Fluorescence resonance energy transfer from tryptophan in human serum albumin to a bioactive indoloquinolizine system

Evaluation of Antifungal Activity and Potential Application as Fluorescent Probes of Indolenine and Benzo[e]Indole-Based Squarylium Dyes

The antifungal performance and the possible use as fluorescent probes of a series of squarylium dyes derived from indolenine and benzo[e]indole previously synthesized was evaluated. Some photophysical properties were performed in ethanol and phosphate buffer, and the type of aggregates form in phosphate buffer was analyzed. Using the 1,3-diphenylisobenzofuran assay, a qualitative assessment of the capacity of dyes to produce singlet oxygen after irradiation was performed. Regarding the antifungal activity, this was studied through a broth microdilution assay using Saccharomyces cerevisiae PYCC 4072 as a biological model. The effect of irradiation of the dyes, with an appropriate light emitting diode system, on the antifungal activity was also evaluated, and it was verified that some of the dyes improve their activity after irradiation. Using fluorescence microscopy techniques, the colocalization of dyes in S. cerevisae cells was investigated and it was possible to verify that some of the squarylium dyes with a barbituric moiety in the four-membered central ring stained and accumulated preferentially in the mitochondrial web and perinuclear membrane of the cells. The possible use as a fluorescent probe for the detection of HSA was also evaluated for one of the dyes of the series, demonstrating a linear variation in the fluorescence intensity accompanied by the increase in the protein concentration.

Binding interactions and FRET between bovine serum albumin and various phenothiazine-/anthracene-based dyes: a structure-property relationship

The present study demonstrates binding interactions and Förster resonance energy transfer (FRET) between bovine serum albumin (BSA) and a series of structurally and electronically diverse phenothiazine (PTZ) and anthracene (ANT) dyes. Upon selective excitation of tryptophan (Trp) residues of BSA, radiationless energy transfer to a dye takes place, resulting in fluorescence quenching of the former. Fluorescence quenching mechanisms, FRET parameters, possible locations, and binding constants of dyes with the BSA have been examined to deduce a structure–property relationship. The mechanism of quenching is apparently static in nature. PTZ dyes with heteroatoms and a pentyl tail (C5-PTZ) attached to them were found to have a stronger binding affinity with BSA as compared to ANT dyes. Stronger binding affinities of C5-PTZ dyes with BSA result in greater energy transfer efficiencies (E_T). A dye with a strong electron-withdrawing group present in it has shown better energy accepting capability. A FRET study with dicyanoaniline (DCA) analogs of PTZ and ANT dyes (C5-PTZDCA and ANTDCA, respectively) revealed that E_T depends on electronic and structural factors of molecules. An almost orthogonal geometry between ANT and DCA moieties (∼79°) in ANTDCA induces the greater extent of electron transfer from ANT to DCA, showing a higher E_T for this dye as compared to C5-PTZDCA in which the torsion angle is only ∼38°. Further, the observed facts have been validated by experimentally determined bandgaps (using cyclic voltammetry experiments) for all the dyes. Thus, the hydrophobic character and the presence of interactive substituents along with the electron-accepting abilities majorly control the FRET for such dyes with BSA.

The present study demonstrates binding interactions and Förster resonance energy transfer (FRET) between bovine serum albumin (BSA) and a series of structurally and electronically diverse phenothiazine (PTZ) and anthracene (ANT) dyes.

Fluorescence Resonance Energy Transfer, Small-Angle Neutron Scattering, and Dynamic Light Scattering Study on Interactions of Gemini Surfactants Having Different Spacer Groups with Protein at Various Regions of Binding Isotherms

The binding interactions of three gemini surfactants having different spacer groups (12-4-12, 12-8-12, and 12-4(OH)-12) with a high concentration (150 μM) of bovine serum albumin (BSA) at various regions of binding isotherms have been studied by means of steady-state fluorescence and fluorescence anisotropy, time-correlated single-photon counting fluorescence of trans-2-[4-(dimethylamino)styryl]benzothiazole, small-angle neutron scattering (SANS), and dynamic light scattering (DLS) measurements. The fluorescence resonance energy transfer phenomenon between the twisted intramolecular charge transfer fluorescent molecule, trans-2-[4-(dimethylamino)styryl]benzothiazole as an acceptor, and tryptophan 213 (Trp-213) of BSA as a donor has been successfully used to probe the binding interactions of gemini surfactants with protein at all regions of binding isotherms. The increasing order of energy transfer efficiency at a higher concentration range of surfactants is 12-8-12 > 12-4-12 > 12-4(OH)-12. Stronger binding of micelles of gemini surfactant molecules having a comparatively more hydrophobic spacer group with the hydrophobic segments of the protein results in closer approach of trans-2-[4-(dimethylamino)styryl]benzothiazole molecules solubilized in micelles to Trp-213. The average excited-state lifetimes become shorter with a trend of increase in contribution from the fast component and decrease in contribution from the slow component to the decay with increasing concentration of a surfactant. The nonradiative rate constant of trans-2-[4-(dimethylamino)styryl]benzothiazole increases with increasing concentration of a surfactant because the average microenvironment around it in protein-surfactant aggregates is more polar as compared to that in native protein. SANS and DLS measurements were carried out for the study of the structural deformations in the protein, on enhancement of the concentration of the gemini surfactants. The necklace and bead model has been used for the analysis of SANS data for the protein-surfactant complexes. At a higher concentration range, 12-8-12 and 12-4-12 have a slightly smaller fractal dimension and a larger correlation length as compared to 12-4(OH)-12. DLS data show that the increasing order of hydrodynamic diameter for the complexes of protein with three gemini surfactants in their high concentration range is 12-4(OH)-12 < 12-4-12 < 12-8-12.

Effects of Visible-Light Irradiation of Protoporphyrin IX on the Self-Assembly of Tubulin Heterodimers

The formation and the effects of laser irradiation of the complex formed by protoporphyrin IX (PPIX) and tubulin was investigated. We have used tubulin as a model protein to investigate whether docked photoactive ligands can affect the structure and function of polypeptides upon exposure to visible light. We observed that laser irradiation in the Soret band prompts bleaching of the PPIX, which is accompanied by a sharp decrease in the intensity and average fluorescence lifetime of the protein (dominated by the four tryptophan residues of the tubulin monomer). The kinetics indicate non-trivial effects and suggest that the photosensitization of the PPIX bound to tubulin prompts structural alterations of the protein. These modifications were also observed through changes in the energy transfer between Trp residues and PPIX. The results suggest that laser irradiation produces localized partial unfolding of tubulin and that the changes prompt modification of the formation of microtubules in vitro. Measurements of singlet oxygen formation were inconclusive in determining whether the changes are prompted by reactive oxygen species or other excited state mechanisms.

Potentiating the cellular targeting and anti-tumor activity of Dp44mT via binding to human serum albumin: two saturable mechanisms of Dp44mT uptake by cells

Di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT) demonstrates potent anti-cancer activity. We previously demonstrated that ¹⁴C-Dp44mT enters and targets cells through a carrier/receptor-mediated uptake process. Despite structural similarity, 2-benzoylpyridine 4-ethyl-3-thiosemicarbazone (Bp4eT) and pyridoxal isonicotinoyl hydrazone (PIH) enter cells via passive diffusion. Considering albumin alters the uptake of many drugs, we examined the effect of human serum albumin (HSA) on the cellular uptake of Dp44mT, Bp4eT and PIH. Chelator-HSA binding studies demonstrated the following order of relative affinity: Bp4eT≈PIH>Dp44mT. Interestingly, HSA decreased Bp4eT and PIH uptake, potentially due to its high affinity for the ligands. In contrast, HSA markedly stimulated Dp44mT uptake by cells, with two saturable uptake mechanisms identified. The first mechanism saturated at 5-10 μM (B_max:1.20±0.04 × 10⁷ molecules/cell; K_d:33±3 μM) and was consistent with a previously identified Dp44mT receptor/carrier. The second mechanism was of lower affinity, but higher capacity (B_max:2.90±0.12 × 10⁷ molecules/cell; K_d:65±6 μM), becoming saturated at 100 μM and was only evident in the presence of HSA. This second saturable Dp44mT uptake process was inhibited by excess HSA and had characteristics suggesting it was mediated by a specific binding site. Significantly, the HSA-mediated increase in the targeting of Dp44mT to cancer cells potentiated apoptosis and could be important for enhancing efficacy.

DNA Binding, Photonuclease Activity and Human Serum Albumin Interaction of a Water-Soluble Freebase Carboxyl Corrole

The DNA binding property of 5,10,15-Tris(4-carboxyphenyl) corrole (TCPC) was studied by UV-Visible, fluorescence and circular dichroism (CD) spectroscopic methods. TCPC can bind to ct-DNA via an outside binding mode with the binding constant of K_b = 1.05 × 10⁵ M⁻¹. TCPC also displayed good photonuclease activity, which involves singlet oxygen species (¹O₂). The binding constant between TCPC and human serum albumin (HSA) is K_A = 2.24 × 10⁵ M⁻¹ with a simulated binding distance of 2.06 nm. The fluorescence quenching of HSA by TCPC followed a static quenching process. Site marker competitive displacement experiments indicated that warfarin site I is the main binding site. The secondary structure of HSA was changed upon interaction with TCPC, which was confirmed by UV-Visible and CD spectroscopy.

Time resolved spectroscopy and gain studies of Fullerenes C60 and C70

The fluorescence decay time of Fullerenes C60 and C70 in pure form as well as in mixture with Coumarin C440 and Quinizarine dyes are studied. Results indicate that the decay of pure fullerenes is constant throughout the solute concentration and it is also independent of excitation wavelength, whereas in the case of mixture with dyes different behavior is noticed. We have also calculated the Stern-Volmer quenching constant and optical gain of both the fullerenes from which it is found that the optical gain is positive for Fullerene C70 only in a very narrow range of concentration.

Comparison of Förster-resonance-energy-transfer acceptors for tryptophan and tyrosine residues in native proteins as donors

Homogenous bioaffinity analysis with tryptophan/tyrosine residues in native proteins as FÖrster-resonance-energy-transfer (FRET) donors is feasible when suitable fluorophors can act as FRET acceptors in ligands (FRET probes) and FRET efficiency in complexes of proteins and FRET probes is high enough. In complexes of proteins and FRET probes, suitable acceptors should have excitation peaks around 335 nm and high rotation freedom, are preferred to have sufficient quantum yields and excitation valleys around 280 nm. In protein binding sites mimicked with mixtures of neutral phosphate buffer and organic solvents, quantum yields of candidate acceptors are altered inconsistently but their excitation peaks show tiny changes. Fluorophores as acceptors in such FRET probes are buried inside glutathione-S-transferase and have low rotation freedom, but are localized on streptavidin surface and display high rotation freedom; FRET efficiency in complexes of streptavidin and its FRET probes is much stronger than that in complexes of glutathione-S-transferase and its FRET probes. Specially, the quantum yield is about 0.70 for free 1-naphthylamine probe in neutral phosphate buffer, about 0.50 for 1-naphthylamine probe bound by streptavidin, and about 0.15 for that bound by glutathione-S-transferase. The quantum yield is about 0.06 for free dansylamide probe, about 0.11 for dansylamide probe bound by streptavidin and about 0.27 for that bound by glutathione-S-transferase. Therefore, 1-naphthylamine and dansylamide are effective acceptors when they localize on surfaces of complexes of proteins and FRET probes.

Biological evaluation and interaction of a newly designed anti-cancer Pd(II) complex and human serum albumin

The pharmacokinetics and pharmacodynamics of any drug will depend, largely, on the interaction that has with human serum albumin (HSA), the most abundant plasma protein. The interaction between newly synthesized Pd(II) complexe, 2,2'-bipyridin Butylglycinato Pd(II) nitrate, an anti-tumor component, with HSA was studied at different temperatures by fluorescence, far UV circular dichroism (CD), UV-visible spectrophotometry and theoretical approaches. The Pd(II) complex has a strong ability to quench the intrinsic fluorescence of HSA through a dynamic quenching procedure. The binding parameters and thermodynamic parameters, including δH°, δS° and δG° were calculated by fluorescence quenching method, indicated that hydrophobic forces play a major role in the interaction of Pd(II) complex with HSA. Based on Autodock, FRET (fluorescence resonance energy transfer) and fluorescence quenching data, it may be concluded that one of the binding sites in the complex of HSA is near the only one Trp of HSA (Trp214) in sub domain IIA of the protein. Far-UV-CD results indicated that Pd(II)-complex induced increase in the α-helical content of the protein. The anti-tumor property of the synthesized Pd(II) complex was studied by testing it on human tumor cell line K562. The 50% cytotoxic concentration (Cc₅₀) of complex was determined using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay. Also, fluorescence staining with DAPI (4,6-diamidino-2-phenylindole) revealed some typical nuclear changes that are characteristic of apoptosis which is induced at Cc₅₀ concentration of Pd(II) complex in K562 cell line after 24 h incubation. Our results suggest that Pd(II) complex is a promising anti-proliferative agent and should execute its biological effects by inducing apoptosis.

Förster's resonance energy transfer between Fullerene C60 and Coumarin C440

The interaction between Coumarin C440 with Fullerene C60 has been studied by fluorescence and time resolved spectroscopic techniques. The Coumarin C440-Fullerene C60 pair shows Forster's resonance energy transfer (FRET) from Coumarin C440 (donor) to Fullerenes C60 (acceptor). The FRET efficiency of this pair increases with the increase of the acceptor concentration. The critical energy transfer distance (R0) at which transfer efficiency is 50% is found to be 34Ǻ. Stern-Volmer plot indicates static as well as dynamic quenching. However, the FRET studies show highest efficiency at the critical stage of dimer formation.

Homogeneous competitive assay of ligand affinities based on quenching fluorescence of tyrosine/tryptophan residues in a protein via Főrster-resonance-energy-transfer

A new homogeneous competitive assay of ligand affinities was proposed based on quenching the fluorescence of tryptophan/tyrosine residues in a protein via Főrster-resonance-energy-transfer using a fluorescent reference ligand as the acceptor. Under excitation around 280 nm, the fluorescence of a protein or a bound acceptor was monitored upon competitive binding against a nonfluorescent candidate ligand. Chemometrics for deriving the binding ratio of the acceptor with either fluorescence signal was discussed; the dissociation constant (K(d)) of a nonfluorescent candidate ligand was calculated from its concentration to displace 50% binding of the acceptor. N-biotinyl-N'-(1-naphthyl)-ethylenediamine (BNEDA) and N-biotinyl-N'-dansyl-ethylenediamine (BDEDA) were used as the reference ligands and acceptors to streptavidin to test this new homogeneous competitive assay. Upon binding of an acceptor to streptavidin, there were the quench of streptavidin fluorescence at 340 nm and the characteristic fluorescence at 430 nm for BNEDA or at 525 nm for BDEDA. K(d) of BNEDA and BDEDA was obtained via competitive binding against biotin. By quantifying BNEDA fluorescence, K(d) of each tested nonfluorescent biotin derivative was consistent with that by quantifying streptavidin fluorescence using BNEDA or BDEDA as the acceptor. The overall coefficients of variation were about 10%. Therefore, this homogeneous competitive assay was effective and promising to high-throughput-screening.

Binding interaction of indomethacin with human serum albumin

The interaction between indomethacin and human serum albumin (HSA) was investigated by fluorescence quenching technique and UV-vis absorption spectroscopy. The results of fluorescence titration revealed that indomethacin, strongly quench the intrinsic fluorescence of HSA by static quenching and nonradiative energy transfer. The binding site number n and the apparent binding constant K(A), were calculated using linear and nonlinear fit to the experimental data. The distance r between donor (HSA) and acceptor (indomethacin) was obtained according to fluorescence resonance energy transfer (FRET). The study suggests that the donor and the acceptor are bound at different locations but within the quenching distance.