Time-resolved fluorescence relaxation of 3-methyllumiflavin in polar solution

We have studied the fluorescent properties of a well-defined model flavin compound (3-methyllumiflavin) in a relatively polar solvent like propylene glycol or ethanol. Inhomogeneous spectral broadening effects were directly time-resolved by detection at the extreme blue and red edges of the fluorescence band of 3-methyllumiflavin using excitation in the main absorption band. At the high-energy side of the emission band a rapid decay component (tens of picoseconds) was resolved indicative for the disappearance of the initially prepared, nonequilibrium state with a characteristic dipolar relaxation time. At the low-energy side the rise of a solvent relaxed fluorescent species could be time-resolved. The wavelength-dependent effects on the dipolar relaxation were abolished when excitation was at the low-energy side of the absorption band. The experimental decays of the flavin solvate at different energies of fluorescence and excitation are presented as they represent an easy diagnosis for energy dependent solvation dynamics. Wavelength dependent rotation of 3-methyllumiflavin, examined by fluorescence anisotropy decay, turned out to be absent for 3-methyllumiflavin in propylene glycol between 263 and 293 K, probably because of the small change in dipole moment upon flavin excitation.     

Kinetic-catalytic determination of osmium by fluorescence quenching with salicylfluorone and hydrogen peroxide

A highly sensitive and selective fluorescence-quenching kinetic method is proposed for the determination of trace osmium(IV), based on the catalytic effect of osmium(IV) on the salicylfluorone (_ex = 510 nm, _em = 535 nm)-H₂O₂ system at pH 9.3–9.6. Using the fixed time method, osmium(IV) in the range 0.008–0.6 ng/ml can be determined. The detection limit is 0.006 ng/ml. Over thirty anions and cations, including other platinum metal ions, do not interfere, even when present in large excess. The method has been applied successfully for the determination of osmium in a series of synthetic mixtures and refined ore with relative standard deviations of 2–6%.     

Distance-dependent fluorescence quenching ofN-acetyl-l-tryptophanamide by acrylamide

We examined the time-dependent intensity decays ofN-acetyl-l-tryptophanamide (NATA) when collisionally quenched by acrylamide in propylene glycol over a range of temperatures. The intensity decays of NATA became increasingly heterogeneous in the presence of acrylamide. The NATA intensity decays were not consistent with the Collins-Kimball radiation boundary condition (RBC) model for quenching. The steady-state Stern-Volmer plots show significant upward curvature, and quenching of NATA by acrylamide was observed even in vitrified propylene glycol, where translational diffusion cannot occur during the lifetime of the excited state. These frequencydomain and steady-state data indicate a through-space quenching interaction between NATA and acrylamide, and the results are consistent with a rate constant for quenching that depends exponentially on the fluorophore-quencher separation distance. The exponential distance-dependent rate of quenching also explains the upward curvature of the Stern-Volmer plot, and the steady-state data aid in determining the interaction distance between NATA and acrylamide. These results suggest that the distance-dependent quenching rates need to be considered in the interpretation of acrylamide quenching of proteins.     

Determination of aluminum in serum by capillary zone electrophoresis with laser-induced fluorescence detection

Summary A novel method of separating and detecting trace aluminum by capillary zone electrophoresis is described. Aluminum is reacted with lumogallion [4-chloro-3-(2,4-dihydroxyphenylazo)-2-hydroxybenzen-1-sulphonic acid] so that the complex can be selectively and sensitively detected by a laser-induced fluorescence detector after capillary electrophoretic separation. Using the proposed method, limits of detection in the sub parts per billion range are achieved. The technique is applied to the determination of aluminum in human serum.     

Microenvironmental differences amongst micelles of various shapes and structures

Micelles of different amphiphiles adopt different shapes and internal packing arrangements in water, depending on their chemical structures and the conditions of the medium. Two microenvironmental features, namely the polarity and the microviscosity that the aggregate offers to a solubilized molecule, have been monitored using extrinsic fluorescence probes. While the differences between micelles of spherical and rod-like shapes are not always distinct, stacked micelles and peptide micelles offer distinctly lower polarity and higher microviscosity to solubilizates than the others.     

Coupled-column liquid chromatography method with photochemically induced derivatization for the direct determination of benzoylureas in vegetables

The coupled-column (LC-LC) system, consisting of a first column packed with internal surface reversed phase (ISRP) (50 x 4.6mm ID) and a Chrompack C18 (100 x 4.6 mm ID) as second column, allowed the simultaneous determination of five benzoylurea insecticides in dichloromethane (CH2Cl2) extracts of vegetable samples without any clean-up step. This system was combined with a photochemically induced fluorescence (PIF) post-column derivatization in order to provide strongly fluorescent photoproducts from the non-fluorescent benzoylureas. Limits of detection ranged from 0.21 to 0.98 microg L(-1) of pesticide (equivalent to 0.14-0.65 microg kg(-1) in vegetable samples) and limits of determination ranged from 4.0 to 10.0 microg L(-1) (equivalent to 2.7-6.7 microg kg(-1)). Linearity of the method was established between 2 and 1800 microg L(-1), depending upon the compound. Validation of the total method was performed by randomly analyzing recoveries of four vegetable samples (aubergine, cucumber, green bean, and tomato) spiked at two levels of concentration (10.0 and 33.3 microg kg(-1)). The combination of the LC-LC system with PIF detection provides a sensitive, selective, and rapid method for the determination of pesticides in vegetable samples at levels lower than the maximum residue levels (MRLs) established for these compounds by Spanish legislation.     

Mercury species immobilized on sulphydryl cotton: A new candidate reference material for mercury speciation

A batch of sulphydryl cotton microcolumns was prepared and charged with a mixed Hg standard solution (methyl-, ethyl- and inorganic Hg, 10 g l^–1 as Hg, 3 ml) and stored at 4 °C in a light-tight box. At regular time intervals over a 4 month period microcolumns were removed and Hg species were quantified by gas chromatography microwave-induced plasma atomic emission spectrometry (after elution, extraction and derivatization steps). It was found that analyte recoveries for methyl- and inorganic Hg were quantitative over the 4 month period while ethyl-Hg species appeared to be stable for up to 2 months.     

Measurements of HO2 uptake coefficient on aqueous (NH4)2SO4 aerosol using aerosol flow tube with LIF system

The γ(HO₂) was elevated with increase of Cu(II) concentrations in aqueous (NH₄)₂SO₄ aerosol. The threshold of Cu(II) concentration was 10^-3 mol/L for the dramatic increase of γ(HO₂) to 0.1, suggesting sensitive γ(HO₂) value to concentration of transition metal ions in aerosol.     

The study of terbium generated bacterirhodopsin

The localization of Terbium (Tb³⁺) cations binding to deionized bacteriorhodopsin (bR) has been studied by using spectroscopic methods. It was found that adding Tb³⁺ cations to deionized bR affects the fluorescence lifetimes of tryptophan (Trp) in bR, the wavelength of fluorescence peak shifts “blue” and the peak value of fluorescence decreases. It was also found that adding one Tb³⁺ cation to deionized bR can restore the purple state from its blue state obviously. The measurements of absorbance, fluorescence and lifetime of fluorescence also show that when more than three Tb³⁺ cations are added, no further changes can be found. It is suggested that one Tb³⁺ specific binding site for the color-controlling is located on the exterior of the bR trimer structure to negatively charged lipids near Trp-10 and Trp-12. Three Tb³⁺ cations binding per bR is needed for the regenerated bR.