Spectroscopic study of molecular associations between riboflavin and some (dihydro) β-carboline derivatives

The spectrophotometric and thermodynamic properties of molecular complexes of riboflavin (RFN) with some dihydro β-carboline derivatives have been investigated by using electronic absorption and fluorescence spectroscopic methods in aqueous solution. The molecular associations have been examined by means of eletronic absorption spectra, since in each, a new charge transfer-like band has been located, and also by observing the variation of the fluorescence emission of RFN on the solutions. The formation constants for the molecular complexes were determined from absorption data, using the Forster—Hammick—Wardley method. The quenching phenomenon observed in RFN fluorescence is related to the concentration of the dihydro β-carboline derivatives, allowing the calculation of the quenching constants for RFN—β-carboline complexes. Thermodynamic parameters have been determined from the values of association constants for the molecular complexes at various temperatures. The influence of substituents in the dihydro β-carboline molecule on the stability of the complexes formed was also investigated.     

Spectroscopic study of molecular associations between FMN and β-carbolines

The spectrophotometric and thermodynamic properties of molecular complexes of flavin mononucleotide (FMN) (riboflavin 5′-phosphate) with some β-carboline derivatives have been investigated in aqueous solution. The molecular associations have been examined by means of electronic absorption spectra, since in each a new charge-transfer band has been located, and also the variation of the fluorescence emission of FMN on the solutions has been observed. The formation constants for the molecular complexes were determined from absorption data using the Foster—Hammick—Wardley method. The quenching phenomenon observed in FMN fluorescence is related to the concentration of the β-carboline derivatives, allowing the calculation of the quenching constants for FMN-β-carboline complexes. Thermodynamic parameters have been determined from the values of association constants for the molecular complexes at various temperatures. The influence of substituents in the β-carboline molecule on the stability of the complexes formed was also investigated.     

Spectroscopic study of intermolecular complexes between FAD and some β-carboline derivatives

The formation of molecular complexes between flavine adenine dinucleotide (FAD) and some β-carboline derivatives [antidepressant drugs that have a pronounced inhibition of monoamine oxidase (MAO)] has been studied by using electronic absorption and fluorescence spectroscopic methods. Thermodynamic parameters have been determined from the values of association constants for the molecular complexes at various temperatures. The influence of substituents in the β-carboline molecule on the stability of the complexes formed was also investigated.     

COMPLEXING OF RIBOFLAVIN AND ITS 2-SUBSTITUTED ANALOGS WITH ADENOSINE AND OTHER 6-SUBSTITUTED PURINE DERIVATIVES*

Abstract Fluorescence properties of riboflavin and its 2-substituted analogs and the quenching of fluorescence which occurs upon complexing with adenosine and other 6-substituted purine derivatives have been examined at different concentrations of interactants and hydrogen ion. The 6-substituted purines and their ribosides show varying degrees of association with riboflavin. Successive N-alkylation of the 6-amino group of purine ribosides leads to more stable complexing in the order ethyl > methyl > propyl with riboflavin and its analog. These results suggest that such groups, including the 6-amino group of adenine or adenine moiety of FAD, do not directly interact with flavins by hydrophobic or hydrogen bonding. Lowering the pH to above that required to protonate the isoalloxazine system disrupts the intermolecular complexes. Hence, only the unprotonated bases can form such complexes. The 2-substituted aminoriboflavins, as well as FAD, exhibit fluorescence optima upon changing pH. The species involved appear to be a non-fluorescent form with protonated isoalloxazine and amine or adenine portions at low pH, a fluorescent form with only the amine or adenine portions protonated at moderately acid pH, and a non-fluorescent form which is intramolecularly quenched at higher pH. The general electron donating properties of the amines may be the common reason for their quenching of fluorescence in inter- and intramolecular complexes with flavins.     

Fluorescence quenching of carbazole and indole by ethylenetrithiocarbonate

The fluorescence of quenching of carbazoles and indoles by ethylenetrithiocarbonate has been studied. Both steady-state and transient fluorescence experiments were carried out in order to determine rate constants in the case of carbazoles. The results obtained support the charge-transfer mechanism. Ground-state complex formation is assumed to explain the higher rate constants for indole and its derivatives, which appear as positive curvatures in Stem-Volmer plots.     

A FLUORESCENCE STUDY OF THE INTERACTION OF INDOLIC COMPOUNDS WITH SYNTHETIC POLYELECTROLYTES

Abstract— –The interaction of indole derivatives with synthetic polyelectrolytes was investigated using UV absorption and fluorescence spectroscopy. The presence of both sodium poly(styrene sulfonate) (PSS) and sodium polyvinyl sulfonate) (PVS) inhibits the fluorescence quenching of 1-pyrene sulfonic acid by tryptamine. The effect is more marked for PSS than for PVS. There was no polyelectrolyte effect on the quenching by tryptophan. It was also found that aromatic polyelectrolytes strongly quench the fluorescence of indole derivatives of opposite charge by a static mechanism. This is accompanied by a new absorption in the red extreme of the UV spectrum of the mixtures. The systems investigated were tryptamine-PSS and polyvinyl benzyl trimethylammonium chloride) with the anions of indole-3-alkanoic acids. Equilibrium constants for the binding of the indole derivatives to the polyelectrolytes were determined. The fluorescence of zwitterionic tryptophan is not affected by the presence of the polyelectrolytes.     

Thermodynamic and Kinetic Study of the Interaction between Alkylpyridinium Ions and Pyrene Derivatives in Aqueous Solution

Abstract— The kinetics of fluorescence quenching of pyrene and three of its derivatives by a series of n -alkylpyridinium ions has been studied. The quenching process is diffusion controlled for all the systems studied, independent of the charge in the aromatic molecule and the size of the alkyl chain in the pyridinium ions. Pyrensulfonate, PS, and pyranine form ground-state complexes with these quenchers. The association constants were determined by using a kinetic scheme where both dynamic and static quenching were included. A linear relationship between the free energy change and number of carbon atoms was found for PS, and a methylene contribution equal to -1.1 kJ/mol was determined from the slope. A similar correlation was established for pyranine, but a slope change was observed when the alkyl chain has more than 10 carbon atoms. From the initial slope, an incremental free energy of -0.35 kJ/mol was obtained. The ground-state complex formation is determined mainly by an electrostatic interaction, but there is a hydrophobic effect on the value of the measured association constants.     

核苷磷酸盐对含双芘侧链荧光敏感器化合物的荧光猝灭与分子识别

合成了一种在水溶液中对核苷磷酸盐ATP,ADP,AMP阴离子具有识别能力的荧光化学敏感器分子---化合物1。通过检测化合物1在水溶液中芘激基缔合物荧光发射强度随核苷磷酸盐、腺嘌呤等的加入而引起的变化,求出不同核苷磷酸盐及腺嘌呤对芘激基缔合物荧光发射的猝灭常数,并进行了比较研究。利用荧光发射强度随不同核苷磷酸盐引入而发生的变化计算出化合物1对不同核苷磷酸盐进行配位的配位稳定常数。结果表明所合成的化合物1对ATP有着良好的识别力选择功能。此外,还利用分子力学的计算方法对化合物1及其与核苷磷酸盐形成配合物后的分子构象及几何尺寸作了计算,并结合实验结果进行了初步讨论。     

Fluorescence quenching of anthracene by indole derivatives in phospholipid bilayers

The quenching of anthracene fluorescence by indole, 1,2-dimethylindole (DMI), tryptophan (Trp) and indole 3-acetic acid (IAA) in palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers was investigated. A very efficient quenching of the anthracene fluorescence in the lipid membrane is observed. Stern-Volmer plots are linear for DMI but present a downward curvature for the other quenchers. This was interpreted as an indication of the presence of an inaccessible fraction of anthracene molecules. By a modified Stern-Volmer analysis the fraction accessible to the quenchers and the quenching constant were determined. The changes in the fluorescence emission spectrum of indole and DMI have been used to calculate the partition constants of these probes into the membranes, and bimolecular quenching rate constants were determined in terms of the local concentration of quencher in the lipid bilayer. The rate constants are lower than those in homogeneous solvents, which may be ascribed to a higher viscosity of the bilayer. No changes in the emission spectra of Trp and IAA are observed in the presence of vesicles, indicating that these probes locate preferentially in the aqueous phase, or in close proximity to the vesicular external interface in a medium resembling pure water. In these cases quenching rate constants were determined in terms of the analytical concentration. In the quenching by DMI a new, red shifted, emission band appears; it is similar to that observed in non-polar solvents and it is ascribable to an exciplex emission. The exciplex band is absent in the quenching by IAA and Trp and only very weakly present when the quencher is indole. From the position of the maximum of the exciplex emission, a relatively high local polarity could be estimated for the region of the bilayer where the quenching reaction takes place.     

Fluorescence quenching of flavins by reductive agents

The fluorescence behaviour of the flavins riboflavin, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and lumiflavin in aqueous solution at pH 8 in the presence of the reducing agents β-mercaptoethanol (β-ME), dithiothreitol (DTT), and sodium nitrite (NaNO₂) is studied under aerobic conditions. The fluorescence quantum yields and fluorescence lifetimes are determined as a function of the reducing agent concentration. For all three reducing agents diffusion controlled dynamic fluorescence quenching is observed which is thought to be due to photo-induced reductive electron transfer. For DTT additionally static fluorescence quenching occurs.     

THE PHOTOPHYSICS OF 5-METHOXYINDOLE. A NON-EXCIPLEX FORMING INDOLE: ADDITIONAL EVIDENCE FOR TWO CLASSES OF EXCIPLEXES*

Abstract— 5-Methoxyindole is a non-exciplex forming indole, and its excited state behavior is qualitatively different from that of indole and its methyl substituted derivatives. This fact supports the idea that there are two limiting classes of exciplexes, charge-transfer and dipole-dipole stabilized. The fluorescence quantum yield in water is 0.29 with a lifetime of 4.0 ns at 25d?C. The activation energy for fluorescence quenching in water is 15.9 pM 0.5 kJ/mol, which is smaller than for indole and the methyl substituted indoles which have been measured. In cyclohexane at 25d?C, the fluorescence quantum yield is 0.63 with a lifetime of 6.7 ns. The fluorescence is efficiently quenched by electron scavengers, as is the case for other indoles. Some electron ejection to solvent probably occurs in both solvents.     

两种组氨酸酰胺衍生物的合成及其与人血清白蛋白的结合

L-组氨酸对生物有机体有着良好的亲和能力,通过修饰其化学结构以期寻找药理活性和生物利用度高的衍生物。本文将L-组氨酸分别与反式肉桂酸和对甲氧基肉桂酸反应,合成了两种组氨酸酰胺类衍生物,利用傅里叶变换红外光谱、质谱、氢谱/碳谱核磁共振谱进行了结构表征。采用分子操作环境(MOE)软件分子对接技术、荧光光谱法、同步荧光光谱法(SFS)、紫外-可见光谱法(UV-Vis),共同研究了两种衍生物分别和人血清白蛋白(HSA)相结合的机理。MOE对接结果显示,这两种衍生物与HSA的模拟结合能分别为-13.82和-16.25 kcal/mol,主要是通过范德华力和疏水作用结合在HSA亚结构域ⅡA(即siteⅠ)的疏水腔内。荧光猝灭数据表明,衍生物与HSA相互作用并形成了新的基态配合物,荧光猝灭过程为静态猝灭;不同温度(300、305和310 K)下衍生物与HSA相互作用的结合常数分别为1.773×104、6.354×10~3、1.260×10~3和5.314×10~4、4.614×10~3、1.420×10~3;由热力学参数得到衍生物与HSA的结合过程是由范德华力驱动;SFS表明,衍生物使得HSA的二级结构发生了变化。结合UV-Vis的结果可以确定,在体外生理条件下,组氨酸酰胺类衍生物均可以通过范德华力与HSA结合,并对HSA内源荧光产生静态猝灭及构象影响,这与分子对接结果一致,从而为组氨酸酰胺类衍生物药物的进一步开发提供了参考。     

Quenching of tryptophan fluorescence in various proteins by a series of small nickel complexes

A series of twelve anionic, cationic, and neutral nickel(II) complexes have been synthesized and characterized. The interaction of these complexes with bovine serum albumin (BSA), human serum albumin (HSA), lysozyme (Lyso), and tryptophan (Trp) has been studied using steady-state fluorescence spectroscopy. Dynamic and static quenching constants have been calculated, and the role played in quenching by the ligand and complex charge investigated. The nickel complexes showed selectivity towards the different proteins based on the environment surrounding the Trp residue(s). Only small neutral complexes with hydrophobic ligands effectively quenched protein fluorescence via static quenching, with association constants ranging from 10(2) M(-1) (free Trp) to 10(10) M(-1) (lysozyme), indicating a spontaneous and thermodynamically favorable interaction. The number of binding sites, on average, was determined to be one in BSA, HSA and free Trp, and two in lysozyme.     

Photophysics of anthracene-indole systems in unilamellar vesicles of DMPC and POPC: Exciplex formation and temperature effects

The quenching of anthracene fluorescence by indole (IN), 1,2-dimethylindole (DMI), tryptophan (Trp) and indole 3-acetic acid (IAA) in dimiristoylphophatidylcholine (DMPC) and palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers was investigated. The studies were carried out at 25 degrees C in POPC vesicles and below (15 degrees C) and above (35 degrees C) the phase transition temperature (24 degrees C) of DMPC. A very efficient quenching of the anthracene fluorescence by IN and DMI in the lipid membrane is observed in all cases. It is less efficient in the case of Trp and IAA. Stern-Volmer plots are linear for DMI but present a downward curvature for the other quenchers. This was interpreted as an indication of the presence of an inaccessible fraction of anthracene molecules. By a modified Stern-Volmer analysis the fraction accessible to the quenchers and the quenching constant were determined. Partition constants of the quenchers were obtained from the changes in the fluorescence emission of the indole moiety caused by the presence of the phospholipid. Using the partition constants bimolecular quenching rate constants were determined in terms of the local concentration of quencher in the lipid bilayer. These corrected rate constants are lower than those in homogeneous solvents. In the case of DMPC values the gel phase are higher than in the liquid-crystalline phase. In the quenching by IN and DMI a new, red shifted, emission band appears which could be assigned to an exciplex emission. The exciplex band is absent in the quenching by IAA and Trp.     

咔唑及其衍生物对 9-氰基蒽的荧光猝灭机理的研究

本文研究咔唑及其衍生物对9-氰基蒽(9CNA)的荧光猝灭机理。结果表明, 猝灭过程有以下三种方式:(1)一系列N-烷基咔唑及1,4-二咔唑丁烷、反式1,2-二咔唑环丁烷、N-苄基咔唑等对9CNA的荧光猝灭是通过形成激基复合物。(2)咔唑对9CNA的荧光猝灭是通过形成氢键。(3)1,3-二咔唑丙烷及N-痖烯基咔唑对9CNA的荧光猝灭是属于一般碰撞猝灭过程。以上所有猝灭过程主要都是来自电荷转移相互作用。另外, 还讨论了空间位阻对形成激基复合物的影响。并由稳态和动态荧光实验结果,应用Ware关于激基复合物的形成和解离的动力学公式计算出一系列光物理速率常数。     

A fluorimetric study of pindolol and its complexes with cyclodextrins

Spectrofluorimetric characteristics of pindolol have been investigated with the aim of using this technique for analytical determinations. Other monosubstituted indole derivatives, 4-methoxy and 5-methoxyindole, have been also studied for comparative purposes. Corrected excitation and emission wavelengths in different solvents are reported and the effect of solvent on the Stokes shifts of these compounds has been analysed using the Lippert equation. In addition, the Stokes shift of pindolol has been determined in dioxan-water solvent mixtures and the presence of specific solvent effects is discussed. The fluorescence of pindolol is pH dependent, the quantum yields determined in water are lower than those in other solvents. With respect to the sensitivity, it has been found that the detection limits in aqueous solutions are improved in the presence of beta and methyl-beta-cyclodextrin. Finally, a fluorimetric analysis of the interaction between pindolol and different cyclodextrins has been carried out in order to determine the apparent stability constants of the complexes and the thermodynamic parameters associated to complexation.     

DYNAMICS OF EXCITED FLAVOPROTEINS—PICOSECOND LASER PHOTOLYSIS STUDIES

Abstract— Molecular mechanism of fluorescence quenching of flavins in flavodoxin from Desulfovibrio vulgaris , strain Miyazaki, and riboflavin binding protein from egg white has been investigated by means of picosecond laser photolysis technique. In the case of flavodoxin, a transient absorption band characteristic of the non-fluorescent exciplex formed by electron transfer from indole to excited flavins in model systems has been observed around 600 nm at the delay time of 33 ps from exciting ps pulse pulse width, 25 ps). In the case of riboflavin binding protein, the transient absorption spectra were different from those of flavin-indole exciplex and rather similar to the spectra of the model system of flavin-phenol. These results suggest that tryptophan residue exists near the isoalloxazine nucleus in flavodoxin, and in riboflavin binding protein, tyrosine residue exists near the flavin. Direct measurements of the ultrafast process of the electron transfer in flavoproteins as developed here could provide useful information for elucidating protein dynamics, associated with redox reaction, in the picosecond time region.     

Specific Sequestering Agents for the Actinides. 29. Stability of the Thorium(IV) Complexes of Desferrioxamine B (DFO) and Three Octadentate Catecholate or Hydroxypyridinonate DFO Derivatives: DFOMTA, DFOCAMC, and DFO-1,2-HOPO. Comparative Stability of the Plutonium(IV) DFOMTA Complex(1)

The metal complex stability constants of Th(IV) with desferrioxamine B (DFO) and three octadentate derivatives [N-(2,3-dihydroxy-4-carboxybenzoyl)desferrioxamine B (DFOCAMC), N-(1,2-dihydro-1-hydroxy-2-oxopyridin-6-yl)carbonyl)desferrioxamine B (DFO-1,2-HOPO) and N-(2,3-dihydroxy-4-(methylamido)benzoyl)desferrioxamine B (DFOMTA)] have been determined. The formation constant of the Pu(IV)/DFOMTA complex has also been determined, and the formation constants have been estimated for the other Pu(IV) complexes of octadentate DFO derivatives. The DFO derivatives form 1:1 complexes with Th(IV) in aqueous solution. The solution chemistry of the Th(IV) complexes has been studied by spectrophotometric, potentiometric and proton NMR titrations. The Th(IV) formation constants are as follows (log K(f) values and esd's): DFO, 26.6(1); DFOMTA, 38.55(5); DFOCAMC, 37.2(3); DFO-1,2-HOPO, 33.7(4). The Pu(IV)/DFOMTA formation constant, determined by competitive spectrophotometric titration is (log K(f) value) 41.7(2). The estimation of the other Pu(IV) formation constants are as follows (log K(f) values): DFOCAMC, 40.4; DFO-1,2-HOPO, 36.9. The selectivity of DFO and the three derivatives for actinide(IV) ions is discussed.     

Fluorescence quenching of acridinium and 6-methoxyquinolinium ions by Pb2+, Hg2+, Cu2+, Ag+ and hydrogen sulphide

The quenching of the fluorescene of cationic derivatives of 6-methoxyquinoline and acridine by various heavy metal cations and hydrogen sulphide ion have been studied in detail. Stern—Volmer constants were determined and quenching was found to be exclusively dynamic, except for the hydrogen sulphide ion, which acts as both a dynamic and static quencher. The highly efficient static quenching process is interpreted in terms of nucleophilic attack of hydrogen sulphide to the 9-position of acridinium ion, thereby destroying the conjugated system of the fluorophore.     

Stereodifferentiation in the formation and decay of the encounter complex in bimolecular electron transfer with photoactivated acceptors

Experimental evidence has been obtained for the involvement of encounter complexes between both enantiomers of a pi,pi* triplet excited ketone and a chiral phenol or indole. Determination of the pre-equilibrium constants (K(EC)) and the intrinsic decay rate constants (kd) indicates a significant stereodifferentiation in both steps of the quenching process.