Mechanisms and energetics of free radical initiated disulfide bond cleavage in model peptides and insulin by mass spectrometry

We investigate the mechanism of disulfide bond cleavage in gaseous peptide and protein ions initiated by a covalently-attached regiospecific acetyl radical using mass spectrometry (MS). Highly selective S–S bond cleavages with some minor C–S bond cleavages are observed by a single step of collisional activation. We show that even multiple disulfide bonds in intact bovine insulin are fragmented in the MS2 stage, releasing the A- and B-chains with a high yield, which has been challenging to achieve by other ion activation methods. Yet, regardless of the previous reaction mechanism studies, it has remained unclear why (1) disulfide bond cleavage is preferred to peptide backbone fragmentation, and why (2) the S–S bond that requires the higher activation energy conjectured in previously suggested mechanisms is more prone to be cleaved than the C–S bond by hydrogen-deficient radicals. To probe the mechanism of these processes, model peptides possessing deuterated β-carbon(s) at the disulfide bond are employed. It is suggested that the favored pathway of S–S bond cleavage is triggered by direct acetyl radical attack at sulfur with concomitant cleavage of the S–S bond (S_H2). The activation energy for this process is substantially lower by ∼9–10 kcal mol^–1 than those of peptide backbone cleavage processes determined by density functional quantum chemical calculations. Minor reaction pathways are initiated by hydrogen abstraction from the α-carbon or the β-carbon of a disulfide, followed by β-cleavages yielding C–S or S–S bond scissions. The current mechanistic findings should be generally applicable to other radical-driven disulfide bond cleavages with different radical species such as the benzyl and methyl pyridyl radicals.     

Photochemical reactions of aromatic compounds XLV: controlling factors for reactivities and mechanisms in photoelectron transfer reactions of some selected diarylcyclobutanes with electron acceptors

We have attempted to explore mechanistic aspects of the photosensitized ring-cleavage reactions of cis-1,2-diphenylcyclobutane (1), cis-transoid-cis-cyclobutal[1,2-a:4,3-a′] diindene (2) and r-1,c-2-dimethyl-t-3,t-4-di(4-methoxyphenyl)cyclobutane (3) by electron acceptors (A) in acetonitrile. The experimental results demonstrate that the ring cleavage of 1 and 2 occurs as a consequence of the rapid geminate recombination of ion-radical pairs occurring at a rate of well over 10⁹ s⁻¹ without ionic dissociation. In the case of 3, however, the photoreactions proceed by way of a chain-reaction mechanism involving the free cation radical of 3 which undergoes ring cleavage at much less than 10⁷ s⁻¹. The rapid ring cleavage of 1⁺ and 2⁺ is attributed to significant perturbations of the cyclobutane ring by the population of positive charge on the orbital array of the two π-electron systems and the cyclobutane-ring σ framework because of strong through-bond couplings. It is presumed that the cyclobutane ring of 3⁺ is much less distorted since the positive charge is mostly localized on the aryl group. The rapid geminate recombination of the A⁻⁻−1⁺ and A⁻⁻−2⁺ pairs is discussed in terms of a very efficient transition from the “distorted” and “ring-opened” minima of the A⁻⁻−D⁺ surface to the A–D surface. In the case of 3, this mechanism cannot be expected to operate in the geminate recombination.     

Two paths for stabilization of intermediate radical arylation of thioamides

Conclusions On the example of the radical phenylation of the N-phenyl and N,N-diethyldiphenylthioacetates it was shown that the intermediate radicals can be stabilized by the cleavage of a hydrogen atom from them from both NH and the CH group.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1167–1169, May, 1981.     

Structural studies in main group chemistry : XIX. Organotin(IV) derivatives of tetracyanoethylene, 7,7,8,8-tetracyanoquinodimethane and 2,3,5,6-tetrachlorobenzoquinone. The isolation of stable organotin-substituted radicals

The reaction of organotin chlorides with the lithium salt of 7,7,8,8-tetracyanoquinodimethane (TCNQ) or hexaalkylditins with TCNQ yield stable organotin-substituted free radicals of the types R₃SnTCNQ^. (R = Me, n-Pr, n-Bu) and Me₂Sn(TCNQ^.)₂. The reaction of hexaphenylditin with TCNQ yields a (σ → π) charge transfer complex of stoichiometry (Ph₃SnSnPh₃)·TCNQ, whilst [Me₂SnCl(terpyridyl)⁺](TCNQ^-·) was isolated from the reaction of [Me₂SnCl(terpyridlyl)⁺][Me₂SnCl₃^-] and LiTCNQ. The oxidation of hexaalkylditins by tetracyanoethylene (TCNE) yields stable free radicals of the type R₃SnTCNE·, but treatment with 2,3,5,6-tetrachlorobenzoquinone yields either R₃SnOC₆Cl₄O·-p (R = Me) or R₃SnOC₆Cl₄OSnR₃-p (R = n-Bu, Ph). Tin-119 Mössbauer spectroscopy shows that the derivatives R₃SnTCNQ· and R₃TCNE· have trigonally-bipyramidally coordinated tin with planar [SnC₃] skeletons and bridging [TCNQ·] and [TCNE·] groups forming infinite one-dimensional chain structures. Me₃SnOC₆Cl₄O·-p was inferred to possess a similar structure but with oxy bridges forming chains with a Sn---O---Sn---O backbone. Me₂Sn(TCNQ·)₂ has a structure intermediate between tetrahedral and octahedral with a non-linear MeSnMe unit and anisobidentate chelation by two TCNQ groups. The TCNQ derivatives were of two types: (i) “green” or “brown”, indicative of delocalisation of the Ione electron over the cyanoquinone ligand, and (ii) a “blue” form in which spin-pairing of the Ione electron between adjacent organic groups takes place. Me₃SnTCNQ· may exist in both forms depending upon the mode of preparation.     

Cleavage of the siloxane bond in poly(dialkylsiloxanes) by alkyl orthotitanates

Conclusions Linear polydialkylsiloxanes undergo cleavage upon heating with alkyl orthotitanates containing primary and secondary alkyl radicals, and the reactions are independent of the structure of radicals attached to the Si atoms, molecular weight, and the nature of the end groups. The factor which seems to determine the reactivity of polydialkysiloxanes with respect to cleavage at room temperature is the presence of CH end groups. The rate and degree of conversion in these reactions is decreased by increasing steric hindrance at the Si or Ti atoms.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 886–890, April, 1987.     

Dissociation and rearrangements of the molecular ions of sulfides,sulfoxides, and sulfones generated by a strong electric field

The field mass spectra of 18 sulfides, sulfoxides, and sulfones were investigated, and the principles of charge localization in the dissociation and rearrangements of the molecular ions of these compounds were established. A new type of fragmentation leading to the elimination of S^+., SO^+., and SO₂. was observed. A new mechanism, according to which cleavage of two C-S bonds and the formation of one new C-C bond due to radical recombination cocur in the cyclic transition complex, is proposed. Prior migration of alkyl radicals from sulfur to oxygen with subsequent cleavage of the S-O and C-S bonds via the above-mentioned mechanism may also occur in the molecular ions of sulfoxides and sulfones.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 2, pp. 172–181, March–April, 1987.     

Synthesis of triazenes of the benzothiazole,benzimidazole, and pyridine series,and investigation of the kinetics of their photochemical decomposition

Disubstituted alkyl(aryl)heteryltriazenes were synthesized by the reaction of the azides of benzothiazole, benzimidazole, and pyridine with Grignard reagents. The kinetics of their cleavage by UV irradiation were investigated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 201–204, February, 1971.     

Amine groups functionalized gel-type resin supported Pd catalysts: Physicochemical and catalytic properties in hydrogenation of alkynes

Palladium catalysts (0.125–0.5 wt.% Pd) supported by amine groups—functionalized gel-type resin (FCN) were studied in the hydrogenation of alkynes reagents, 2-butyne-1,4-diol and phenylacetylene. The catalysts were prepared by two routes. The first, “OAc” is based on the immobilization of Pd-precursor in the pre-swollen resin from THF solution of Pd(OAc)₂, followed by chemical reduction of the Pd-centers. This method produces Pd particles of size in nano-scale. The second procedure, “aq” implies the deposition of Pd-species on dry resin beads using aqueous solution of PdCl₂. Reduction of these Pd-species gives relatively large Pd particles, dominating are 30–50 nm in size. The SEM studies performed over the cross-section of catalysts grains showed location of Pd in outer shell of polymer beads in both “OAc” and “aq” catalysts; however, thinner layer of Pd appears in “aq” series catalysts. In the presence of all catalysts, prepared by “OAc” and “aq” methods the selectivity towards alkenes is high, above 90%. The catalysts of “aq’ series are much more active and more selective than “OAc” analogues giving selectivity to alkene ca. 94% at almost complete conversion of alkynes. Moreover, catalytic performance of “aq’ series catalyst is unchanged under recycling use. The catalyst was recovered and reused 4 times, maintaining its catalytic efficiency.     

Preparation of alpha-elastin nanoparticles by gamma irradiation

Nanoparticles were prepared by utilizing the thermosensitive aggregation of alpha-elastin and gamma ray crosslinking. Three different heating process, “Slow heating”, “Fast heating”, and “Heat shock”, were applied for the aggregation of the alpha-elastin and examined to yield nanoparticles by gamma rays crosslinking. As a result, only “Slow heating” process yielded nanoparticles with diameters of about ca. 300 nm above cloud point (CP) and about ca. 100 nm below CP, and a narrow size distribution above 1.0 mg/ml concentration (exclude 1.0 mg/ml).     

Diffuse reflectance spectroscopy of powders

Diffuse reflectance (DRIFT) spectra in the mid-IR region, especially of inorganic powders, often reveal inverted or derivative-like structures (“reststrahlen” bands) which make their straightforward interpretation in terms of the Kubelka-Munk theory difficult. A simulation technique based on a combination of single scattering (Mie theory) and radiative transfer (with Kubelka-Munk as a special case) is reported that allows measured DRIFT spectra to be described without use of any free “fit” parameters. The results for silica glass microspheres and silicon carbide powder show that not specular reflectance but prominent structures in the single particle scattering cross-section are responsible for the reststrahlen features which cannot be suppressed even by diluting the powder in an excess of KBr. The efficiency of a “blocker” device in DRIFTS is also discussed.     

Investigation of the interactions between ginsenosides and amino acids by mass spectrometry and theoretical chemistry

In order to evaluate the essence of the interactions of ginsenosides and proteins which are composed by α-amino acids, electrospray ionization mass spectrometry was employed to study the noncovalent interactions between ginsenosides (Rb₂, Rb₃, Re, Rg₁ and Rh₁) and 18 kinds of α-amino acids (Asp, Glu, Asn, Phe, Gln, Thr, Ser, Met, Trp, Val, Gly, Ile, Ala, Leu, Pro, His, Lys and Arg). The 1:1 and 2:1 noncovalent complexes of ginsenosides and amino acids were observed in the mass spectra. The dissociation constants for the noncovalent complexes were directly calculated based on peak intensities of ginsenosides and the noncovalent complexes in the mass spectra. Based on the dissociation constants, it can be concluded that the acidic and the basic amino acids, Asp, Glu, Lys and Arg, bound to ginsenosides more strongly than other amino acids. The experimental results were verified by theoretical calculations of parameters of noncovalent interaction between ginsenoside Re and Arg which served as a representative example. Two kinds of binding forms, “head–tail” (“H–T”) and “head–head” (“H–H”), were proposed to explain the interaction between ginsenosides and amino acids. And the interaction in “H–T” form was stronger than that in “H–H” form.     

Semiempirical Analysis of the Homolytic Decomposition of Peresters with the Concerted Cleavage of Two Bonds

The parabolic model of a bimolecular reaction is modified to study the monomolecular decomposition of molecules into radicals by the cleavage of several bonds. Together with the oscillation model of molecule decomposition with the concerted cleavage of several bonds, this model is used to analyze the kinetic data on the decomposition of 16 peresters with the simultaneous cleavage of C–C and O–O bonds. Parameters characterizing this decomposition are obtained and multiple variants in representing such decomposition in terms of the parabolic model are discussed.     

An experimental study on the “sequential order” rules in generalized two-dimensional correlation spectroscopy

Following our theoretical analysis on the “sequential order” rules in generalized two-dimensional (2D) correlation spectroscopy (H. Huang, Anal. Chem. 79 (2007) 8281–8292), an experimental study was conducted to test the “sequential order” rules using the FT-NIR data of poly(3-hydroxybutyrate) (PHB)/poly(l-lactic acid) (PLA) blends under uniaxial elongation and parallel polarization. The local sequential order concept proposed for the generalized two-dimensional (2D) correlation spectroscopy is now more clearly stated; “the intensity change at ν1 occurs predominantly before ν2” means that the starting time of the intensity change at ν1 is prior to that at ν2. It is this local sequential order which reflects the real and intuitive sequential order between two events in generalized situations. It has been found that the integrated/overall sequential order results obtained from the 2D correlation analysis may be contradictory to the intuitive local sequential order. In addition, different integrated/overall sequential orders could be obtained by selection of different sampling intervals from a certain set of experimental data, or choosing different number of the contours for the same sampling interval. These new experimental findings are a perfect reinforcement to our previous theoretical study and have further demonstrated the uncertainty of applying the “sequential order” rules in generalized 2D correlation spectroscopy.     

Azasteroids. Synthesis by Diels-Alder Reaction between Maleimides, Citraconimide, and Triazolindiones and 1-(1-Trialkylsiloxyvinyl)-3,4-dihydronaphthalene Derivatives

Summary. 18-Nor-16-azaestrane derivatives with 8β, 13β, and 14β orientation were isolated from Diels-Alder reactions between maleimides or citraconimide and 1-(1-siloxyvinyl)naphthalene derivatives. (8RS)-13,14,16-Triazaestrane derivatives were synthesized from 1,2,4-triazolin-3,5-diones. The parent 11-oxo derivatives were obtained by desilylation, and they were transformed into 11-hydroxyimino derivatives. 3-Hydroxy derivatives, finally were synthesized by cleavage of the 3-methoxy group with BBr₃. During these transformations the stereochemistry of the steroidal skeleton was not changed. The stereochemistry of these “unnatural” steroids was elucidated by spectroscopic methods, and compared with results from calculations, and with the configuration of natural estrane derivatives. Finally, an improved method for the synthesis of the starting material, 6-methoxy-1-[(1-trialkylsiloxy)-vinyl]-3,4-dihydronaphthalene was developed.     

Preparation and catalytic activity of titanium silicalite-1 zeolite membrane with TPABr as template

The preparation of the supported titanium silicalite-1 (TS-1) zeolite membrane with inexpensive tetrapropylammonium bromide (TPABr)/weak base synthesis system was studied by three methods, and the catalytic activity of the obtained TS-1 zeolite membrane was evaluated with the oxidation of 2-propanol (IPA) under pervaporation condition. It was found that TS-1 zeolite membrane could be successfully prepared with “seeding” or “seeding in situ” method, but could not be achieved with “in situ” method. Adding a little amount of promoter ions of PO₄³⁻ into the synthesis gel was of benefit to the catalytic activity of the prepared TS-1 zeolite membrane, but had no obvious effect on the membrane layer formation on the mullite porous support. For “seeding” method, the membrane prepared with the synthesis gel having molar composition of SiO₂:0.1TPABr:0.9Et₂NH:0.03TiO₂:80H₂O:0.06H₃PO₄ at 150 °C for 48 h showed the highest oxidation conversion of IPA of 72% accompanied by a flux of 0.35 kg/m² h. Further more, much higher IPA oxidation conversion of 76% accompanied by a flux of 0.65 kg/m² h was obtained for the TS-1 zeolite membrane prepared with the same synthesis gel by “seeding in situ” method at 150 °C for 72 h.     

Efficient cleavage of tertiary amide bonds via radical–polar crossover using a copper(ii) bromide/Selectfluor hybrid system

A novel approach for the efficient cleavage of the amide bonds in tertiary amides is reported. Based on the selective radical abstraction of a benzylic hydrogen atom by a CuBr₂/Selectfluor hybrid system followed by a selective cleavage of an N–C bond, an acyl fluoride intermediate is formed. This intermediate may then be derivatized in a one-pot fashion. The reaction proceeds under mild conditions and exhibits a broad substrate scope with respect to the tertiary amide moiety as well as to nitrogen, oxygen, and carbon nucleophiles for the subsequent derivatization. Mechanistic studies suggest that the present reaction proceeds via a radical–polar crossover process that involves benzylic carbon radicals generated by the selective radical abstraction of a benzylic hydrogen atom by the CuBr₂/Selectfluor hybrid system. Furthermore, a synthetic application of this method for the selective cleavage of peptides is described.

A novel approach for the efficient cleavage of the amide bonds in tertiary amides is reported.     

Experimental studies on the applicability of the Kelvin equation to highly curved concave menisci

The thermodynamic properties of liquids trapped in microscopic pores are described in theory by the Kelvin equation, which relates the equilibrium meniscus curvature to the relative vapor pressure. We report here two series of experiments designed to test the validity of the Kelvin equation by direct measurement of the mean radius of curvature of the surface of cyclohexane condensed between crossed mica cylinders. In one series of experiments, the relative vapor pressure of the volatile cyclohexane was controlled by mixing it with a relatively involatile solute (n-dodecane or n-hexadecane). We found that the mean radius of curvature rapidly reached that predicted by the Kelvin equation at each relative vapor pressure of the volatile liquid, but that there was also a slow, but continuous, accumulation of the “involatile” solute at the point of condensation as the system approached true equilibrium. Such accumulation of very low vapor pressure materials may be one factor responsible for the discordant results reported by earlier workers. We find that the process of impurity buildup is complex, and suggest that studies of real porous systems may be affected by accumulation of “involatile” impurities through the vapor phase and by surface diffusion. The other series of experiments was designed to eliminate the impurity problem by maintaining the vapor pressure by temperature control of the pure liquid. The results from this series of experiments were not time dependent, and no evidence of contamination was found. The measured radii were within ±6% of those predicted by the Kelvin equation, for radii in the range 4–20 nm. We conclude that the thermodynamic basis of the Kelvin equation is valid in principle for menisci with radii as low as 4 nm.     

Conventional and unconventional infrared spectrometry and their quantitative interpretation

In condensed matter, optical properties can be described by a dielectric function (DF), and the structures observed in spectra are then related to the poles and zeros of the DF. As an example, model functions are calculated by a fit to measured spectroscopic data for polystyrene and silica. The first material shows weak, narrow bands and the latter strong, broad bands and a negative real part of the DF.Based on these model DFs, spectra are simulated which are expected to be obtained by “conventional” methods such as transmittance or reflectance measurements, or by “unconventional” methods such as reflectance at oblique incidence, diffuse reflectance, photoacoustic spectroscopy and attenuated total reflectance. A variety of simulated, typical spectra are plotted as a small “atlas”. Conditions are discussed that allow a straightforward procedure for interpreting the spectra quantitatively, i.e., the evaluation of the resonance frequency and the concentration of the oscillators under consideration.It is shown that for systems characterized by weak, narrow oscillator lines, mostly an intuitive interpretation is possible, looking only at the position and strength of “lines” in the spectra. Materials showing strong polar vibrations, however, require more sophisticated procedures for interpreting the spectra.     

A thermodynamic interpretation of the behavior of higher fatty alcohols and acids upon the chromatographic paper

An idea was presented of treating the chromatographed substance as a “solute,” and the chromatographic system, composed of the stationary and the mobile phase as a “solvent.” Moreover the concept of “local equilibrium” was introduced, allowing to regard a given chromatographic spot as a “binary solution.” Thus a possibility arose to apply the classical thermodynamic approach, normally used for binary solutions, and namely: μ_i = μ_i + RT ln x_iƒ_i, where μ_i—chemical potential of the “i”-th compound in the solution, μ_i—chemical potential of the pure “i”-th compound, x_i-molar fraction of the “i”-th compound, ƒ_i—its activity coefficient, in a modified form, suitable for the chromatographic purpose.     

Nanotechnologies for drug delivery: Application to cancer and autoimmune diseases

Polymer-based nanotechnologies are now proposed as an alternative to classical formulations for drug administration, delivery and targeting. Therapeutic applications of the first generation of nanotechnologies include the treatment of cancer liver diseases. Avoiding the recognition by the liver is also possible by developing long circulating polymeric colloidal carriers (“stealth” systems) able to avoid the opsonization process and the recognition by the macrophages. The design of such carriers of second generation is based on the physico-chemical concept of the “steric repulsion”: by grafting polyethyleneglycol chains at the surface of nanoparticles, the adsorption of seric proteins may be dramatically reduced due to steric hindrance. Such an approach allows maintaining the drug carrier for a longer time into the circulation and the resulting extravasation towards non reticuloendothelial-located cancers may become possible. Now, new applications and exciting perspectives are proposed for the delivery of drugs to previously non accessible diseased sanctuaries, like the brain (treatment of glioma and autoimmune diseases of the brain) or the ocular tissues (treatment of the autoimmune uveitis). Finally, the use of nanotechnologies for the delivery of nucleic acids (oligonucleotides) is also discussed in this review.