Synthesis, Structure and (Photo)Catalytic Behavior of Ce-MOFs Containing Perfluoroalkylcarboxylate Linkers: Experimental and Theoretical Insights.

Cerium-based Metal-Organic frameworks (Ce-MOFs) are attracting increasing interest due to their similar structural features to zirconium MOFs. The redox behavior of Ce(III/IV) adds a range of properties to the compounds. Recently, perfluorinated linkers have been used in the synthesis of MOFs to introduce new characteristic into the structure. We report the synthesis and structural characterization of Ce(IV)-based MOFs constructed using two perfluorinated alkyl linkers. Their structure, based on hexanuclear Ce6O4(OH)4 12+ clusters linked to each other by the dicarboxylate ions, has been solved ab-initio from X-ray powder diffraction data and refined by the Rietveld method. The crystallization kinetics and the MOF formation mechanism was also invesitigated by Synchrotron radiation with XAS spectroscopies (EXAFS and XANES). The MOFs present the same fcu cubic topology as observed in MOF-801 and UiO-66, and they showed good stability in water at different pH conditions. The electronic structure of these MOFs has been studied by DFT calculations in order to obtain insights into the density of states structure of the reported compounds, resulting in band gaps in the range of 2.8-3.1 eV. Their catalytic properties were tested both thermally and under visible light irradiation for the degradation of methyl orange (MO) dye.

Effect of Surfactant Structure on the Superactivity of Candida rugosa Lipase.

In this work, we present the effects of ionic and zwitterionic surfactants on the hydrolytic activity of Candida rugosa lipase (CRL), one of the most important and widely used microbial lipases. A series of amine N-oxide surfactants was studied to explore the relationship between their molecular structures and their effect on catalytic properties of CRL. These zwitterionic amphiphiles are known for their ability to form aggregates that can increase their size, thanks to a sphere-rod transition, without any additive. Enzyme activity seemed to be improved by morphological changes of micelles from spherical to rod-like, and the structure of the monomers played a crucial role in this transition. In fact, all the amine oxides investigated provoked superactivation, but the CRL activity increased by lengthening the alkyl chain of N-oxide surfactants, whereas it decreased in the presence of bulky head groups. Superactivity was mainly because of an increase in kcat (0.57 s-1 in buffer, 0.80-1.99 s-1 in surfactant solutions) and, in some cases, a decrease in KM (2 × 10-3 M in buffer, 1.08-4.28 × 10-3 M in surfactant solutions). Micelles seemed to play a dual role: superactivity occurred at surfactant concentrations higher than their critical micelle concentration, but, on the other hand, micelles subtracted the substrate from the bulk, making it unavailable for the catalysis.

Electron Transfer Mechanism in the Oxidation of Aryl 1-Methyl-1-phenylethyl Sulfides Promoted by Nonheme Iron(IV)-Oxo Complexes: The Rate of the Oxygen Rebound Process.

The oxidation of aryl 1-methyl-1-phenylethyl sulfides promoted by the nonheme iron(IV)-oxo complexes [(N4Py)FeIV═O]2+ and [(Bn-TPEN)FeIV═O]2+ occurs by an electron transfer-oxygen rebound (ET-OT) mechanism leading to aryl 1-methyl-1-phenylethyl sulfoxides accompanied by products derived from Cα-S fragmentation of sulfide radical cations (2-phenyl-2-propanol and diaryl disulfides). For the first time, the rate constants for the oxygen rebound process (kOT), which are in the range of <0.8 × 104 to 3.5 × 104 s-1, were determined from the fragmentation rate constants of the radical cations (kf) and the S oxidation/fragmentation product ratios.

Oxidation of Aryl Diphenylmethyl Sulfides Promoted by a Nonheme Iron(IV)-Oxo Complex: Evidence for an Electron Transfer-Oxygen Transfer Mechanism.

The oxidation of a series of aryl diphenylmethyl sulfides (4-X-C6H4SCH(C6H5)2, where X = OCH3 (1), X = CH3 (2), X = H (3), and X = CF3 (4)) promoted by the nonheme iron(IV)-oxo complex [(N4Py)Fe(IV)═O](2+) occurs by an electron transfer-oxygen transfer (ET-OT) mechanism as supported by the observation of products (diphenylmethanol, benzophenone, and diaryl disulfides) deriving from α-C-S and α-C-H fragmentation of radical cations 1(+•)-4(+•), formed besides the S-oxidation products (aryl diphenylmethyl sulfoxides). The fragmentation/S-oxidation product ratios regularly increase through a decrease in the electron-donating power of the aryl substituents, that is, by increasing the fragmentation rate constants of the radical cations as indicated by a laser flash photolysis (LFP) study of the photochemical oxidation of 1-4 carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate (MeOP(+)PF6(-)).

Photoinduced One-Electron Oxidation of Benzyl Methyl Sulfides in Acetonitrile: Time-Resolved Spectroscopic Evidence for a Thionium Ion Intermediate.

The photo-oxidation of 4-methoxybenzyl methyl sulfide (1a), benzyl methyl sulfide (1b), and 4-cyanobenzyl methyl sulfide (1c) has been investigated in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP(+)PF6(-)) under nitrogen in CH3CN. The steady-state photolysis experiments showed for the investigated sulfides exclusively the formation of the corresponding benzaldehyde as the oxidation product, reasonably due to a deprotonation of the sulfide radical cations. Photo-oxidation of 1a-1c occurs through an electron transfer process. Indeed, laser flash photolysis measurements showed an efficient formation of sulfide radical cations, detected in their dimeric form [(4-X-C6H4CH2SCH3)2(+•)] at ≈520 nm. At longer delay times, the absorption of the dimer radical cation was replaced by an absorption band assigned to the (α-thio)benzyl cation (thionium ion, λmax = 420-400 nm), formed by oxidation of the benzyl radical and not by that of the (α-thiomethyl)benzyl radical, as expected if a Cα-H bond cleavage is operative. This finding highlights a particular stability of this kind of cation never reported before, even though its involvement in one-electron oxidation mechanisms of various sulfides has already been invoked. Density functional theory calculations allowed identification of a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom of the radical cations.

Photosensitized oxidation of aryl benzyl sulfoxides. Evidence for nucleophilic assistance to the C-s bond cleavage of aryl benzyl sulfoxide radical cations.

The radical cations of a series of aryl benzyl sulfoxides (4-X-C6H4CH2SOC6H4Y(+•)) have been generated by photochemical oxidation of the parent sulfoxides sensitized by 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ(+)ClO4(-)). Steady-state photolysis experiments showed the prevailing formation of benzylic products deriving from the C-S fragmentation in the radical cations, together with sulfur-containing products. Formation of sulfoxide radical cations was unequivocally established by laser flash photolysis experiments showing the absorption bands of 3-CN-NMQ(•) (λmax = 390 nm) and of the radical cations (λmax = 500-620 nm). The decay rate constants of radical cations, determined by LFP experiments, decrease by increasing the electron-donating power of the arylsulfinyl Y substituent and to a smaller extent by increasing the electron-withdrawing power of the benzylic X substituent. A solvent nucleophilic assistance to the C-S bond cleavage has been suggested, supported by the comparison of substituent effects on the same process occurring in aryl tert-butyl sulfoxide radical cations. DFT calculations, performed to determine the bond dissociation free energy in the radical cations, the transition state energies associated with the unimolecular C-S bond cleavage, and the charge and spin delocalized on their structures, were also useful to endorse the nucleophilic assistance to the C-S scission.

An acridinium-based sensor as a fluorescent photoinduced electron transfer probe for proton detection modulated by anionic micelles.

A water-soluble fluorescent pH sensor of 9-amino-10-methylacridinium chromophore with the 2-(diethylamine)ethyl chain as a receptor shows an "off-on" response going from basic to acidic solution. Photoinduced electron transfer has been directly demonstrated to be the quenching mechanism by the observation of the long-lived acridinyl radical. The interaction of the protonated sensor with anionic micelles causes a significant increase in the detection sensitivity of pH.

Structural effects on the C-S bond cleavage in aryl tert-butyl sulfoxide radical cations.

The oxidation of a series of aryl tert-butyl sulfoxides (4-X-C6H4SOC(CH3)3: 1, X = OCH3; 2, X = CH3; 3, X = H; 4, X = Br) photosensitized by 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ(+)) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Products deriving from the C-S bond cleavage in the radical cations 1(+•)-4(+•) have been observed in the steady-state photolysis experiments. By laser irradiation, the formation of 3-CN-NMQ(•) (λ(max) = 390 nm) and 1(+•)-4(+•) (λ(max) = 500-620 nm) was observed. A first-order decay of the sulfoxide radical cations, attributable to C-S bond cleavage, was observed with fragmentation rate constants (k(f)) that decrease by increasing the electron donating power of the arylsulfinyl substituent from 1.8 × 10(6) s(-1) (4(+•)) to 2.3 × 10(5) s(-1) (1(+•)). DFT calculations showed that a significant fraction of the charge is delocalized in the tert-butyl group of the radical cations, thus explaining the small substituent effect on the C-S bond cleavage rate constants. Via application of the Marcus equation to the kinetic data, a very large value for the reorganization energy (λ = 62 kcal mol(-1)) has been calculated for the C-S bond scission reaction in 1(+•)-4(+•).

Competition of C-H and C-O fragmentation in substituted p-methoxybenzyl ether radical cations generated by photosensitized oxidation.

Laser and steady-state photolysis, sensitized by 2,4,6-triphenylpyrylium tetrafluoroborate (TPP(+)BF(4)(-)), of 4-methoxybenzyl ethers [4-CH(3)O-C(6)H(4)CH(OR(1))R, 1a: R = H, R(1) = CH(3); 1b: R = H, R(1) = C(CH(3))(3); 1c: R = CH(3), R(1) = C(CH(3))(3); 1d: R = 4-CH(3)O-C(6)H(4), R(1) = CH(3)] was carried out in CH(3)CN in the presence of oxygen. In particular, steady-state irradiation of 1a, b and d produced benzylic alcohols (together with a small amount of acetamide for 1a and 1b) and the oxidation carbonyl compounds (esters and ketones); 4-methoxy-α-methylbenzyl alcohol was the only product observed with 1c. Time-resolved investigations of 1c and 1d gave evidence of the intermediate benzylic carbocation, coming from the ether radical cation formed within the laser pulse by an electron transfer process from the ether to the TPP(+) excited state. These results suggested that, besides the deprotonation of the benzyl carbon, the cleavage of the C-OR(1) bond is also operative in the reaction pathways of the ether radical cation. The comparison of these results with those obtained in the photolysis of 1c with the uncharged 9,10-dicyanoanthracene (DCA) upholds a particular behavior of TPP(+)BF(4)(-), probably due to the specific electrostatic interactions of BF(4)(-) with the radical cation. This is also supported by quantum mechanical calculation results performed at the B3LYP/6-31G(d) level to obtain the charge and spin density distributions of radical cations, free and complexed with BF(4)(-).

Structural and solvent effects on the C-S bond cleavage in aryl triphenylmethyl sulfide radical cations.

Steady-state and laser flash photolysis (LFP) studies of a series of aryl triphenylmethyl sulfides [1, 3,4-(CH(3)O)(2)-C(6)H(3)SC(C(6)H(5))(3); 2, 4-CH(3)O-C(6)H(4)SC(C(6)H(5))(3); 3, 4-CH(3)-C(6)H(4)SC(C(6)H(5))(3); 4, C(6)H(5)SC(C(6)H(5))(3); and 5, 4-Br-C(6)H(4)SC(C(6)H(5))(3)] has been carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate in CH(3)CN, CH(2)Cl(2), CH(2)Cl(2)/CH(3)CN, and CH(2)Cl(2)/CH(3)OH mixtures. Products deriving from the C-S bond cleavage in the radical cations 1(•+)-5(•+) have been observed in the steady-state photolysis experiments. Time-resolved LFP showed first-order decay of the radical cations accompanied by formation of the triphenylmethyl cation. A significant decrease of the C-S bond cleavage rate constants was observed by increasing the electron-donating power of the arylsulfenyl substituent, that is, by increasing the stability of the radical cations. DFT calculations showed that, in 2(•+) and 3(•+), charge and spin densities are mainly localized in the ArS group. In the TS of the C-S bond cleavage an increase of the positive charge in the trityl moiety and of the spin density on the ArS group is observed. The higher delocalization of the charge in the TS as compared to the initial state is probably at the origin of the observation that the C-S bond cleavage rates decrease by increasing the polarity of the solvent.

Structure and C-S bond cleavage in aryl 1-methyl-1-arylethyl sulfide radical cations.

Steady state and laser flash photolysis (LFP) of a series of p-X-cumyl phenyl sulfides (4-X-C(6)H(4)C(CH(3))(2)SC(6)H(5): 1, X = Br; 2, X = H; 3, X = CH(3); 4, X = OCH(3)) and p-X-cumyl p-methoxyphenyl sulfides (4-X-C(6)H(4)C(CH(3))(2)SC(6)H(4)OCH(3): 5, X = H; 6, X = CH(3); 7, X = OCH(3)) has been carried out in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP(+)PF(6)(-)) under nitrogen in MeCN. Steady state photolysis showed the formation of products deriving from the C-S bond cleavage in the radical cations 1(+•)-7(+•) (2-aryl-2-propanols and diaryl disulfides). Formation of 1(+•)-7(+•) was also demonstrated by LFP experiments evidencing the absorption bands of the radical cations 1(+•)-3(+•) (λ(max) = 530 nm) and 5(+•)-7(+•) (λ(max) = 570 nm) mainly localized in the arylsulfenyl group and radical cation 4(+•) (λ(max) = 410, 700 nm) probably mainly localized in the cumyl ring. The radical cations decayed by first-order kinetics with a process attributable to the C-S bond cleavage. On the basis of DFT calculations it has been suggested that the conformations most suitable for C-S bond cleavage in 1(+•)-4(+•) and 7(+•) are characterized by having the C-S bond almost collinear with the π system of the cumyl ring and by a significant charge and spin delocalization from the ArS ring to the cumyl ring. Such a delocalization is probably at the origin of the observation that the rates of C-S bond cleavage result in very little sensitivity to changes in the C-S bond dissociation free energy (BDFE). A quite large reorganization energy value (λ = 43.7 kcal mol(-1)) has been calculated for the C-S bond scission reaction in the radical cation. This value is much larger than that (λ = 12 kcal mol(-1)) found for the C-C bond cleavage in bicumyl radical cations, a reaction that also leads to cumyl carbocations.

Steady-state and time-resolved investigations of a crown thioether conjugated with methylacridinium and its complexes with metal ions.

The crown thioether 9-[4-(4,7,10,13-tetrathia-1-azacyclopentadecyl]phenyl-N-methylacridinium perchlorate (TCMA) was synthesized and characterized with the aim to verify its ability to interact selectively with metal ions and substantiate the possibility to detect easily the presence of heavy metals in fluid samples. The spectroscopic properties of TCMA, alone and in the presence of metal ions, were therefore studied in polar solvents (MeCN and H(2)O); in particular, steady-state UV-Vis spectrophotometric and fluorimetric techniques were used together with transient absorption spectroscopy with fs time resolution to investigate the spectral and dynamic properties of the lowest excited singlet state of TCMA and of TCMA/metal ion complexes. The absorption in the Vis region is characterized by a charge-transfer nature with the methylacridinium moiety acting as the electron-acceptor and the anilic group as the electron-donor. No emission from the S(1) was detected both in MeCN and H(2)O, while a small S(2)→ S(0) fluorescence emission (λ(max) = 485 nm and ϕ(F) = 0.0011) was detected in water. Time-resolved measurements with fs resolution of TCMA in MeCN have shown that the relaxed S(1) state is reached ∼0.6 ps after the laser pulse, while the S(1)→ S(0) time constant is 3.7 ps. Among the investigated metal ions, only Fe(3+) (in MeCN) and Hg(2+) (in MeCN and H(2)O) were able to form stable complexes (association constant, K(ass) = 1-11 × 10(4) M(-1)) with TCMA. The S(1) state of the TCMA/M(n+) complexes emits with low quantum yield (ϕ(F) = 0.0023-0.014) and decays with time constants much longer than TCMA itself, at least in the case of TCMA/Hg(2+) in MeCN. This study showed that TCMA is a good candidate for colorimetric/fluorimetric sensing of Hg(2+) in aqueous media owing to its high selectivity towards metal ions.

Competition Between Cα -S and Cα -Cß Bond Cleavage in ß-Hydroxysulfoxides Cation Radicals Generated by Photoinduced Electron Transfer.

A kinetic and product study of the 3-cyano-N-methyl-quinolinium photoinduced monoelectronic oxidation of a series of ß-hydroxysulfoxides has been carried out to investigate the competition between Cα -S and Cα -Cß bond cleavage within the corresponding cation radicals. Laser flash photolysis experiments unequivocally established the formation of sulfoxide cation radicals showing their absorption band (λ max ≈ 520 nm) and that of 3-CN-NMQ• (λ max ≈ 390 nm). Steady-state photolysis experiments suggest that, in contrast to what previously observed for alkyl phenyl sulfoxide cation radicals that exclusively undergo Cα -S bond cleavage, the presence of a ß-hydroxy group makes, in some cases, the Cα -Cß scission competitive. The factors governing this competition seem to depend on the relative stability of the fragments formed from the two bond scissions. Substitution of the ß-OH group with -OMe did not dramatically change the reactivity pattern of the cation radicals thus suggesting that the observed favorable effect of the hydroxy group on the Cα -Cß bond cleavage mainly resides on its capability to stabilize the carbocation formed upon this scission.

Evidences in favour of a single electron transfer (SET) mechanism in the TiO2 sensitized photo-oxidation of α-hydroxy- and α,ß-dihydroxybenzyl derivatives in water.

The TiO(2) photosensitized oxidation in water of a series of X-ring substituted benzyl alcohols gives the corresponding benzaldehyde. Kinetic evidence (from competitive experiments) suggests a single electron transfer (SET) mechanism with a changeover of the electron abstraction site from the aromatic moiety (X=4-OCH(3), 4-CH(3), H and 3-Cl) to the hydroxylic group (X=3-CF(3) and 4-CF(3)), probably due to the preferential adsorption of the above OH group on the TiO(2) surface. The same photo-oxidation of a series of 1-(X-phenyl)-1,2-ethanediols and of 2-(X-phenyl)-1,2-propanediols gives the corresponding benzaldehyde and acetophenone, respectively, accompanied by formaldehyde, whereas a series of symmetrically X-ring-substituted 1,2-diphenyl-1,2-ethanediols yields the corresponding benzaldehyde (substrate/product molar ratio=0.5). The relative rate values suggest a SET mechanism in all of the series, with electron abstraction from one of the two OH groups of all the considered diols, probably due to the much higher adsorption of the above groups (due to the chelation effect) on the semiconductor. Further confirmation of this mechanistic behaviour has been obtained from laser flash photolysis experiments.

Photophysics of aromatic thiourea derivatives and their complexes with anions. Fast and ultrafast spectroscopic investigations.

In order to collect detailed information on the interaction mechanism between fluorescent thiourea derivatives and anions, 9-[4-(trifluoromethyl)phenylthioureidomethyl]anthracene (1) and the corresponding 10-cyanoanthracene derivative (2) were synthesized and investigated in DMSO and MeCN by using absorption and emission steady state techniques, both in the absence and in the presence of different anions (AcO(-), H(2)PO(4)(-), HSO(4)(-), and Br(-)). A wide examination of the mechanism of anion recognition was also performed by time-resolved transient absorption spectroscopy, with nano- and femto-second time resolution. A complete picture of the excited state deactivation pathways of 1 and 2, where the main operative processes were fluorescence, intersystem crossing and internal conversion, was obtained. Even if steady-state measurements suggest that 1 and 2 selectively interact in the ground state with the anions H(2)PO(4)(-) and AcO(-), time-resolved investigations demonstrate that the substrates are able to complex all the four anions. The photophysics of such complexes was fully characterized. The anions mainly modify the lifetime of the lowest excited singlet state and, especially in the cases of H(2)PO(4)(-) and AcO(-), the efficiencies of fluorescence emission and triplet formation. In particular, no evidence was found of further deactivation processes such as photoinduced electron transfer, photodissociation, and photoionization.

Solvent-Free Synthetic Route for Cerium(IV) Metal-Organic Frameworks with UiO-66 Architecture and Their Photocatalytic Applications.

A near solvent-free synthetic route for Ce-UiO-66 metal-organic frameworks (MOFs) is presented. The MOFs are obtained by energetically grinding the reagents, cerium ammonium nitrate (CAN) and the carboxylic linkers, in a mortar for a few minutes with the addition of a small amount of acetic acid (AcOH) as a modulator (8.75 equiv, 0.5 mL). The slurry is then transferred into a 2 mL vial and heated at 120 °C for 1 day. The MOFs have been characterized for their composition, crystallinity, and porosity and employed as heterogeneous catalysts for the photo-oxidation reaction of substituted benzylic alcohols to benzaldaldehydes under near-ultraviolet light irradiation. The catalytic performances, such as selectivity, conversion, and kinetics, exceed those of similar systems studied by chemical oxidation using similar Ce-MOFs as a catalyst. Moreover, the MOFs were found to be reusable up to three cycles without loss of activity. Density functional theory (DFT) calculations were used to fully describe the electronic structure of the best performing MOFs and to provide useful information on the catalytic activity experimentally observed.

Photosensitized oxidation of alkyl phenyl sulfoxides. C-S bond cleavage in alkyl phenyl sulfoxide radical cations.

The 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ(+)ClO4(-))-photosensitized oxidation of phenyl alkyl sulfoxides (PhSOCR1R2R3, 1, R1 = R2 = H, R3 = Ph; 2, R1 = H, R2 = Me, R3 = Ph; 3, R1 = R2 = Ph, R3 = H; 4, R1 = R2 = Me, R3 = Ph; 5, R1 = R2 = R3 = Me) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Steady-state photolysis showed the formation of products deriving from the heterolytic C-S bond cleavage in the sulfoxide radical cations (alcohols, R1R2R3COH, and acetamides, R1R2R3CNHCOCH3) accompanied by sulfur-containing products (phenyl benzenethiosulfinate, diphenyl disulfide, and phenyl benzenethiosulfonate). By laser irradiation, the formation of 3-CN-NMQ(*) (lambda(max) = 390 nm) and sulfoxide radical cations 1(*+) , 2(*+), and 5(*+) (lambda(max) = 550 nm) was observed within the laser pulse. The radical cations decayed by first-order kinetics with a process attributable to the heterolytic C-S bond cleavage leading to the sulfinyl radical and an alkyl carbocation. The radical cations 3(*+) and 4(*+) fragment too rapidly, decaying within the laser pulse. The absorption band of the cation Ph2CH(+) (lambda(max) = 440 nm) was observed with 3 while the absorption bands of 3-CN-NMQ(*) and PhSO(*) (lambda(max) = 460 nm) were observed just after the laser pulse in the LFP experiment with 4. No competitive beta-C-H bond cleavage has been observed in the radical cations from 1-3. The C-S bond cleavage rates were measured for 1(*+), 2(*+), and 5(*+). For 3(*+) and 4(*+), only a lower limit (ca. >3 x 10(7) s(-1)) could be given. Quantum yields (Phi) and fragmentation first-order rate constants (k) appear to depend on the structure of the alkyl group and on the bond dissociation free energy (BDFE) of the C-S bond of the radical cations determined by a thermochemical cycle using the C-S BDEs for the neutral sulfoxides 1-5 obtained by DFT calculations. Namely, Phi and k increase as the C-S BDFE becomes more negative, that is in the order 1 < 5 < 2 < 3, 4, which is also the stability order of the alkyl carbocations formed in the cleavage. An estimate of the difference in the C-S bond cleavage rate between sulfoxide and sulfide radical cations was possible by comparing the fragmentation rate of 5(*+) (1.4 x 10(6) s(-1)) with the upper limit (10(4) s(-1)) given for tert-butyl phenyl sulfide radical cation (Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Org. Lett. 2006, 8, 641-644). It turns out that sulfoxide radical cations undergo C-S bond breaking at a rate at least 2 orders of magnitude faster than that of corresponding sulfide radical cations.

Trifluoroacetylated tyrosine-rich D-tetrapeptides have potent antioxidant activity.

The term "oxidative stress" indicates a set of chemical reactions unleashed by a disparate number of events inducing DNA damage, lipid peroxidation, protein modification and other effects, which are responsible of altering the physiological status of cells or tissues. Excessive Reactive Oxygen Species (ROS) levels may accelerate ageing of tissues or induce damage of biomolecules thus promoting cell death or proliferation in dependence of cell status and of targeted molecules. In this context, new antioxidants preventing such effects may have a relevant role as modulators of cell homeostasis and as therapeutic agents. Following an approach of peptide libraries synthesis and screening by an ORACFL assay, we have isolated potent anti-oxidant compounds with well-defined structures. Most effective peptides are N-terminally trifluoroacetylated (CF3) and have the sequence tyr-tyr-his-pro or tyr-tyr-pro-his. Slight changes in the sequence or removal of the CF3 group strongly reduced antioxidant ability, suggesting an active role of both the fluorine atoms and of peptide structure. We have determined the NMR solution structures of the active peptides and found a common structural motif that could underpin the radical scavenging activity. The peptides protect keratinocytes from exogenous oxidation, thereby from potential external damaging cues, suggesting their use as skin ageing protectant and as cell surviving agents.

Dual pathways for the desilylation of silylamines by singlet oxygen.

[reaction: see text] A kinetic and product study has been carried out for the reactions of silylamines 1a and 1b with (1)O(2) in MeCN and (80:20) MeCN-MeOH. Indications suggesting an electron-transfer step following exciplex (I) formation have been obtained. However, the fate of the radical cation is solvent dependent. The radical cation undergoes desilylation in MeCN-MeOH and deprotonation in MeCN.