in 5OH122W.

Photophysics of hypericin and hypocrellin A in complex with subcellular components: interactions with human serum albumin.

Time-resolved fluorescence and absorption measurements are performed on hypericin complexed with human serum albumin, HSA (1:4, 1:1 and approximately 5:1 hypericin: HSA complexes). Detailed comparisons with hypocrellin A/HSA complexes (1:4 and 1:1) are made. Our results are consistent with the conclusions of previous studies indicating that hypericin binds to HSA by means of a specific hydrogen-bonded interaction between its carbonyl oxygen and the N1-H of the tryptophan residue in the IIA subdomain of HSA. (They also indicate that some hypericin binds nonspecifically to the surface of the protein.) A single-exponential rotational diffusion time of 31 ns is measured for hypericin bound to HSA, indicating that it is very rigidly held. Energy transfer from the tryptophan residue of HSA to hypericin is very efficient and is characterized by a critical distance of 94 A, from which we estimate a time constant for energy transfer of approximately 3 x 10(-15) s. Although it is tightly bound to HSA, hypericin is still capable of executing excited-state intramolecular proton (or hydrogen atom) transfer in the approximately 5:1 complex, albeit to a lesser extent than when it is free in solution. It appears that the proton transfer process is completely impeded in the 1:1 complex. The implications of these results for hypericin (and hypocrellin A) are discussed in terms of the mechanism of intramolecular excited-state proton transfer, the mode of binding to HSA and the light-induced antiviral and antitumor activity.

Subcellular distributions and excited-state processes of hypericin in neurons.

The photodynamic drug, hypericin, is studied in fetal rat neurons using fluorescence microscopy. Hypericin has an extremely high affinity for the cell membrane and is found to a smaller extent in the nucleus. Fluorescent excitation of hypericin is shown to cause irreversible damage to the cell membranes of living neurons. Fixed cells were used to make ultrafast time-resolved measurements to avoid the deleterious effects of long-term exposure to intense light and room temperatures. To our knowledge, these are the first ultrafast time-resolved measurements of the fluorescence lifetime of hypericin in a subcellular environment. Nonexponential fluorescence decay is observed in hypericin in the neurons. This nonexponential decay is discussed in terms of other examples where nonexponential decay is induced in hypericin upon its binding to biomolecules. The nonradiative processes giving rise to the nonexponential hypericin decay are attributed to excited-state electron transfer, excited-state proton transfer or both.

Photoactivated virucidal properties of tridentate 2,2'-dihydroxyazobenzene and 2-salicylideneaminophenol platinum pyridine complexes.

The potent photoactivated virucidal activity of tridentate 2,2'-dihydroxyazobenzene- and 2-salicylideneaminophenol platinum pyridine complexes 1, 2, 4, 6, 7, 9, and 10 against enveloped viruses (e.g., EIAV, HIV, and HSV) is described.

Domain structure of the Staphylococcus aureus collagen adhesin.

Sequence analysis of surface proteins from Gram-positive bacteria indicates a composite organization consisting of unique and repeated segments. Thus, these proteins may contain discrete domains that could fold independently. In this paper, we have used a panel of biophysical methods, including gel permeation chromatography, analytical ultracentrifugation, circular dichroism, and fluorescence spectroscopy, to analyze the structural organization of the Staphylococcus aureus collagen adhesin, CNA. Our results indicate that the structure, function, and folding of the ligand-binding domain (A) are not affected by the presence or absence of the other major domain (B). In addition, little or no interaction is observed between the nearly identical repeat units within the B domain. We propose that CNA is indeed a mosaic protein in which the different domains previously indicated by sequence analysis operate independently.

The role of oxygen in the antiviral activity of hypericin and hypocrellin.

The light-induced antiviral activity of hypericin and hypocrellin in the presence and absence of oxygen was examined under experimental conditions where the effect of oxygen depletion could be quantified. There was a significant reduction of light-induced antiviral activity of hypericin and hypocrellin under hypoxic conditions. Interestingly, antiviral activity of hypocrellin was not observed at low oxygen levels at which hypericin retained measurable virucidal activity. This suggests that additional pathways, such as the generation of protons from excited states of hypericin, may enhance the biological activity of activated oxygen species.