Fluorimetric determination of reducing carbohydrates with malonamide

Samples are heated for 90 min with malonamide in carbonate buffer (pH 9.2) for measurement at 383 nm (excitation)/425 nm (emission). For D-glucose, relative standard deviations are 1–3% at the 10^-6–10^-9 mol level. Relative fluorescence intensities are reported for 36 carbohydrates.     

Study on the new fluorescence enhancement system of Zn-bis-(trimethylolaminomethane)-4-tert-butyl-disalicylicimine in the presence of beta-cyclodextrin and its analytical application

A new bis-Schiff base ligand, bis-(trimethylolaminomethane)-4-tert-butyl-disalicylicimine (HL), was synthesized. The fluorescence intensity of HL-Zn²⁺ complex was increased by about 8-fold upon addition of β-cyclodextrin (β-CD). The spectrofluorimetric determination of trace amounts of Zn²⁺ based on the phenomenon was carried out. The excitation and emission wavelengths are 405 and 465 nm, respectively. Under optimal conditions, the fluorescence intensities vary linearly with the concentration of Zn²⁺ in the range 0-317 μg l⁻¹ with a detection limit of 1.0 μg l⁻¹. The interferences of some inorganic ions were described. The method is a selective, sensitive, rapid, and simple analytical procedure for the determination of Zn²⁺ in crops. The mechanism for the fluorescence enhancement was also discussed.     

Photoluminescence of Acenaphthenone in Organic Solvents and Aqueoug Media

The configuration of the lowest excited state of acenaphthenone, S₁(π, π^*) or T₁(π, π^*), depending on the solvent, dominates photoluminescence. The T₁(n, π^*) state in aprotic organic solvents is responsible for the phosphorescence of acenaphthenone. The wavelengths of the phosphorescence measured in benzene are 576 nm and 635 nm (vibronic) with 3.3 × 10^?4 quantum efficiency. However, the S₁(π, π^*) state in protic solution which dominates the fluorescence emission depending upon acidity is the most distinctive feature of acenaphthenone. The wavelengths of the emissions are 446 nm under water solvation with 0.185 quantum efficiency and 538 nm with 0.097 quantum efficiency under high acidity. The emission at 446 nm is assigned from a H-bonded keto-form excited state, whereas the emission at 538 nm is probably due to the excited state of protonated keto-form. The pK_a value in aqueous solution measured by diminution of fluorescence in basic solutions is 12.5 ± 0.4.     

Electronic structure of pyrazine: a study of the geometries in the lowest excited singlet π *← n and π*←π states

Electronic, excitation energies, charge distributions and geometries of pyrazine in the lowest excited singlet π^*←π and π^*←n states have been studied by the VE—PPP, CNDO/2 and CNDO/s-CI molecular orbital methods. Study of the change of geometry in the π^*←n excitation requires localization of the density matrices in the ground and excited states, and with the help of these σ-bond orders are defined. Charge distributions and bond orders in the lowest excited singlet π^*←π and π^*←n states are compared. Whereas in the lowest singlet π*←π excitation the pyrazine ring expands uniformly, in the case of the π^*←n excitation C-C bonds contract whereas C-N bonds elongate. The predictions of theory are in agreement with experimental results, showing that the method used can be employed to obtain reliably the trends of geometry changes following a π^*←π excitation of a molecule before a more complete theoretical or experimental study is performed.     

Emission from the 1(n, π*) state of azobenzene: Spectrum and ultrashort decay time

Picosecond lifetime and depolarization studies of ¹(n, π^*) emission from trans-azobenzene indicate that fluorescence contributes substantially- to the emission decay pattern. The ¹(n, π^*) state's decay time is ≈25 ps, in contrast to the nanosecond lifetimes exhibited by fluorescing singlet states in stilbenes and styrenes.     

Triplet states of fluorocarbonyl compounds determined by trapped electron spectroscopy

Ion cyclotron resonance trapped electron spectra for carbonylfluoride, trifluoroacetyl fluoride, hexafluoroacetone, acetyl fluoride and I,I.I-trifluoroacetone are reported. Comparisons of data obtained for ³(π → π^*) vertical excitation energies are used in conjunction with photoelectron spectroscopic data to deduce the effects of fluorine and trifluoromethyl groups on the energies of σ, π and π^* orbitals associated with the carbonyl group.     

Intramolecular charge delocalization in the singlet excited state of blocked pyrylium ions

The dual fluorescence emission of the pyrylium ion 3 and of the partly blocked 4 has been studied extensively under various conditions. The short-wavelength emitting species N^* of 3 is short-lived (≤200 ps at room temperature) while the long-wavelength emitting species A^* is long-lived (>3 ns, except in acetic acid). This long-wavelength fluorescence undergoes an important solvatochromic shift and the difference Δ between the absorption and fluorescence maxima versus Lippert’s solvent polarity function Δf is linear. Increasing the viscosity of the medium, or decreasing the temperature, decreases the long-wavelength emission quantum yield while that of the short-wavelength fluorescence and its lifetime (from <100 ps to >4 ns) both increase, indicating that A^* is formed from N^*. Introducing an ortho methyl group on the paraanisyl substituent (compound 4) blocks its rotation and reduces the fluorescence I_A./I_N. ratio, but it does not suppress completely the long-wavelength emission. This favors a ground state configuration where the phenyl substituent would be orthogonal to the xanthylium moiety. A strong interaction of 3 and 4 with aliphatic nitriles is characterized from the quenching of the fluorescence emission (with rate constants of ca. 2×10⁸ M⁻¹ s⁻¹). A static quenching process also occurs indicating a ground state interaction with the solvent. In pure aliphatic nitriles, this interaction is the main deactivation pathway of the singlet excited state, and practically no fluorescence nor triplet formation can be observed.     

Electronic to vibrational energy transfer and relaxation in matrices. II. Hg in mixed N2/Kr matrices

The electronic fluorescence spectrum of Hg-doped N₂/Kr mixed matrices obtained by laser excitation of the Hg³P₁ level is composed of the Hg “atomic” (³P₁—¹S₀ and ³P₀—¹S₀) fluorescence and the exciplex fluorescence due to the (Hg—N₂)^* complex stable in the excited state. The temperature dependence of their intensities and lifetimes was studied in the temperature range 12–24 K. It is argued that the essential part of the ³P₀ and exciplex emission is due to two types of Hg sites with one N₂ nearest neighbor, differing probably in the orientation of the N₂ molecular axis. Strong irreversible effects due to the diffusion of N₂ molecules induced by laser irradiation are observed.     

Application of the MC SCF method to the π → π* excitation energies of ethylene

The MC SCF method is employed to calculate the N → T and N → V π → π^* vertical excitation energies of ethylene. To obtain accurate excitation energies it is found to be necessary to utilize an expanded valence space containing two π and two π^* orbitals. Relatively small MC SCF calculations, allowing at most one-electron excitations from the sigma space, are found to yield excitation energies and spatial extents of the excited states in excellent agreement with the predictions of large multi-reference or iterative-natural-orbital CI calculations. These results show that within an MC SCF framework σ-σ correlation is unimportant for describing the π → π^* processes. We also conclude that the neglect of the effects of unlinked cluster terms in some of the CI calculations may have introduced small, but important, errors in the excitation energies and predictions of the spatial extent of the V state.     

Optically detected magnetic resonance study of the phosphorescent states of thiouracils

The triplet states of 1-methyl-2-thiouracil (1-Me-s²U), 1-methyl-4-thiouracil (1-Me-s²s⁴U) and 1-methyl-2,4-dithiouracil (1-Me-s² s⁴) have been investigated by optically detected magnetic resonance in zero magnetic field. The zero field splittings (ZFS) and the individual sublevel kinetic parameters are reported. The ZFS (|D|, |E|) values (in cm^?1) are found to increase in the order: 1-Me-s² U (0.2895, 0.0728) < 1-Me-s⁴U, (0.605, 0.0500) < 1-Me-s²s⁴ U (0.870, 0.0458). The triplet state lifetimes decrease in the same order, and both effects are attributed to an internal heavy atom effect of sulfur substitution. The vibronic structure of the phosphorescence emission indicates that the thiouracil phosphorescent states are ³(π, π^*). The low phosphorescence quantum yields of 1-Me-s⁴ U and of 1-Me-s²s⁴U result from radiationless decay of the triplet state rather than from inefficient intersystem crossing from the excited singlet state. The efficient radiationless decay of the triplet state appears to be a feature of the S-substitution at the 4-position of uracil. Phosphorescence polarization measurements of the individuals triplet sublevel emissions at ca. 1.2 K are consistent with 1-Me-s²U and 1-Me-s⁴U being non-planar in the phosphorescent state; the thiouracil phosphorescence from each triplet sublevel is polarized in the average plane of the distorted molecule. In the absence of σπ separability, spin—orbit mixing of ¹(π, π^*) and ³(π, π^*) states is enhanced and the radiative properties of the triplet state may be dominated by this mechanism rather than by the mixing of ¹(n, π^*), ¹(σ, π^*), or ¹(π, σ^*) with ³(π, π^* states which generally is the dominant mechanism for planar aromatic molecules.     

Synthesis and photophysical behaviors of temperature/pH-sensitive polymeric materials. I. Vinyl monomer bearing 9-aminoacridine and polymers

A novel acrylic monomer bearing acridinyl group, acridine-9-N-acrylamide (Ac-9AA) was synthesized from 9-aminoacridine (9AA) and acryloyl chloride in the presence of triethylamine in dry dichloromethane (CCl₂) at room temperature. The synthesized Ac-9AA was identified by IR, MS and ¹H NMR spectra. Homopolymer of Ac-9AA was obtained using AIBN as a thermal initiator in THF under 65-70 °C and the average molecular weights (M_w) of poly(Ac-9AA) obtained was very low, being in the order of ca. 10³. Copolymer of Ac-9AA and acrylamide was synthesized with thermal initiator and poly(Ac-9AA-co-AM) was characterized by the method of IR, UV-vis and DSC. The photophysical behaviors of Ac-9AA and its polymers were explored by recording the fluorescence spectra in solution, solid and film. In addition, the pH and temperature dependence on fluorescence of the water-soluble poly(Ac-9AA-co-AM) were investigated in detail. The results showed that the relative fluorescence intensity of poly(Ac-9AA-co-AM) had an excellent linear response to temperature in the range of 0-60 °C. Moreover, the fluorescence intensity increased continuously from low pH to high pH while the excitation maxima at 388 nm and emission maxima at 400 nm had redshift after the addition of HCl or NaOH, which results from the fact that the predominance of tautomeric forms of Ac-9AA changed at different medium. This investigation may provide a convenient way to prepared multifunctional macromolecule biomaterial bearing aminoacridine to probe pH and temperature in biological system.     

FLUORESCENCE PROPERTIES OF PORPHYRIN-GLOBIN FROM HUMAN HEMOGLOBIN

Fluorescence excitation and emission spectra, decays, and quantum yields are reported for the porphyrin-globin of hemoglobin (Hb^desFe) in aqueous solution of pH 8, at 4°C. A very weak fluorescence was observed in the UV (maximum at 334 nm), due to tryptophan and tyrosine residues, in addition to the strong porphyrin emission in the visible (maxima at 624 and 692 nm) reported previously. The absorption and fluorescence properties of the porphyrins of Hb^desFe were compared to those for free porphyrin in organic solvents and in aqueous solution. The close similarity of the fluorescence decays and quantum yields in Hb^desFe and in solution indicate the absence of stronger, specific porphyrin-protein interactions; however, slight spectral shifts point to the existence of water molecules in the Hb^desFe porphyrin environment. The fluorescence study also demonstrates the existence of efficient Trp-porphyrin energy transfer of Förster type. The extent of transfer is in satisfactory agreement with the value expected from crystallographic data for hemoglobin. The results are discussed and compared to previous fluorescence studies of hemoglobin and apohemoglobin. An improved method for the preparation of Hb^desFe is reported.     

FLUORESCENCE RELAXATION KINETICS AND QUANTUM YIELD FROM THE PHYCOBILISOMES OF THE BLUE-GREEN ALGA NOSTOC SP. MEASURED AS A FUNCTION OF SINGLE PICOSECOND PULSE INTENSITY*

Abstract— A detailed experimental study of the effect of intensity of a 6 ps excitation pulse on the decay kinetics and yield from phycobilisomes (PBsomes) is presented. The fluorescence from the c-phycoerythrin (PE) emission from PBsomes was found to decay as a single exponential with a time of 31 ± 4ps for an excitation intensity <10¹⁴ photons/cm² per pulse. The risetime of the c-phycocyanin (PC) and allophycocyanin (APC) emission from PBsomes was found to be 34 ± 13 ps. Therefore, at low excitation intensities, the energy transfer time between the constituent phycobiliproteins, PE and PC, is measured to be 34 ± 13ps from the fluorescence decay time of PE and the fluorescence risetime of the PC and APC emission. The fluorescence yield from the PE emission component in PBsomes was found to be intensity dependent for excitation intensities >10¹⁴ photons/cm². The decrease in yield with increased intensity in this case occurred at a higher intensity than in the isolated phycobiliprotein PE. The fluorescence yield of the PC and APC emission component was also found to decrease markedly with increasing excitation intensity. This is in contrast to the case of the isolated phycobiliprotein APC which showed only a slight quenching of the fluorescence. The higher quenching observed for the APC emission in the PBsome evidences the higher effective absorption of APC via energy transfer from PE to PC and APC.     

On-line separation, preconcentration and determination of trace amounts of gold in mineral sample by flow injection catalytic kinetic spectrofluorimetry

2-Pyridine carboxaldehyde furfuralhydrazone (PCFH) was newly synthesized and its ionization, IR and elemental analysis were established. An on-line separation and preconcentration system was developed for the flow injection (FI) catalytic kinetic spectrofluorimetric determination of trace amounts of gold. The method was based on the fluorescence enhancing reaction of PCFH with potassium bromate, which was catalysed by Au³⁺ in aqueous medium at pH 4.20 and 35 °C. A micro polyamide resin column was used to separate and collect gold from sample solution. Under these experimental conditions, the oxidized product of PCFH had excitation and emission maxima at 296 and 404 nm, respectively. The linear range of this method was 0.53-144 ng ml⁻¹ with the RSD of 1.6%. The detection limit was 0.16 ng ml⁻¹. A high analysis rate of 15 samples h⁻¹ was obtained. The proposed method had been applied successfully to the determination of gold in synthetic mixture and mineral sample with satisfactory results.     

High-resolution electronic spectra and luminescence properties of some benzonaphthyridines at 77 K. absorption and fluorescence spectra of 1,5-, 1,6- a

Absorption and fluorescence spectra of 1,5-, 1,6- and 4,6-benzo[h]naphthyridines (BN) were examined in Shpolskii matrices and in n-butanol at 77 K. Vibrational analysis of quasilinear spectra of 1,6- and 4,6-BN in n-hexane matrices was performed. Calcula- tions of the electronic structure of the isomers examined were done using a modified INDO CI method. The results of the experiments and calculations prove the π, π^* state to be the lowest excited singlet state of 1,6- and 4,6-BN molecules; in 1,5-BN molecule the S₁ (n, π^*) state is strongly perturbed by the nearby s₂(π π^*) state.     

TIME-RESOLVED FLUORESCENCE SPECTRA OF TRYPTOPHAN IN MONOMERIC GLUCAGON*

Abstract— The fluorescence decay of tryptophan-25 in monomeric glucagon at pH 8.2 was measured at a series of emission wavelengths using pulsed laser excitation and single photon counting techniques. Double exponential kinetics were consistently observed, with time constants 3.26 and 1.11 ns. This allowed the steady-state emission spectrum to be resolved into two components with differing intensities but similar emission maxima near 350 nm. Decay parameters were almost unchanged in the presence of 5.5 M guanidinium chloride. The dual emission is thought to reflect different conformers of the indole ring or of the peptide chain.     

Development of a new fluorescent probe for transition metal cations based upon fluorescence resonance energy transfer

A novel fluorescent probe for metal cations, which has a large Stokes shift, was synthesized from the reaction of N-(3-carboxy-2-naphthyl)-ethylenediamine-N,N′,N′-triacetic acid (CNEDTA) with 4-(N,N-dimethylaminosulfonyl)-7-(2-aminoethylamino)-2,1,3-benzoxadiazole (DBD-ED). The large Stokes shift is due to the FRET phenomenon between a donor (CNEDTA) and an acceptor (DBD-ED) fluorophore. When the fluorescent probe, DBD-ED-CNEDTA, was excited at 240, 340 and 440 nm, an emission maximum was observed only at 560 nm. However, the fluorescence (FL) at 480 nm, based upon the CNEDTA moiety, was not detected with excitation at 340 nm. The FL intensity of DBD-ED-CNEDTA was dependent upon the acidity of the medium and highest at pH 4.1. DBD-ED-CNEDTA reacted with metal cations, i.e., Zn, Cd, Al, Y, and La, in aqueous medium to form chelates. The spectral change of FL excitation and emission was small before and after the addition of the metal ions. However, the FL intensity was dependent upon the concentrations of the metal ions. In the case of Zn²⁺, the molar ratio bound with DBD-ED-CNEDTA was calculated as 1:1. The FL intensities after chelate formation of Zn/DBD-ED-CNEDTA (1:1) were enhanced by 3.8-fold (excitation at 340 nm, emission at 560 nm), 4.2-fold (excitation at 440 nm, emission at 560 nm), and 5.9-fold (excitation at 240 nm, emission at 560 nm), respectively. The FL probe was applied to the determination of Zn in a food supplement.     

Photochemistry of planar four-coordinate palladium(II) complexes—IV. Photochemical behaviour of trans-[Pd(PPrnn)(pyridine-4-aldehyde-4- nitrophenylhydrazone)Cl2] in various solvents

The photochemical and photophysical behaviour of the complex trans-[Pd(PPr_nⁿ (4-PAP-NO₂)Cl₂] (4-PAP-NO₂ = pyridine-4-aldehyde-4-nitrophenylhydrazone) has been investigated in various solvents. Under irradiations of different wavelengths, in the region of the lowest intraligand transition, photoisomerization of the coordinated ligand from a syn to an anti configuration occurs, and for each excitation wavelength a photoequilibrium is gained. Quantum yields for the process depend both on the energy of irradiation and on the solvent used; in all cases the quantum yields are lower than those corresponding to the free ligand. The complex emits at room temperature in acetonitrile solution, and emission spectra (also at 77K), excitation spectra and emission lifetime have been studied. The emission can probably be ascribed to a π-π* fluorescence.     

Broken orbital symmetry study of low-lying excited and N15 ionized states of pyrazine

Ab initio ΔE_SCF calculations of the low-lying n-π^* and π-π^* transitions in pyrazines are reported with D_2h and C_2v symmetry adapted molecular orbitals. The use of broken orbital symmetry is essential for interpreting the emission properties of pyrazine at the computational level used. The energy of the N_is orbital is calculated using these two symmetry constraints with C_2v orbitals leading to results in better agreement with experiment.     

Spectroscopic studies of dipyridamole derivatives in homogeneous solutions: Effects of solution composition on the electronic absorption and emission

Electronic absorption and fluorescence spectra of three different dipyridamole (DIP) derivatives, RA 39, RA 14 and RA 25, were monitored in aqueous solution as a function of pH in the range 2–13. Extinction coefficients, quantum yields and fluorescence lifetimes were obtained in aqueous solutions at different pHs and in several other solvents. The absorption spectra are characterized by two broad bands centered at 405–415 nm and 290 nm for RA 39 and RA 14 and at 370 and 270 nm for RA 25. The first band used for fluorescence excitation has a value of ε ≌ 5 × 10³ M⁻¹ cm⁻¹ characteristic of a π → π* transition. An increase in pH is accompanied by both fluorescence and absorption sharp changes around pH 6 giving a pK ≌ 5.8 for RA 39 and RA 14, similar to the one observed for DIP. In the case of RA 25 two pKs are observed below 6.5, namely a pK₁ ≌ 2.25 and pK₂ = 5.20. These changes are associated to protonation equilibria of nitrogens in the π-conjugated system of the pyrimido—pyrimidine ring. At high pH a second transition is observed only in fluorescence with pK ≌ 12.5 for RA 39 which is similar to DIP and pK ≌ 12.1 for RA 25. For RA 14 this transition was not observed. Estimates of the pK* for the first singlet excited state showed that in comparison to the ground state pK ≌ 5.8 or 5.2 (RA 25), the excited state pK* is more acid by almost a unit. In the case of RA 25 and the lowest pK of 2.25, an excited state reaction, possibly of proton transfer, seems to take place since emission and absorption data give opposite effects for pK*.