Different Interactions between Isomeric Tetrakis-(N-Hexadecylpyridiniumyl) Porphyrins and CdS Nanoparticles

Using porphyrin amphiphiles TC(16)PyP(2), TC(16)PyP(3), and TC(16)PyP(4) as photosensitizers, the interaction between amphiphilic porphyrins and colloidal CdS nanoparticles was studied by observing their absorption spectra, fluorescence spectra, and fluorescence lifetimes. The experimental results reveal that upon addition of CdS nanoparticles to a TC(16)PyP(3) or TC(16)PyP(4) solution, TC(16)PyP(3) or TC(16)PyP(4) is adsorbed onto the surface of the colloidal nanoparticles due to electrostatic action. The absorption spectra display the characteristic absorption of metalloporphyrin. Moreover, this adsorption also leads to red-shifted fluorescence spectra and the quenching of fluorescence emission. These changes are related to the formation of complexes. Nearly 90% of the fluorescence emission of 5x10(-6) mol/L TC(16)PyP(4) can be quenched with 6.8x10(-4) mol/L CdS colloid nanoparticles. Only 60% of the fluorescence emission of 5x10(-6) mol/L TC(16)PyP(3) can be quenched with 6.8x10(-4) mol/L CdS nanoparticles. The fluorescence quenching is attributable mainly to static quenching. According to the fluorescence quenching curves, the apparent association constants of TC(16)PyP(4) and TC(16)PyP(3) with colloidal CdS nanoparticles are 1.42x10(3) (mol/L)(-1) and 6.76x10(2) (mol/L)(-1), respectively. However, TC(16)PyP(2) does not adsorb onto the surface of colloid nanoparticles due to its larger steric hindrance; its absorption and fluorescence spectra are unchanged. Copyright 2000 Academic Press.     

Photoinduced interaction between fluorescein ester derivatives and CdS colloid

The photoinduced interaction of fluorescein ester derivatives, fluorescein's butyl ester (FL4) and fluorescein's anthraquinone-methyl ester (FL-AQ), and colloidal CdS was examined by absorption, fluorescence spectroscopy, and photoinduced ESR spectroscopy. It is found that FL4 and FL-AQ molecules are adsorbed on the surfaces of CdS colloid by an electrostatic interaction forming the surface complex of the type CdS-FL4 or CdS-FL-AQ. The apparent association constant (K(app)) and the degree of association (alpha) of CdS-FL4 obtained from absorption spectra are 2.25 x 10(4) M(-1) and 0.78, respectively. The values of K(app) and alpha of CdS-FL4 as determined from fluorescence spectra are 1.54 x 10(4) M(-1) and 0.82, respectively, which matches well with that determined from the absorption spectra changes. And the values of K(app) and alpha of CdS-FL-AQ obtained from absorption spectra are 4.18 x 10(4) M(-1) and 0.83, respectively. These data indicate that there is a strong interaction between the dye and the CdS particle surface. But there was no evidence for interfacial electron transfer from FL4 or FL-AQ to colloidal CdS by photoinduced ESR experiments. The fluorescence quenching is due to the formation of a nonfluorescent complex. The related phenomena are discussed in this paper.     

An investigation on electron transfer quenching of zinc(II) meso-tetraphenylporphyrin (ZnTPP) by colloidal TiO(2)

The interaction of zinc(II) meso-tetraphenylporphyrin (ZnTPP) with colloidal TiO(2) was studied by absorption, steady state and time-resolved fluorescence spectroscopy. The quenching was found to obey the Stern-Volmer equation and the corresponding Stern-Volmer plots were linear in the range of quencher concentration used 0-5 x 10(-4)M. The bimolecular quenching rate constants (k(q)) were 20.5 x 10(10)M(-1)s(-1) (steady-state) and 2.85 x 10(10)M(-1)s(-1) (time resolved). The quenching process is suggested to involve electron transfer from the ZnTPP to TiO(2) considering the experimental evidences obtained.     

Cyanobacterial chlorophyll as a sensitizer for colloidal TiO2

Chlorophyll has been extracted from cyanobacteria. The adsorption of chlorophyll on the surface of colloidal TiO(2) through electrostatic interaction was observed. The apparent association constant (K(app)) of chlorophyll-TiO(2) obtained from absorption spectra is 3.78x10(4)M(-1). The K(app) value of chlorophyll-TiO(2) as determined from fluorescence spectra is 1.81x10(4)M(-1), which matches well with that determined from the absorption spectra changes. These data indicate that there is an interaction between chlorophyll and colloidal TiO(2) nanoparticle surface. The dynamics of photoinduced electron transfer from chlorophyll to the conduction band of colloidal TiO(2) nanoparticle has been observed and the mechanism of electron transfer has been confirmed by the calculation of free energy change (DeltaG(et)) by applying Rehm-Weller equation as well as energy level diagram. Lifetime measurements gave the rate constant (k(et)) for electron injection from the excited state chlorophyll into the conduction band of TiO(2) is 4.2x10(8)s(-1).     

Interaction of water‐soluble tetrakis‐(1‐methyl‐4‐pyridiniyl)‐porphyrin and CdS naonoparticles

The interaction between positively charged porphyrin TMPyP(4) (tetrakis‐(1‐methyl‐4‐pyridiniyl) porphyrin) and negatively charged surface of colloidal CdS nanoparticles has been studied by absorption spectrum, fluorescence spectroscopy, and time‐correlated single‐photon‐counting methods. Addition of CdS colloid to a TMPyP(4) solution leads to TMPyP(4) adsorption onto the surface of CdS colloid with an apparent association constant of 4643 L/mol. This adsorption results in distinct changes in absorption spectrum of TMPyP (4) and the quenching of its′ fluorescence, but the biexponential fluorescence decay changes hardly. Nearly 90% of the fluorescence of 5 × 10^?6 mol/L TMPyP(4) can be quenched with 1.5 × 10^?4 mol/L CdS colloid. These spectral changes are attributed to the formation of a certain form complex TMPyP(4)‐CdS.     

Dye induced quenching of firefly luciferase-luciferin bioluminescence

The quenching of firefly bioluminescence (BL) in presence of xanthene dyes and tetratolylporphyrin was investigated. The BL intensity was quenched with an altered decay pattern in presence of xanthene dyes and tetratolylporphyrin. The electronic absorption spectra indicate that there is no significant interaction occurring between the dyes and the BL components in the ground state. The BL quenching decay rate and fluorescence quenching studies of luciferin by the dyes suggest an energy transfer through an exciplex, involving oxyluciferin, in the excited state and the dyes, in the ground state. The bimolecular quenching rate constant (K(q)) values obtained from fluorescence studies varied between 7.7 x 10(12) and 19.8 x 10(12)M(-1)s(-1). The magnitude of the bimolecular quenching rate constants confirmed the complex formation between dye and excited oxyluciferin. The exciplex subsequently undergoes a non-radiative decay to the ground state via a combination of heavy atom induced and F?rster-type energy transfer. The decay rate constants in presence and in absence of dyes vary between 7.47 x 10(-4) and 7.6 x 10(-2)s(-1). In the presence of dyes the effective decay rate constants (k(eff)) increased while the lifetime of light emitting species decreased. The kinetic studies in presence of singlet oxygen scavengers, like beta-carotene and NaN(3), prove that there is no significant quenching of the firefly BL due to the formation of singlet oxygen.     

Fluorescence quenching of bovine serum albumin.

The fluorescence emission spectra and 3D fluorescence spectra of bovine serum albumin (BSA) in cetyltrimethylammonium bromide (CTAB) reversed micelles were affected by the microenvironment. Blue shifts of the fluorescence emission peaks were found when BSA was present in CTAB reversed micelles. The fluorescence intensity changed with the water content. Similar changes in the peak regions of the 3D fluorescence spectra were also observed. CdS nanoparticles prepared in CTAB reversed micelles quenched the fluorescence of BSA significantly. The fluorescence of BSA was more effectively quenched by negative CdS nanoparticles than by positive or neutral CdS ones. The quenching degree increased linearly with increasing the concentration of negative CdS nanoparticles over the range of 5.0 x 10(-6) - 3.0 x 10(-5) mol L(-1). The quenching mechanism is discussed and the quenching constant is 1.32 x 10(4) L mol(-1).     

Fluorescence quenching of meso-tetrakis (4-sulfonatophenyl) porphyrin by colloidal TiO(2)

A stable colloidal TiO(2) has been prepared. The interaction of meso-tetrakis (4-sulfonatophenyl) porphyrin (TSPP) with colloidal TiO(2) was studied by absorption and fluorescence spectroscopy. Upon excitation of its absorption band, the fluorescence emission of TSPP was quenched by colloidal TiO(2). The bimolecular quenching rate constant (k(q)) is 1.78 x 10(11)M(-1)s(-1). The porphyrin can participate in the quenching process by injecting electrons from its excited states into the conduction band of TiO(2). The quenching mechanism is discussed on the basis of the quenching rate constant as well as the reduction potential of the colloidal TiO(2). Rehm-Weller equation was applied for the calculation of free energy change (DeltaG(et)).     

Study on the interaction of new water-soluble porphyrin with DNA

A porphyrin meso-tetrakis{[4-(1-pyridyl)propoxy]phenyl}porphyrin (TPyPP) and its Ni complex (TPyPP(Ni)) have been synthesized and characterized by 1H NMR, UV-vis spectra. The interaction of two porphyrins with calf thymus-DNA (CT-DNA) has been explored by UV-vis, fluorescence and circular dichroic spectroscopy and viscosity measurements. The results suggest that these porphyrins can bind to DNA by the same binding mode. TPyPP outside binds by self-stack with DNA both at low drug load r (=[porphyrin]/[DNA]) and high drug load. Though TPyPP(Ni) has center metal nickel, binding mode with DNA has little difference compared with TPyPP, dominating out-binding mode with different direction along DNA. The binding constants of the TPyPP and TPyPP(Ni) to DNA were 4.65 x 10(5) M(-1) and 3.2 x 10(5) M(-1), respectively. A colored precipitate was found after time in two porphyrin's viscosity measurement. The reasonable interpretation is the porphyrins with alkyl connected N-position of pyridine can strongly interact with the anionic phosphates of DNA and lead to hydrophobic complex.     

DNA binding studies of a solvatochromic fluorescence probe 3-methoxybenzanthrone

A fluorescence probe of 3-methoxybenzanthrone (MBA) exhibits significant solvatochromic characteristics correlated with the polarity of solvents. The interaction of the solvatochromic fluorescence probe with calf thymus DNA (ct-DNA) has been investigated. In the presence of ct-DNA the fluorescence of MBA is strongly quenched with a blue-shift of emission peak and a hypochromism in absorption spectra. The absorption spectra, fluorescence quenching and fluorescence polarization experiments show that the MBA molecule as an intercalator is inserted into the base-stacking domain of the ct-DNA double helix, and the interaction of the nucleobases with the MBA molecule causes quenching of fluorescence and hypochromism in the absorption spectra. The intrinsic binding constant and the binding site number were determined to be 1.70 x 10(5) mol l-1 in base pairs and six, respectively. The I0/I versus [ct-DNA] plot shows linear relationship in the range covering 4.3 x 10(-7)-1.02 x 10(-4) mol l-1 in base pairs which can be used for ct-DNA determination. The limit of detection was found to be 4.3 x 10(-7) mol l-1 in base pairs (0.5 microgram ml-1).     

Photo processes on self-associated cationic porphyrins and plastocyanin complexes 1. Ligation of plastocyanin tyrosine 83 onto metalloporphyrins and electron-transfer fluorescence quenching

The spectroscopic properties of the self-associated complexes formed between the anionic surface docking site of spinach plastocyanin and the cationic metalloporphyrins, in which the tyrosine 83 (Y83) moiety is placed just below the docking site, tetrakis(N-methyl-4-pyridyl)porphyrin (Pd(II)TMPyP(4+) and Zn(II)TMPyP(4+)), have been studied and reported herein. The fluorescence quenching phenomenon of the self-assembled complex of Zn(II)TMPyP(4+)/plastocyanin has also been discovered. The observed red-shifting of the Soret and Q-bands of the UV-visible spectra, ca. 9 nm for Pd(II)TMPyP(4+)/plastocyanin and ca. 6 nm for the Zn(II)TMPyP(4+)/plastocyanin complexes, was explained in terms of exciton theory coupled with the Gouterman model. Thus, the hydroxyphenyl terminus of the Y83 residue of the self-associated plastocyanin/cationic porphyrin complexes was implicated in the charge-transfer ligation with the central metal atoms of these metalloporphyrins. Moreover, ground-state spectrometric-binding studies between Pd(II)TMPyP(4+) and the Y83 mutant plastocyanin (Y83F-PC) system proved that Y83 moiety of plastocyanin played a critical role in the formation of such ion-pair complexes. Difference absorption spectra and the Job's plots showed that the electrostatic attractions between the cationic porphyrins and the anionic patch of plastocyanin, bearing the nearby Y83 residue, led to the predominant formation of a self-associated 1:1 complex in the ground-state with significantly high binding constants (K = (8.0 +/- 1.1) x 10(5) M(-1) and (2.7 +/- 0.8) x 10(6) M(-1) for Pd(II)TMPyP(4+) and zinc variant, respectively) in low ionic strength buffer, 1 mM KCl and 1 mM phosphate buffer (pH 7.4). Molecular modeling calculations supported the formation of a 1:1 self-associated complex between the porphyrin and plastocyanin with an average distance of ca. 9 A between the centers of mass of the porphyrin and Y83 positioned just behind the anionic surface docking site on the protein surface. The photoexcited singlet state of Zn(II)TMPyP(4+) was quenched by the Y83 residue of the self-associated plastocyanin in a static mechanism as evidenced by steady-state and time-resolved fluorescence experiments. Even when all the porphyrin was complexed (more than 97%), significant residual fluorescence from the complex was observed such that the amplitude of quenching of the singlet state of uncomplexed species was enormously obscured.     

Interaction of colloidal AgTiO2 nanoparticles with bovine serum albumin

The interaction between colloidal AgTiO₂ nanoparticles and bovine serum albumin (BSA) was studied by using absorption, steady state, time resolved and synchronous fluorescence spectroscopy measurements. Absorption spectroscopy proved the formation of a ground state BSA?AgTiO₂ complex. Upon excitation of BSA, colloidal AgTiO₂ nanoparticles effectively quenched the intrinsic fluorescence of BSA. The number of binding sites (n = 1.06) and apparent binding constant (K = 3.71 × 10⁵ M⁻¹) were calculated by the fluorescence quenching method. A static mechanism and conformational changes of BSA were observed.     

基于GSH-CdTe/CdS量子点的荧光变化研究hsDNA与盐酸洛美沙星-Cu(Ⅱ)配合物的相互作用

采用水相法合成了谷胱甘肽(GSH)修饰的CdTe/CdS量子点(GSH-CdTe/CdS QDs). 透射电子显微镜表征结果表明, GSH-CdTe/CdS QDs的粒径分布均匀, 分散性好. 在Tris-HCl(pH=7.6)缓冲液中, 由于静电引力作用, 带正电的盐酸洛美沙星(LMFH)-Cu(Ⅱ)配合物[LMFH-Cu(Ⅱ)]吸附到带负电的GSH-CdTe/CdS QDs表面形成基态复合物, 导致GSH-CdTe/CdS QDs的荧光猝灭. 随后, 向GSH-CdTe/CdS QDs-LMFH-Cu(Ⅱ)配合物体系中加入鲱鱼精DNA(hsDNA), hsDNA可诱导LMFH-Cu(Ⅱ)配合物从GSH-CdTe/CdS QDs表面脱落而嵌入到hsDNA的双螺旋结构中, 使GSH-CdTe/CdS QDs的荧光恢复. 通过对GSH-CdTe/CdS QDs荧光的可逆调控, 利用荧光光谱、 紫外-可见吸收光谱和共振瑞利散射光谱研究了hsDNA与LMFH-Cu(Ⅱ)配合物的相互作用. 通过对比GSH-CdTe/CdS QDs与LMFH相互作用的光谱性质, 讨论了GSH-CdTe/CdS QDs-LMFH-Cu(Ⅱ)-hsDNA的相互作用机理, 模拟了作用过程, 从而建立了一种研究氟诺喹酮类药物的金属配合物与核酸相互作用机制的光谱方法.     

Surface-modified CdS quantum dots as luminescent probes for sulfadiazine determination

A novel, sensitive and convenient determine technology based on the quenching of the fluorescence intensity of functionalized CdS quantum dots by sulfadiazine was proposed. Luminescent CdS semiconductor quantum dots (QDs) modified by thioglycollic acid (TGA) were synthesized with the microwave method. The modified CdS QDs are water-soluble, stable and highly luminescent. The possible mechanism for the reaction was also discussed. When sulfadiazine was added into the CdS QDs colloid solution, the surface of CdS QDs generates the electrostatic interaction in aqueous medium, which induces the quenching of fluorescence emission at 489 nm. Under optimum condition, the fluorescence intensity versus sulfadiazine concentration gave a linear response according Stern-Volmer equation with an excellent 0.9981 correlation coefficient. The linearity range of the calibration curve was 1.2 x 10(-5) to 2.13 x 10(-3) mol L(-1). The limit of detection (3delta) is 8.0 micromol L(-1). The relative standard deviation for five determinations of 0.13 x 10(-3)mol L(-1) sulfadiazine is 1.4%. The concentrations of sulfadiazine injections were determined by the proposed method with a satisfactory result.     

Chlorobenzylidine-herring sperm DNA interaction: binding mode and thermodynamic studies

The interaction of chlorobenzylidine with herring sperm DNA has been investigated by fluorescence, absorption, DNA melting experiment and differential scanning calorimetry (DSC). When bound to DNA, chlorobenzylidine shows hypochromism and red shift in absorption spectra, fluorescence quenching and polarization increasing in fluorescence spectra and increasing in DNA melting temperature. These spectral characteristics strongly support intercalation of chlorobenzylidine into herring sperm DNA. Scatchard plots constructed from fluorescence titration data give a binding constant of 3.2 x 10(4) M(-1) and a binding site size of six base pairs per bound drug molecule. The intercalative interaction is exothermic with a van't Hoff enthalpy of -30.6 kJ mol(-1). This result is obtained from DSC experiment. In addition, DeltaG degrees =-28.5 kJ mol(-1), and DeltaS degrees =-7.1 J mol(-1) K(-1). These results show that the binding of chlorobenzylidine to herring sperm DNA is exothermic.     

Photooxidation of dibenzothiophene and 4,6-dimethyldibenzothiophene sensitized by N-methylquinolinium tetrafluoborate: mechanism and intermediates investigation

Photooxidation of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) sensitized by N-methylquinolinium tetrafluoborate (NMQ(+)BF4-) has been investigated in O2-saturated acetonitrile solutions. Nearly 100% oxidation of DBT and DMDBT was observed, and the oxidized products are predominantly composed of sulfoxides and sulfones, which are formed via photoinduced electron transfer (ET). Such ET processes were studied with fluorescence quenching of NMQ+, time-resolved transient absorption measurement, and ESR experiments. The fluorescence of NMQ+ is efficiently quenched by DBT and DMDBT via diffusion-controlled processes, with bimolecular quenching constants of 1.6 x 10(10) M(-1) s(-1) for DBT and 2.3 x 10(10) M(-1) s(-1) for DMDBT. The electron-transfer nature of the quenching is evidenced by the transient absorption measurement of the neutral radical NMQ*, which is formed by electron transfer from the substrates (DBT or DMDBT) to the excited singlet state of NMQ+. The ESR spectra of the superoxide radical anion (O2*-) trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in the photooxygenation of DBT and DMDBT as well as their sulfoxides manifest that O2 traps an electron from NMQ* to form O2*-. The fact that the formation of sulfoxides and sulfones is greatly suppressed in the presence of benzoquinone (BQ), an efficient electron trap for NMQ* and O2*-, further indicates an ET process in the photooxidation of DBT and DMDBT. As inferred from the control experiments, the role of singlet oxygen (1O2) in the photooxidation is negligible. The intermediates responsible for the formation of sulfoxides and sulfones have been examined in detail.     

Functionalization of Diameter-Sorted Semiconductive SWCNTs with Photosensitizing Porphyrins: Syntheses and Photoinduced Electron Transfer

Covalent functionalization of diameter sorted SWCNTs with porphyrins (MP), and photochemistry to establish nanotube diameter-dependent charge separation efficiencies are reported. The MP-SWCNT(n,m) [M=2?H or Zn, and (n,m)=(7,6) or (6,5)] nanohybrids are characterized by a variety of spectroscopic, thermogravimetric, TEM imaging techniques, and also by DFT MO calculations. The thermogravimetric, Raman and fluorescence studies reveal the presence of a moderate number of porphyrins on the SWCNT surface. The MO results suggest charge separation (CS) via the excited state of MP. Time-resolved fluorescence studies reveal quenching of the singlet excited state of the MP with SWCNT(n,m), giving the rate constants of charge separation (k(CS) ) in the range of (4-5)×10(9) s(-1) . Nanosecond transient absorption measurements confirm the charge-separated radical cation and the radical anion as [MP(.+) -SWCNT(.-) ] with their characteristic absorption bands in the visible and near-IR regions. The charge separated states persist for about 70-100?ns thus giving an opportunity to utilize them to build photoelectrochemical cells, which allowed us to derive the structure-reactivity relationship between the nature of porphyrin and diameter of the employed nanotubes.     

Interaction of monosulfonate tetraphenyl porphyrin (H2TPPS1) with plant-esterase: determination of the binding mechanism by spectroscopic methods

The interaction of monosulfonate tetraphenyl porphyrin (H(2)TPPS(1)) with plant-esterase was investigated using fluorescence and UV-vis absorption spectroscopy. Fluorescence quenching, from which the binding parameters were evaluated, revealed that the quenching of the esterase by H(2)TPPS(1) resulted from the formation of a dye-esterase complex. According to the modified Stern-Volmer equation, the effective quenching constants (K(a)) between H(2)TPPS(1) and plant-esterase at four different temperatures (297 K, 300 K, 303 K, and 306 K) were obtained to be 14.132×10(5), 5.734×10(5), 2.907×10(5), and 2.291×10(5) M(-1), respectively. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) for the reaction were calculated to be -181.67 kJ M(-1) and -0.49 kJ M(-1)K(-1), indicating that van der Waals force and hydrogen bonds were the dominant intermolecular force in stabilizing the complex. Site marker competitive experiments showed that the binding of H(2)TPPS(1) to plant-esterase primarily took place in the active site. The binding distance (r) was obtained to be 5.99 nm according to F?rster theory of non-radioactive energy transfer. The conformation of plant-esterase was investigated by synchronous fluorescence and UV-vis absorption spectroscopy, and the results confirmed some micro-environmental and conformational changes of plant-esterase molecules.     

UV and fluorescence spectral changes induced by neodymium binding of N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine] and N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N' diacetic acid

In 0.01 M 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (Hepes), pH 7.4 and room temperature, the binding of neodymium to N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine] (EHPG), or N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N' diacetic acid (HBED) had been studied from 210 to 330 nm by means of difference UV spectra. Two peaks at 240 and 292 nm appear in difference UV spectra after neodymium binding to EHPG or HBED. The 1:1 stable complex can be confirmed from spectral titration curves. The molar extinction coefficient of Nd-EHPG and Nd-HBED complexes are Deltaepsilon(Nd-EHPG)=(12.93+/-0.21) x 10(3)cm(-1)M(-1), Deltaepsilon(Nd-HBED)=(14.45+/-0.51) x 10(5)cm(-1)M(-1) at 240 nm, respectively. Using EDTA as a competitor, the conditional equilibrium constants of the complexes are logK(Nd-EHPG)=11.89+/-0.09 and logK(Nd-HBED)=12.19+/-0.15, respectively. At the same conditions, fluorescence measurements show that neodymium binding to EHPG leads to a quenching of the fluorescence of EHPG at near 310 nm. However, there is no obvious fluorescence change of HBED at 318 nm with the binding of neodymium to HBED.     

Interaction of zinc tetrasulfonated phthalocyanine with cytochrome C in water and Triton-X 100 micelles

The interaction of a zinc tetrasulfonated phthalocyanine with cytochrome c was studied using steady-state spectroscopic techniques and time-correlated single photon counting in water and Triton-X 100 micelles. The dye forms dimers in water with a high equilibrium constant (70 x 10(6) M(-1)). Because of a specific electrostatic interaction, the presence of cytochrome c does not lead to a dissociation of this dimer, but increases its formation, with an equilibrium constant of about 7.9 x 10(9) M(-1). Triton-X 100 micelles dissociate the dimer, creating two populations of dye molecules: one in a hydrophilic media, probably on the surface of the micelles, another on a hydrophobic environment, probably inside the micelles. However, when cytochrome c is added the dye aggregation is again induced leading to a strong fluorescence quenching. This fluorescence quenching may also be caused by a photoinduced electron-transfer due to the formation of a 1:1 complex between the dye and the protein, but the present work does not give direct evidence of such an effect because the fluorescence decays did not show the presence of an extra component. The results presented here are quite different from those reported for aluminum sulfonated phthalocyanines, where aggregation does not occur and the fluorescence quenching is solely due to photoinduced electron-transfer reactions.