Nitrite Reduction in Aqueous Solution Mediated by Amavadin Homologues: N2 O Formation and Water Oxidation.

Reactions of two vanadium(IV) complex anions that are homologues of amavadin, [V(HIDPA)2 ]2- and [V(HIDA)2 ]2- (HIDPA=N-oxyiminodipropionate, HIDA=N-oxyiminodiacetate), with the nitrite ion (NO2- ) in aqueous solution were investigated by experimental (absorption spectroscopy in the visible range, through measurements of dioxygen formed in solution from water oxidation and identification of nitrogen oxide species of a gaseous atmosphere from nitrite reduction by using an IR analyser) and theoretical methods. Two reactions, mediated by the vanadium complexes, with environmental and biological significance, were observed in this system, namely, reduction of nitrite to N2 O and oxidation of water to molecular oxygen. The reduction of nitrite, as studied by DFT calculations, occurs through the formation of NO (ΔG≠ =14.3 kcal mol-1 ), which is strongly dependent on pH and slightly endergonic, and is then easily converted into N2 O, with an overall activation barrier of ΔG≠ =11.8 kcal mol-1 . The later process includes dimerisation of NO assisted by one molecule of the V complex, protonation and oxidation of the formed ONNO.- ligand by another amavadin molecule or by nitrite, and NO bond cleavage/proton transfer in the ONNOH- ligand. The results indicate that amavadin exhibits an unusual nitrite reductase type activity that could be involved in nitrogen metabolism of Amanita muscaria and other fungi containing this vanadium complex.

Electrophilic reactivity of tetrabromorhodamine 123 is bromine induced: convergent interpretation through complementary molecular descriptors.

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 2-(3,6-diamino-2,4,5,7-tetrabromo-9-hydroxy-9H-xanthen-9-yl)xanthyl benzoate, HO4BrR123 and 2-(3,6-diamino-2,4,5,7-tetrabromo-9-methoxy-9H-xanthen-9-yl)xanthyl 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 extremely enhances the 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.

Structural effects of biologically relevant rhodamines on spectroscopy of fluorescence fluctuations.

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.

The near-mid-IR HOMO-LUMO gap in amide linked porphyrin-rhodamine dyads.

Novel amide linked porphyrin-rhodamine dyads yield utmost intense red-shifted electronic transitions beyond the near-infrared region.

Electronic excited-state behavior of rhodamine 3B in AOT reverse micelles sensing contact ion pair to solvent separated ion pair interconversion.

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.

Fluorescence lifetime imaging microscopy and fluorescence resonance energy transfer from cyan to yellow fluorescent protein validates a novel method to cluster proteins on solid surfaces.

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.

Compaction of ribosomal protein S6 by sucrose occurs only under native conditions.

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.

Ion pairing in Ti(IV) trisamidotriazacyclononane compounds.

[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.

Activated radiationless decay of rhodamine-3B: nonequilibrium polarization effects in viscous solvents.

Nonequilibrium polarization effects that arise in high viscous polar solvents are discussed as regards to the rhodamine-3B-activated radiationless process. Rate constants are interpreted using dipole isomerization theories which enable the recovery of a barrier top region wave number identical to that previously obtained in less viscous solvents [J. Phys Chem. A 104, 11909 (2000)]. The Onsager-frequency-dependent reaction field can model the friction effect on the rate constants that in glycerol were estimated also from an adiabatic charge-transfer model. The cusp barrier height is half the electronic coupling, as expected from the equality found for the frequencies of the reactant well and barrier top in this process. Coupling to solvent polarization modes can control the friction effect on the reactive mode. A two-dimensional reaction surface explains the photophysical features detected in the radiationless decay and a state energy diagram is proposed for rhodamine-3B.