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ABSTRACT: Nucleophilic addition of water and of methanol to 3,6-diamino-2,4,5,7-tetrabromo-9-[2-(methoxycarbonyl) phenyl]-9H-xanthen-9-ylium, 4BrR123, yields, respectively, xanthyl, 2-(3,6-diamino-2,4,5,7-tetrabromo-9-hydroxy-9H-xanthen-9-yl) benzoate, HO4BrR123 and, xanthyl, 2-(3,6-diamino-2,4,5,7-tetrabromo-9-methoxy-9H-xanthen-9-yl) benzoate, MeO4BrR123. The novel experimental results are addressed theoretically. The Linear Free Energy Relationship, LFER, Second Order Perturbation Theory analysis of the Natural Bond Orbital, NBO, and Quantum Theory of Atoms in Molecules, QTAIM, lead to the same conclusion: the electron-withdrawing effect of bonded Br atoms in 4BrR123 enhances extremely molecular electrophilicity, as compared to 3,6-diamino-9-[2-(methoxycarbonyl) phenyl]-9H-xanthen-9-ylium, R123. The reactivity of these diaminoxanthylium cations is discussed in the context of local and global softness in extended conjugated systems.
The Journal of Physical Chemistry A 11/2012; · 2.95 Impact Factor
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ABSTRACT: The amphiphile Aerosol OT (1,4-bis(2-ethylhexyl)sodium sulfosuccinate, AOT) forms, in reverse micellar nanoaggregates of water, RM, in isooctane, ion pairs (IPs) with the cationic fluorescent probe dye, rhodamine 3B, (3,6-bis(ethylamino)-9-[2-(ethoxycarbonyl)phenyl]-9H-xanthen-9-ylium, R3B), as either contact ion pairs, CIPs, or solvent (water) separated ion pairs, SSIPs. The ground-state AOT R3B ion pairs' equilibria as well as the dynamics of R3B electronic excited states show the progressive hydration of AOT(-) R3B(+) toward solvent separated ion pairs, SSIPs as the characteristic reverse micelle parameter w(0) = [H(2)O]/[AOT] increases. The apparent limiting hydration constant of R3B ion pairs, K(hyd) = 2.8 +/- 0.2, corresponds to full hydration of AOT, consistent with 1-3 water molecules per AOT polar head. Transient relaxations at w(0) = 0.2, with a 375 +/- 15 ps decay at 550 nm decrease to 115 +/- 15 ps at w(0) = 7.2 turning into corresponding rises at 588 nm. At higher w(0), water induced dynamics becomes faster. The lifetime is longer in RM with smaller w(0), due to the presence of CIPs that inhibit intrinsic nonradiative decay processes, which in contrast shorten the decay times at higher w(0), due to the presence of SSIPs. The pairs' electronic excited-state properties are sensitive to viscosity and local polarity of the surrounding environment of the interfacial regions of AOT reverse micellar nanoaggregates.
The Journal of Physical Chemistry B 08/2010; 114(32):10417-26. · 3.70 Impact Factor
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José A B Ferreira
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ABSTRACT: Exciton coupling in pi-pi complexes between the indole ring and other pi systems is known to enhance the efficiency of energy and electron transfer. Rhodamines' xanthylium rings allow the formation of weakly or nonfluorescent complexes with the amino acid tryptophan. Thus, because of the short distance of the participating electronic clouds, intrinsic electron transfer-induced fluorescence quenching occurs. In solution, the rate constant of electron transfer is known to be limited by collision interactions at the contact distance. By contrast, in protein local environments tryptophan residues can be either exposed or buried in hydrophobic regions. Herein, I report on the properties of aromatic derivatized rhodamines, among which is one with a bound phenylalanine amino acid group. Encompassed is the spectroscopic and kinetic information in bulk and at the single-molecule levels both in free solution and in the presence of human serum albumin. Spectroscopic characteristics are focused with special emphasis on enhanced fluorescence that is addressed considering optimized geometries and electronic spectra. The importance of the probes associated with peptides and metal ions both in condensed phase or interfaces and as substrates with proteins is put into perspective.
Annals of the New York Academy of Sciences 02/2008; 1130:85-90. · 3.15 Impact Factor
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ABSTRACT: The effect of osmolyte sucrose on the stability and compaction of the folded and unfolded states of ribosomal protein S6 from Thermus thermophilus was analyzed. Confirming previous results obtained with sodium sulfate and trehalose, refolding stopped-flow measurements of S6 show that sucrose favors the conversion of the unfolded state ensemble to a highly compact structure (75% as compact as the folded state). This conversion occurs when the unfolded state is suddenly placed under native conditions and the compact state accumulates in a transient off-folding pathway. This effect of sucrose on the compaction of the unfolded state ensemble is counteracted by guanidinium hydrochloride. The compact state does not accumulate at higher guanidinium concentrations and the unfolded state ensemble does not display increased compaction in the presence of 6 M guanidinium as evaluated by collisional quenching of tryptophan fluorescence. In contrast, accessibility of the tryptophan residue of folded S6 above 1 M sucrose concentration decreased as a result of an increased compaction of the folded state. Unfolding stopped-flow measurements of S6 reflect this increased compaction of the folded state, but the unfolding pathway is not affected by sucrose. Compaction of folded and unfolded S6 induced by sucrose occurs under native conditions indicating that decreased protein conformational entropy significantly contributes to the mechanism of protein stabilization by osmolytes.
Biochemistry 03/2006; 45(7):2189-99. · 3.42 Impact Factor
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ABSTRACT: [Ti[N(Ph)SiMe2]3-tacn]X complexes (X = Cl, 1; I, 2; PF6, 3; BPh4, 4) were studied by NMR and electron absorption and emission methods, which showed that these compounds exist in bromobenzene and dichloromethane solutions as ion pairs. The significant modifications observed in the proton resonances of tacn in C6D5Br, which follow the sequence BPh(4-) > or = PF(6-) > or = I- approximately Cl-, are a qualitative indication of the strength of the interactions that depend on the anion. The reaction of 2 with LiNMe2 led to [Ti(NPh)[NPh(SiMe2)]2-tacn], 5, that forms upon attack of Me2N- at one SiMe2 group. The formation of 5 is discussed on the basis of the interactions identified in solution.
Inorganic Chemistry 11/2005; 44(24):9017-22. · 4.60 Impact Factor
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ABSTRACT: The intramolecular activated nonradiative decay of the first excited singlet of the Rhodamine 3B cation is studied in a large number of polar solvents covering a wide range of viscosities. The photophysics are interpreted in terms of an isomerization process occurring at the diethyl amino group with adiabatic barrier crossing. The experimental data show that friction effects determine the reaction rates, which are modeled in the framework of Kramers and Grote−Hynes theories. The reactant well wavenumber (ω0/2πc) is estimated as 30 cm-1 and coincides with the wavenumber of the barrier top (ωb/2πc) for the less viscous solvents. The former value is invariant with the solvent and is a good approximation to the reactive molecular mode involved in the nonradiative deactivation. The increase in dipole moment at the barrier top relative to the bottom of the reactant well is inferred from intrinsic barrier heights whose stabilization energies vary according to Onsager's theory of dielectrics.
12/2000;
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ABSTRACT: The solvent kinetic isotope effect in the nonactivated rate constants of Rhodamine 123 and Rhodamine B is discussed in water–H2, relatively to water–D2. In their mixtures the rate constants of the radiationless process in Rhodamine 123 decrease with the deuterium atomic content in the mixture. In water–H2 the process shows activation energy of the order of thermal fluctuations while in water–D2 it is activationless. Batochromic spectral shifts of the electronic absorption and emission are observed in water–H2, relatively to water–D2, and are of the order of the activation energy of nonradiative decay rate constants. The changes cannot be accounted for only in terms of dielectric contributions from solvent mode coupling to the Franck–Condon transitions. The variations are correlated with the molecular structural pattern of rigidified rhodamines and compared considering the energy release efficiency of vibrational donor modes to external acceptor modes. NH stretching vibrations are an order of magnitude more efficient than CH towards the dissipation of the electronic excitation energy of the first excited singlet-state of rhodamines and lower frequency vibrations also contribute.
Chemical Physics.
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ABSTRACT: A novel pH fluorescent chemosensor based on 2-hydroxybenzaldehyde 1-naphthoyl hydrazone (SNH) has been synthesized. This sensor has the ability to respond to both low and high pH values at different wavelengths. In acidic media, the fluorescence of SNH enhanced dramatically at shorter wavelength (λem = 380 nm, ϕf = 0.08) while in basic media the fluorescence at longer wavelength (λem = 500 nm, ϕf = 0.02) increased. At intermediate pH values (5–9), is detected the appearance of a weak emission at 460 nm, likely due to a keto-phototautomer associated to an intramolecular excited-state proton transfer. A complex mechanism tunable in the range of pH 1–11 with at least three fluorescent species at λem = 380, 460 and 500 nm is proposed.
Journal of Photochemistry and Photobiology A: Chemistry.
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ABSTRACT: A novel method to distribute proteins on solid surfaces is proposed. Proteins microencapsulated in the water pool of reverse micelles were used to coat a solid surface with well-individualized round spots of 1 to 3 microm in diameter. The number of spots per unit area can be increased through the concentration of reverse micelles, and networks of spots were obtained at high concentrations of large reverse micelles. Moreover, depending on the pool size of the water reverse micelles, proteins can be deposited far from each other or in close proximity within the range of 50 to 70 A. This proximity obtained with small reverse micelles was proved through fluorescence lifetime imaging microscopy and fluorescence resonance energy transfer (FLIM-FRET) measurements for the most relevant FRET pair in cell biology studies, the cyan and yellow fluorescent proteins. This novel procedure has several advantages and reveals the potential for study of protein-protein interactions on solid surfaces and for developing novel biomaterials and molecular devices based on biorecognition elements.
Journal of Biomedical Optics 14(4):044035. · 3.16 Impact Factor
