W-band transient EPR and photoinduced absorption on spin-labeled fullerene derivatives.

We investigated by W-band (94 GHz) transient electron paramagnetic resonance (TREPR) and photoinduced absorption (PIA) spectroscopy two fullerene derivatives bearing a nitroxide radical unit. After pulsed laser photoexcitation of the molecules in liquid toluene solution, complex EPR spectra are recorded, with lines in absorption and emission. The intrinsic higher spectral and temporal resolution of the W-band frequency leads to the assignment of all the lines in the spectrum and the determination of the sign and the absolute value of the exchange coupling between the fullerene in its photoexcited triplet state (S(T) = 1) and the radical (S(R) = 1/2). The two compounds with different fullerene-nitroxide spacers show opposite-ferromagnetic and antiferromagnetic-exchange couplings. The time evolution of the spectra and the polarization of the lines are interpreted in terms of several possible spin polarization mechanisms. The EPR measurements are complemented with PIA experiments.

Investigation of the inner environment of carbon nanotubes with a fullerene-nitroxide probe.

A fulleropyrrolidine bearing a nitroxide free radical has been inserted into single-walled carbon nanotubes with the aid of supercritical CO2. Thanks to the encapsulated paramagnetic probes, it has been possible to detect and characterize the resulting peapod-like structure through electron paramagnetic resonance (EPR) spectroscopy. In particular, the analysis of spectral parameters derived from extensive EPR studies has elucidated the orientation and the residual rotational dynamics of the molecules embedded in the nanotubes. A limited anisotropic rotational freedom of the encapsulated fullerene nitroxide reveals a rather strong interaction of the probe with the surrounding nanotube walls. The interaction seems to involve the fullerene cage (as confirmed by Raman spectroscopy) and not the nitroxide moiety, whose EPR spectral characteristics, such as the isotropic hyperfine constant and the g-tensor, remain unaltered after encapsulation.

Ab initio modeling of CW-ESR spectra of the double spin labeled peptide Fmoc-(Aib-Aib-TOAC)2-Aib-OMe in acetonitrile.

In this work we address the interpretation, via an ab initio integrated computational approach, of the CW-ESR spectra of the double spin labeled, 310-helical, peptide Fmoc-(Aib-Aib-TOAC)2-Aib-OMe dissolved in acetonitrile. Our approach is based on the determination of geometric and local magnetic parameters of the heptapeptide by quantum mechanical density functional calculations taking into account solvent and, when needed, vibrational averaging contributions. The system is then described by a stochastic Liouville equation for the two electron spins interacting with each other and with two 14N nuclear spins, in the presence of diffusive rotational dynamics. Parametrization of the diffusion rotational tensor is provided by a hydrodynamic model. CW-ESR spectra are simulated with minimal resorting to fitting procedures, proving that the combination of sensitive ESR spectroscopy and sophisticated modeling can be highly helpful in providing 3D structural and dynamic information on molecular systems.

Insulin generates free radicals by an NAD(P)H, phosphatidylinositol 3'-kinase-dependent mechanism in human skin fibroblasts ex vivo.

Oxidative stress may be involved in the development of vascular complications associated with diabetes; however, the molecular mechanism responsible for increased production of free radicals in diabetes remains uncertain. Therefore, we examined whether acute hyperinsulinemia increases the production of free radicals and whether this condition affects proliferative extracellular signal-regulated kinase (ERK-1 and -2) signaling in human fibroblasts in vitro. Insulin treatment significantly increased intracellular superoxide anion (O(2)(-)) production, an effect completely abolished by Tiron, a cell-permeable superoxide dismutase (SOD) mimetic and by polyethylene glycol (PEG)-SOD, but not by PEG catalase. Furthermore, insulin-induced O(2)(-) production was attenuated by the NAD(P)H inhibitor apocynin, but not by rotenone or oxypurinol. Inhibition of the phosphatidylinositol 3'-kinase (PI 3'-kinase) pathway with LY294002 blocked insulin-stimulated O(2)(-) production, suggesting a direct involvement of PI 3'-kinase in the activation of NAD(P)H oxidase. The insulin-induced free radical production led to membranous translocation of p47phox and markedly enhanced ERK-1 and -2 activation in human fibroblasts. In conclusion, these findings provided direct evidence that elevated insulin levels generate O(2)(-) by an NAD(P)H-dependent mechanism that involves the activation of PI 3'-kinase and stimulates ERK-1- and ERK-2-dependent pathways. This effect of insulin may contribute to the pathogenesis and progression of cardiovascular disease in the insulin resistance syndrome.

Time-resolved electron paramagnetic resonance of photoinduced ion pairs in blends of polythiophene and fullerene derivatives.

Substituted polythiophene and triethylenglycolpyrrolidino-C(60) blends are examined by time-resolved electron paramagnetic resonance (TR-EPR) at different temperatures. TR-EPR spectra recorded on the microsecond time scale after a short laser pulse are assigned to polythiophene and fullerene radical ion pairs, generated by electron transfer from the excited state of polythiophene to fullerene. At low temperatures, TR-EPR spectra show polarized lines with an antiphase emission/absorption pattern. The origin of the polarization pattern is described in the frame of spin correlated radical pair theory, in which two unpaired electron spins (on radical cation and anion, respectively) interact through isotropic spin exchange and anisotropic dipolar interactions. The polarization pattern is accounted for assuming a singlet excited state as the precursor of the charge-separated state. Spectral simulations yield dipolar and spin exchange coupling constants between unpaired electrons of the radical ion pair. Their values correspond to a mean distance between opposite charges of 22 A. When the temperature is increased, the spectra gradually loose their antiphase character and eventually consist of a signal totally in emission. This behavior is explained by a polarization mechanism involving a spin-selective charge recombination (ST(-1) mixing). The polarization pattern at different temperatures is examined in detail, and its generating mechanism is discussed.

First observation of the hyperfine structure of an excited quintet state in liquid solution.

After pulsed photoexcitation of a new fullerene-linked bisnitroxide, a well resolved transient EPR spectrum is detected which is assigned to an excited quintet spin state generated by spin coupling of the nitroxides and the fullerene excited triplet.

Solar cells based on a fullerene-azothiophene dyad.

A power conversion efficiency of 0.37%, under white light of 80 mW cm-2 intensity, was obtained when a fullerene-azothiophene dyad was used as the active layer of a photovoltaic cell.

Interaction of 7-azatryptophan and beta-(1-azulenyl)-alanine with a nitroxyl radical.

Aal and 7-Atrp, quasi-isosteric with Trp, have been inserted together with a TOAC residue in two 3(10)-helical, model hexapeptides. The interaction of photoexcited AA1 and 7-Atrp with the nitroxide group of TOAC was investigated by time resolved EPR. Both peptides showed nitroxide spin polarized signals revealing that an intramolecular interaction takes places between the excited chromophore and the free radical moiety. The observation of a spin polarized signal in emission for AA1 is accounted for by the formation of triplet azulene by radical promoted enhanced inter system crossing (EISC).

Time-resolved EPR investigation of [70]fulleropyrrolidine nitroxide isomers.

A novel [70]fulleropyrrolidine functionalized with a nitroxide radical has been synthesized. After pulsed photoexcitation, time-resolved electron paramagnetic resonance (EPR) spectra have been recorded in liquid solution at standard X-band (9.5 GHz) and W-band (95 GHz) microwave frequencies. The spectra exhibit strongly electron spin polarized ground and excited states, the latter being arising from the spin coupling of the nitroxide's electron spin with the fullerene's excited triplet state. The EPR parameters such as g-values and hyperfine coupling constants have been discussed in terms of the spin Hamiltonian for excited doublet, triplet and quartet states. On the basis of the strength of the exchange interaction between the fullerene triplet and the radical group, two, out of a possible four [70]fulleropyrrolidine isomeric monoadducts, have been characterized.

Unraveling solvent-driven equilibria between alpha- and 3(10)-helices through an integrated spin labeling and computational approach.

In this work we present an effective and flexible computational approach, which is the result of an ongoing development in our groups, allowing the complete a priori simulation of the ESR spectra of complex systems in solution. The usefulness and reliability of the method are demonstrated on the very demanding playground represented by the tuning of the equilibrium between 3(10)- and alpha-helices of polypeptides by different solvents. The starting point is the good agreement between computed and X-ray diffraction structures for the 3(10)-helix adopted by the double spin-labelled heptapeptide Fmoc-(Aib-Aib-TOAC)2-Aib-OMe. Next, density functional computations, including dispersion interactions and bulk solvent effects, suggest another energy minimum corresponding to an alpha-helix in polar solvents, which, eventually, becomes the most stable structure. Computation of magnetic and diffusion tensors provides the basic ingredients for the building of complete spectra by methods rooted in the Stochastic Liouville Equation (SLE). The remarkable agreement between computed and experimental spectra at different temperatures allowed us to identify helical structures in the various solvents. The generality of the computational strategy and its implementation in effective and user-friendly computer codes pave the route toward systematic applications in the field of biomolecules and other complex systems.

Optical spectrum of C60 mono-adducts: assignment of transition bands using time-resolved EPR magneto-photo-selection.

The magneto-photo-selection technique implemented in the time-resolved EPR (TR-EPR) experiment is used for studying the characteristics of the optical spectrum of C(60) mono-adducts, in the 410-690 nm wavelength range. The analysis of the shape of the triplet state TR-EPR spectra of the mono-adducts, recorded after laser light pulses having polarization parallel or perpendicular to the magnetic field direction allows to determine the orientation distribution of the excited molecules; whence the direction of transition moments in the molecular frame is inferred. This information provides the assignment of the vibronic states symmetries.

Stochastic modeling of CW-ESR spectroscopy of [60]fulleropyrrolidine bisadducts with nitroxide probes.

In this work, we address the interpretation of continuous wave electron spin resonance (CW-ESR) spectra of fulleropyrrolidine bisadducts with nitroxide addends. Our approach is based on a definition of the spin Hamiltonian which includes exchange and dipolar interactions and on a complete numerical solution of the resulting stochastic Liouville equation, with inclusion of diffusive rotational dynamics. CW-ESR spectra are simulated for a series of C60 bisadducts made up of four trans isomers and the equatorial isomer. A nonlinear least-squares fitting procedure allows extraction directly from the available experimental spectra of a wide range of parameters, namely interprobe relative distances, diffusion tensors, and values of the exchange parameter J. Results are in good agreement with previous, more phenomenological estimates, proving that the combination of sensitive ESR spectroscopy based on multiple spin labeling with nitroxide radicals and sophisticated modeling can be highly helpful in providing structural and dynamic information on molecular systems.

Electron spin polarization of functionalized fullerenes. Reversed quartet mechanism.

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to study two functionalized fullerenes consisting of a C60 moiety covalently linked to TEMPO radical via spacers of different length. Photoinduced electron spin polarization (ESP) reflecting a non-Boltzmann population within the energy levels of the spin system was observed in the electronic ground and excited states. Both fullerenes are characterized by a sign inversion of their TREPR spectra. A new mechanism of ESP generation was suggested to explain the experimental results. This mechanism, termed as the reversed quartet mechanism (RQM), includes the intersystem crossing process, which generates ESP in the excited trip-doublet and trip-quartet (2T1 and 4T1) states. This ISC is accompanied by ESP transfer to the ground state (2S0) by either electron-transfer reaction (in our case via charge transfer state, 2CT, i.e., 2T1--> 2CT --> 2S0 or internal conversion, 2T1--> 2S0.

One-dimensional and two-dimensional coordination polymers from self-assembling of trinuclear triangular Cu(II) secondary building units.

The reactions of pyrazole (Hpz) with some copper(II) carboxylates in the presence of water yield trinuclear copper derivatives characterized by the triangular core [Cu3mu3-OH)(mu-pz)3(RCOO)2] (R = H, C2H5, C3H7). Copper(II) formate gives [Cu3(mu3-OH)(mu-pz)3(HCOO)2(Hpz)2] (1), whereas copper propionate and butyrate afford [Cu3(mu3-OH)(mu-pz)3(C2H5COO)2(EtOH)] (2) and [Cu3(mu3-OH)(mu-pz)3(C3H7COO)2(MeOH)(H2O)] (3), respectively, both containing solvent molecules coordinated to copper atoms. Magnetic susceptibilities are consistent with a single unpaired electron for each trinuclear unit of 1-3, and EPR measurements indicate that higher spin states, generated by exchange coupling between copper atoms, may be populated at room temperature. Density-functional calculations provide the description of the electronic structures of 1-3, allowing, at the same time, the assignment of their UV-vis absorption spectra. X-ray molecular structure determinations show that triangular trinuclear units of 1 are connected to each other through single formate bridges, forming one-dimensional (1D) zigzag coordination polymers, whereas in 2 and 3, two oxygen atoms of two carboxylate ions doubly bridge two copper atoms of different triangles, thus generating hexanuclear units. Moreover, in 2, two other propionate ions link together two hexanuclear units yielding a 12-membered cycle and giving rise to 1D coordination polymers. The supramolecular assemblies of 1-3 are compared to that of the previously reported trinuclear triangular copper(II) derivative [Cu3(mu3-OH)(mu-pz)3(CH3COO)2(Hpz)] (A), where a two-dimensional (2D) coordination polymer is present. The reactions of 3,5-dimethylpyrazole (Hpz) with copper(II) carboxylates in the same conditions yield 1:2 Cu(RCOO)2/Hpz adducts.

Synthesis, photophysics, and photoresponse of fullerene-based azoaromatic dyads.

The synthesis and photophysical characterization of a series of fullerene-based, donor-acceptor dyads is presented, along with a description of their behavior as single molecular components in photovoltaic cells. The spectroscopic and photophysical properties of the dyads, investigated by steady-state fluorescence spectroscopy, pico- and nanosecond transient optical spectroscopy and time-resolved electron paramagnetic resonance (EPR) spectroscopy, revealed that the dyads undergo multiple-step energy transfer from the donor singlet excited state to the fullerene triplet excited state, which in turn decays to the donor triplet state. The inefficient formation of a charge-separated state, both in solution and in the solid state, translates into a poor photovoltaic performance of dyads 2 b-4 b if compared to that of dyad 1 b, in which photoinduced electron transfer is operative in the solid state. In addition, the results of the photophysical investigation suggested that the performance of the solar cells was also limited by the low-lying donor triplet excited state that acts as a photoexcitation energy sink.

A helical peptide receptor for [60]fullerene.

Two terminally blocked nonapeptides, each made up of six Aib residues, a Gly spacer and two L-Tyr residues in positions 2 and 8 (these are substituted in the side chain with either ferrocenoyl or methyl moieties), have been synthesized by solution methods and fully characterized. FT-IR absorption and two-dimensional NMR analyses indicate that a 3(10)-helical conformation is adopted by these rigid peptides in structure-supporting solvents. An X-ray diffraction investigation shows that the bis-L-Tyr(Me) nonapeptide in the crystal state is folded in a regular right-handed 3(10)-helical structure. As five amino acid units separate the two substituted L-Tyr residues in the peptide sequence, the two side chain moieties will-in solution-face each other after two complete turns of the ternary helix. By carrying out a detailed photophysical analysis, we have demonstrated that the electron-rich, hydrophobic and wide cavity generated by the nonapeptide template with two ferrocenoyloxybenzyl walls is able to host [60]fullerene. Further evidence for this superstructure has been provided in the gas phase by a mass spectrometric investigation.

Spontaneous self-assembly of an unsymmetric trinuclear triangular copper(II) pyrazolate complex, [Cu3(micro3-OH)(micro-pz)3(MeCOO)2(Hpz)] (Hpz = pyrazole). synthesis, experimental and theoretical characterization, reactivity, and catalytic activity.

The almost quantitative formation of the triangular trinuclear copper derivative [Cu3(3-OH)(-pz)3(MeCOO)2(Hpz)] (1) (Hpz = pyrazole), has been simply achieved by adding Hpz to an ethanol solution of Cu(MeCOO)2 x H2O. An X-ray molecular structure determination shows that 1 is completely unsymmetric and that trinuclear units result assembled in an extended bidimensional network formed through acetate bridges and hydrogen bonds. EPR and magnetic measurements are consistent with the presence of a single unpaired electron. Theoretical density functional calculations carried out for S = 1/2 provide a thorough description of the electronic structure of 1, allowing a detailed assignment of its UV-vis absorption spectrum. Compound 1 reacts with MeONa, yielding [Cu3(micro3-OH)(micro-pz)3(MeCOO)(MeO)(Hpz)] (2) and [Cu3(micro3-OH)(micro-pz)3(MeO)2(Hpz)] (3) through the substitution of one and two acetate ions, respectively, with MeO- ion(s). The spontaneous self-assembly of the triangular trinuclear Cu3 moiety seems to occur only with pyrazole as can be inferred by the results obtained in the reactions of copper(II) acetate with some substituted pyrazoles leading to the formation of mononuclear [Cu(MeCOO)2(L)2] (4-8) and dinuclear [Cu(MeCOO)2(L)]2 (9-11) (L = substituted pyrazole) compounds. Also the presence of acetate ions seems to play a leading role in determining the formation of the trinuclear triangular arrangement, as indicated by the formation of a mononuclear derivative, [Cu(CF3COO)2(Hpz)]2 (compound 12), in the reaction of copper(II) trifluoroacetate with pyrazole. Compounds 1-3, as well as some other mono- and dinuclear copper(II)-substituted pyrazole complexes, have been tested as catalyst precursors in cyclopropanation reaction, observing the formation of products in a syn:anti ratio opposite that normally reported.