3,4,5-Trimethoxyphenol: A combined experimental and theoretical thermochemical investigation of its antioxidant capacity

The standard (p^° = 0.1 MPa) molar enthalpies of combustion and sublimation of 3,4,5-trimethoxyphenol were measured, respectively, by static bomb combustion calorimetry in oxygen atmosphere and by Calvet microcalorimetry. From these measurements, the standard molar enthalpy of formation in both the crystalline and gaseous phase, at T = 298.15 K, were derived: ?(643.4 ± 1.9) kJ · mol^?1 and ?(518.1 ± 3.6) kJ · mol^?1, respectively. Density functional theory calculations for this compound and respective phenoxyl radical and phenoxide anion were also performed using the B3LYP functional and extended basis sets, which allowed the theoretical estimation of the gaseous phase standard molar enthalpy of formation through the use of isodesmic reactions and the calculation of the homolytic and heterolytic O–H bond dissociation energies. There is good agreement between the calculated and experimental enthalpy of formation. Substituent effects on the homolytic and heterolytic O–H bond dissociation energies have been analysed.     

Synthesis of substituted dipyrido[3,2-a:2′,3′-c]phenazines and a new heterocyclic dipyrido[3,2-f:2′,3′-h]quinoxalino[2,3-b]quinoxaline

Three new α,α′-diimine ligands were synthesized based on condensation of 1,10-phenanthroline-5,6-dione with 1,2-phenylenediamine derivatives using different approaches. All compounds were fully characterized by IR, ¹H and ¹³C NMR, UV-visible, and MS spectroscopies. We report the first example of a dipyrido[3,2-f:2′,3′-h]quinoxalino[2,3-b]quinoxaline, which exhibits a strong absorption at 430 nm and an interesting electrochemical behavior. These new molecules may have biological potential and are of synthetic and technological importance.     

A Cleavable Crosslinking Strategy for Commodity Polymer Functionalization and Generation of Reprocessable Thermosets

Covalently crosslinked polymeric materials, known as thermosets, possess enhanced mechanical strength and thermal stability relative to the corresponding uncrosslinked thermoplastics. However, the presence of covalent inter-chain crosslinks that makes thermosets so attractive is precisely what makes them so difficult to reprocess and recycle. Here, we demonstrate the introduction of chemically cleavable groups into a bis-diazirine crosslinker. Application of this cleavable crosslinker reagent to commercial low-functionality polyolefins (or to a small-molecule model) results in the rapid, efficient introduction of molecular crosslinks that can be uncoupled by specific chemical inputs. These proof-of-concept findings provide one potential strategy for circularization of the thermoplastic/thermoset plastics economy, and may allow crosslinked polyolefins to be manufactured, used, reprocessed, and re-used without losing value. As an added benefit, the method allows the ready introduction of functionality into non-functionalized commodity polymers.     

Eigenvalue Asymptotics for a Schrödinger Operator with Non-Constant Magnetic Field Along One Direction

We consider the discrete spectrum of the two-dimensional Hamiltonian H = H ₀ + V, where H ₀ is a Schrödinger operator with a non-constant magnetic field B that depends only on one of the spatial variables, and V is an electric potential that decays at infinity. We study the accumulation rate of the eigenvalues of H in the gaps of its essential spectrum. First, under certain general conditions on B and V, we introduce effective Hamiltonians that govern the main asymptotic term of the eigenvalue counting function. Further, we use the effective Hamiltonians to find the asymptotic behavior of the eigenvalues in the case where the potential V is a power-like decaying function and in the case where it is a compactly supported function, showing a semiclassical behavior of the eigenvalues in the first case and a non-semiclassical behavior in the second one. We also provide a criterion for the finiteness of the number of eigenvalues in the gaps of the essential spectrum of H.     

Hydroxycruciforms: amine-responsive fluorophores

The synthesis of three hydroxy-substituted cruciforms (XF, 1,4-bis(4'-hydroxystyryl)-2,5-bis(4'-methoxyphenylethynyl)benzene, 1,4-bis(4'-methoxystyryl)-2,5-bis(4'-hydroxyphenylethynyl)benzene, and 1,4-bis(4'-hydroxystyryl)-2,5-bis(4'-hydroxyphenylethynyl)benzene) starts with a Horner reaction followed by a Sonogashira coupling and subsequent deprotection. The three herein described XFs contain either two or four free phenolic hydroxyl groups. All three XFs were subjected to photometric UV/Vis titrations in a methanol/water mixture. The respective pK(a) values were obtained by data deconvolution. As the three XFs display a significant change in emission color upon photoinduced deprotonation, the XFs were taken up in different solvents and exposed to twelve amines. The amine-dependent change in emissivity of the tetrahydroxy XF is sufficiently distinct in the eight solvents that all of the inspected amines are discerned by a linear discriminant analysis. The tetrahydroxy XF in different solvents forms a sensor array, the response of which is based on the excited-state proton transfer (ESPT) to amines and mediated by the choice of the battery of solvents that are utilized.     

Detection of carbonyl-modified proteins in interfibrillar rat mitochondria using N'-aminooxymethylcarbonylhydrazino-D-biotin as an aldehyde/keto-reactive probe in combination with Western blot analysis and tandem mass spectrometry

There is now a large body of supporting data available that links oxidative modifications of proteins to a large number of diseases, degenerative disorders and aging. However, the detailed analysis of oxidative protein modifications remains challenging. Here, we report a new efficient method for identification of oxidatively modified proteins in complex biological samples which is based on the use of an aldehyde-reactive probe, N'-aminooxymethylcarbonylhydrazino-D-biotin (ARP), in combination with Western-type analyses and MS. The biotinylated hydroxylamine derivative forms a chemically stable oxime derivative with the aldehyde/keto group found in carbonyl-modified proteins. The biotin tag is detected by avidin affinity staining. ARP-positive proteins are subsequently subjected to in-gel trypsinization and MS/MS for protein identification. We demonstrate the usefulness of the method for the analysis of protein extracts obtained from interfibrillar heart mitochondria (IFM) from young and old rats. In this study, we identified as putative major protein targets of oxidative modifications the mitochondrial matrix protein, aconitase, the inner mitochondrial membrane protein, ADP/ATP translocase, and constituents of the electron transport chain complexes IV and V. An age-related increase of carbonyl levels was found for aconitase and ATP synthase.     

Quantum mechanical limits to the control of atom-diatom chemical reactions through the polarisation of the reactants

This article considers the extent to which one can control the reactivity of atom-diatom systems through reactant polarisation. Three different limits for reactivity manipulation are defined: "absolute" limits that do not depend on the reaction dynamics but can only be obtained for particular combinations of quantum numbers, "unconstrained" limits that depend on dynamics but not on constraints imposed by any particular experimental setup, and "constrained" limits that depend on dynamics and also on the constraints imposed by a particular experimental setup. Methods for calculation of these limits are presented and applied to the benchmark F + H2 reaction. The variations of the maximum and minimum reactivity one can obtain are analysed in terms of reaction mechanisms and steric constraints. Tables listing the minimum and maximum values of angular momentum polarisation moments of rank up to 4, and integer and half-integer quantum numbers up to 5, are also presented.     

Characterization, reactivity and photosensitizing properties of the triplet excited state of alpha-lapachone

alpha-Lapachone is a natural 1,4-naphthoquinone with promising biological activity. The fused dihydropyran ring present in its structure, acting as formal 2-alkoxy and 3-alkyl substituents to the quinone moiety, endows this compound with milder redox properties and lower toxicity, when compared with other bioactive 1,4-quinones. Its photochemistry, here reported, seems to originate from the triplet state, which shows pipi* character. Triplet quenching in acetonitrile solution with added hydrogen-atom donors such as 1,4-cyclohexadiene or 2-propanol is inefficient, independent of solvent polarity, and leads to formation of the semiquinone radical. With phenol and indole, quenching rate constants are two orders of magnitude higher, but smaller than the value for triethylamine. In the first two cases the semiquinone radical can be detected by laser flash photolysis and in the last case, the anion radical derived from alpha-lapachone is readily detected. The semiquinone radical can also be observed in the quenching of triplet alpha-lapachone by 2'-deoxyguanosine and by the methyl esters of L-tryptophan and L-tyrosine, whereas for L-cysteine methyl ester the quenching rate constant is very slow. Triplet alpha-lapachone is not quenched by thymine, thymidine, 2'-deoxycytosine or 2'-deoxyadenosine; this is probably due to its pipi* character and low energy, which prevents oxetane formation and triplet-triplet energy transfer, respectively. Steady-state photolysis of aerated solutions of these compounds in the presence of alpha-lapachone does not show evidence of decomposition, whereas similar experiments with 2'-deoxyguanosine result in efficient consumption of the nucleoside. Singlet oxygen is formed from triplet alpha-lapachone, and a quantum yield of 0.68 is measured.     

The inducing NO-vasodilation by chemical reduction of coordinated nitrite ion in cis-[Ru(NO(2))L(bpy)(2)](+) complex

The synthesis of [Ru(NO(2))L(bpy)(2)](+) (bpy = 2,2'-bipyridine and L = pyridine (py) and pyrazine (pz)) can be accomplished by addition of [Ru(NO)L(bpy)(2)](PF(6))(3) to aqueous solutions of physiological pH. The electrochemical processes of [Ru(NO(2))L(bpy)(2)](+) in aqueous solution were studied by cyclic voltammetry and differential pulse voltammetry. The anodic scan shows a peak around 1.00 V vs. Ag/AgCl attributed to the oxidation process centered on the metal ion. However, in the cathodic scan a second peak around -0.60 V vs. Ag/AgCl was observed and attributed to the reduction process centered on the nitrite ligand. The controlled reduction potential electrolysis at -0.80 V vs. Ag/AgCl shows NO release characteristics as judged by NO measurement with a NO-sensor. This assumption was confirmed by ESI/MS(+) and spectroelectrochemical experiment where cis-[Ru(bpy)(2)L(H(2)O)](2+) was obtained as a product of the reduction of cis-[Ru(II)(NO(2))L(bpy)(2)](+). The vasorelaxation observed in denuded aortic rings pre-contracted with 0.1 mumol L(-1) phenylephrine responded with relaxation in the presence of cis-[Ru(II)(NO(2))L(bpy)(2)](+). The potential of rat aorta cells to metabolize cis-[Ru(II)(NO(2))L(bpy)(2)](+) was also followed by confocal analysis. The obtained results suggest that NO release happens by reduction of cis-[Ru(II)(NO(2))L(bpy)(2)](+) inside the cell. The maximum vasorelaxation was achieved with 1 x 10(-5) mol L(-1) of cis-[Ru(II)(NO(2))L(bpy)(2)](+) complex.     

Transient absorption spectroscopy for determining multiple site occupancy in drug-protein conjugates. A comparison between human and bovine serum albumins using flurbiprofen methyl ester as a probe

Laser flash photolysis (LFP) has been used to determine the degree of binding of (S)- or (R)-flurbiprofen methyl ester (FBPMe) to human and bovine serum albumins (HSA and BSA, respectively). Regression analysis of the triplet decay of the drug (lambda = 360 nm) in the presence of the proteins led to a satisfactory fitting when considering a set of three lifetimes; the corresponding Afree, AI and AII preexponential coefficients can be correlated with the presence of FBPMe in the bulk solution and within the two known binding sites. The most remarkable differences between HSA and BSA were found under nonsaturating conditions; thus, when the [FBPMe]/[SA] ratio was 1:1, all the drug was bound to HSA, whereas 20-30% of it remained free in the bulk solution in the presence of BSA. The LFP approach was also applicable to the study of more complex FBPMe/HSA/BSA mixtures; the obtained results were in good agreement with the previous findings in FBPMe/HSA and FBPMe/BSA systems. This suggests the possibility of making use of the transient triplet-triplet absorption for investigating the distribution of a drug between several compartments in different host biomolecules.