Time-resolved chemically induced dynamic nuclear polarization studies of structure and reactivity of methionine radical cations in aqueous solution as a function of pH

Using time-resolved chemically induced dynamic nuclear polarization (CIDNP) techniques, we have studied the mechanism of the photoreactions of triplet excited 4-carboxybenzophenone (CBP) with l-methionine (Met) and 3-(methylthio)propylamine (MTPA) in aqueous solution and the details of the formation of CIDNP at pH from 6.7 to 13.6. At a pH below the pKa of the nitrogen atom of Met, the CIDNP is strongly affected by degenerate electron exchange between the S-S cationic radical dimer and the zwitterionic form of Met with the rate constant kex = 3.4 x 10(8) s(-1) providing an exhaustive explanation of the pH dependence of steady-state CIDNP that was previously interpreted as a manifestation of fast interconversion among three different methionine radical species (Goez, M.; Rozwadowski, J. J. Phys. Chem. A 1998, 102, 7945-7953). By analyzing the polarization of different protons formed in geminate recombination as a function of the pH, we obtained the branching ratio between two reaction pathways for oxidative quenching of (T)CBP via electron transfer from the sulfur and nitrogen atoms of Met and MTPA. Nuclear spin-lattice relaxation times were determined in the dimeric cation radical of Met (T1,S = 8.5 micros). In the cyclic radical cation of MTPA with a three-electron two-center S-N bond, the estimated paramagnetic relaxation is comparatively slow for all protons. Fast deprotonation of the primary aminium radical cation of MTPA and Met in strongly basic solution takes place on the submicrosecond time scale leading to efficient formation of CIDNP in the neutral aminyl radical.     

Neighboring pyrrolidine amide participation in thioether oxidation. Methionine as a "hopping" site

Methionine residues have been shown to function as efficient "hopping" sites in long-range electron transfer in model polyprolyl peptides. We suggest that a key to this ability of methionine is stabilization of the transient sulfur radical cation by neighboring proline amide participation. That is, in a model system a neighboring pyrrolidine amide lowers the oxidation potential of the thioether by over 0.5 V by formation of a two-center three-electron SO bond.     

Relay stations for electron hole migration in peptides: possibility for formation of three-electron bonds along peptide chains

Our calculations found that the O thereforeO three-electron (3e) bonds (2.16 approximately 2.27 A) can be formed not only between two neighboring peptide units in a main chain but also between two adjacent peptide units in two different main chains in proteins. This finding may address electron hole migration from one peptide unit to the next in proteins. Evidently, stability of the O thereforeO 3e bonded species is strongly dependent on the component of the oligopeptides and is reduced owing to the steric hindrance of the side chains when the big chains present in oligopeptides. Besides, formation of the O thereforeO 3e bonds competes with the formation of the other forms of three-electron bonds depending on the component of the polypeptides. Formation of the O thereforeS 3e bond is thermodynamically more favorable than that of the O thereforeO 3e bond for the oligopeptides containing sulfur atom in their side chains. Similarly, formation of the O thereforepi 3e bond between aromatic ring of the side chain and the neighboring peptide unit is more stable than that of the O thereforeO 3e bond when the aromatic amino acids present in the oligopeptides. We infer that a series of three-electron bonds may be formed during the electron hole migration along the peptide backbone in proteins and assist electron hole transport as relay stations, supporting the peptide chain as a conduction wire. The ab initio molecular dynamics simulations of the polypeptides also support this conclusion.     

An Isolable Diphosphene Radical Cation Stabilized by Three‐Center Three‐Electron π‐Bonding with Chromium: End‐On versus Side‐On Coordination

Two salts ( 2 and 4 ) containing the radical cations of complexed diphosphenes have been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, IR spectroscopy, and single‐crystal X‐ray diffraction. The P?P bond is coordinated to the Cr center either in an end‐on (in 2 ) or a side‐on (in 4 ) fashion. The spin density of the radical is delocalized over the Cr atom and the two P atoms in 2 whereas the unpaired electron is mainly localized on the Cr atom in 4 . This work provides the first example of a complexed diphosphene radical ( 2 ) featuring novel three‐center three‐electron (3c‐3e) π‐bonding in the Cr‐P‐P unit, and the first example of a 17 e Cr radical with a side‐on π‐bonded ligand ( 4 ).     

Stabilization of sulfide radical cations through complexation with the peptide bond: mechanisms relevant to oxidation of proteins containing multiple methionine residues

The recent study on the *OH-induced oxidation of calmodulin, a regulatory "calcium sensor" protein containing nine methionine (Met) residues, has supported the first experimental evidence in a protein for the formation of S therefore N three-electron bonded radical complexes involving the sulfur atom of a methionine residue and the amide groups in adjacent peptide bonds. To characterize reactions of oxidized methionine residues in proteins containing multiple methionine residues in more detail, in the current study, a small model cyclic dipeptide, c-(L-Met-L-Met), was oxidized by *OH radicals generated via pulse radiolysis and the ensuing reactive intermediates were monitored by time-resolved UV-vis spectroscopic and conductometric techniques. The picture that emerges from this investigation shows there is an efficient formation of the Met (S therefore N) radicals, in spite of the close proximity of two sulfur atoms, located in the side chains of methionine residues, and in spite of the close proximity of sulfur atoms and oxygen atoms, located in the peptide bonds. Moreover, it is shown, for the first time, that the formation of Met(S therefore N) radicals can proceed directly, via H+-transfer, with the involvement of hydrogen from the peptide bond to an intermediary hydroxysulfuranyl radical. Ultimately, the Met(S therefore N) radicals decayed via two different pH-dependent reaction pathways, (i) conversion into sulfur-sulfur, intramolecular, three-electron-bonded radical cations and (ii) a proposed hydrolytic cleavage of the protonated form of the intramolecular, three-electron-bonded radicals [Met(S therefore N)/Met(S therefore NH)+] followed by electron transfer and decarboxylation. Surprisingly, also alpha-(alkylthio)alkyl radicals enter the latter mechanism in a pH-dependent manner. Density functional theory computations were performed on the model c-(L-Met-Gly) and its radicals in order to obtain optimizations and energies to aid in the interpretation of the experiments on c-(L-Met-L-Met).     

Effect of amine ligand bulk on the interaction of methionine with platinum(II) diamine complexes

Molecular mechanics and dynamics calculations have been used in conjunction with experimental data to study the effects of amine ligand bulk on the formation of both guanine and methionine complexes with platinum diamine compounds. The AMBER force field has been supplemented with previous modifications [Yao; et al. Inorg. Chem. 1994, 33, 6061-6077. Cerasino; et al. Inorg. Chem. 1997, 36, 6070-6079] and has been further modified to include parameters for platinum bound to the sulfur atom of methionine. Molecular mechanics calculations with this modified AMBER force field have suggested that a platinum complex with two sulfur-bound methionine ligands and a bulky diamine ligand (N,N,N',N'-tetramethylethylenediamine, Me(4)en) would have severe interligand clashes; such interligand clashes are less pronounced in bis(9-ethylguanine) complexes. Consistent with these observations, NMR studies with [Pt(Me(4)en)(D(2)O)(2)](2+) have indicated that guanine 5'-monophosphate reacts in a 2:1 guanine:platinum ratio while both methionine and N-acetylmethionine react with only a 1:1 stoichiometry. Methionine forms a chelate via the sulfur and nitrogen atoms whereas N-acetylmethionine forms a chelate via the sulfur and oxygen atoms. The oxygen of the latter chelate can be displaced by the addition of guanosine 5'-monophosphate, although complete displacement of the N-acetylmethionine was not observed.     

Photosensitized oxidation of methionine derivatives. Laser flash photolysis studies

The early events in the triplet 4-carboxybenzophenone (CB)-induced oxidation of N-acetyl-methionine methyl ester (N-Ac-Met-OCH₃) are investigated in aqueous solution. Upon electron transfer from the methionine residue of N-Ac-Met-OCH₃ to ³CB*, the resulting sulfur radical cation undergoes further reactions: (1) back electron transfer, (2) escape of the radical ions from the solvent cage, or (3) proton transfer and escape of the radicals. The yields and paths of these reactions are shown to depend strongly on the pH of the solution, and, similar to the previously reported results for dipeptides (Met-Gly and Gly-Met), on the structural nature of the methionine substituents. In the experiments performed in this work, low quencher concentrations were used to avoid formation of intermolecular transients (e.g., dimeric sulfur-centered radical cation (S∴S)⁺). Under these experimental conditions, the one-electron oxidized sulfur does not seem to become stabilized in an (S∴N)⁺ three-electron bonded intramolecular complex. The proposed mechanism is further supported by the stable products analysis. A detailed mechanism involving characterization of the transients is discussed and compared to that of methionine and methionine-containing dipeptides (Met-Gly and Gly-Met). Moreover, a newly installed transient absorption laser system is described in details.     

PHOTOSENSITIZED DEAMINATION OF SULFUR-AMINO ACIDS BY FLAVIN MONONUCLEOTIDE*

Abstract— S-Methylcysteine, methionine and cystine were found to be photochemically deaminated at a much higher rate than methionine sulfoxide, homomethionine and other non-sulfur amino acids in the presence of flavin mononucleotide. These data strongly support the view that the initial oxidation of those sulfur-amino acids by the photoactivated flavin is a one-electron abstraction from the sulfur atom followed by an intramolecular electron transfer from the carboxyl anion of the amino acid to the sulfinium radical, resulting in the decarboxylation-deamination.     

Reactivity and selectivity of charged phenyl radicals toward amino acids in a Fourier transform ion cyclotron resonance mass spectrometer

The reactivity of 10 charged phenyl radicals toward several amino acids was examined in the gas phase in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer. All radicals abstract a hydrogen atom from the amino acids, as expected. The most electrophilic radicals (with the greatest calculated vertical electron affinities (EA) at the radical site) also react with these amino acids via NH(2) abstraction (a nonradical nucleophilic addition-elimination reaction). Both the radical (hydrogen atom abstraction) and nonradical (NH(2) abstraction) reaction efficiencies were found to increase with the electrophilicity (EA) of the radical. However, NH(2) abstraction is more strongly influenced by EA. In contrast to an earlier report, the ionization energies of the amino acids do not appear to play a general reactivity-controlling role. Studies using several partially deuterium-labeled amino acids revealed that abstraction of a hydrogen atom from the α-carbon is only preferred for glycine; for the other amino acids, a hydrogen atom is preferentially abstracted from the side chain. The electrophilicity of the radicals does not appear to have a major influence on the site from which the hydrogen atom is abstracted. Hence, the regioselectivity of hydrogen atom abstraction appears to be independent of the structure of the radical but dependent on the structure of the amino acid. Surprisingly, abstraction of two hydrogen atoms was observed for the N-(3-nitro-5-dehydrophenyl)pyridinium radical, indicating that substituents on the radical not only influence the EA of the radical but also can be involved in the reaction. In disagreement with an earlier report, proline was found to display several unprecedented reaction pathways that likely do not proceed via a radical mechanism but rather by a nucleophilic addition-elimination mechanism. Both NH(2) and (15)NH(2) groups were abstracted from lysine labeled with (15)N on the side chain, indicating that NH(2) abstraction occurs both from the amino terminus and from the side chain. Quantum chemical calculations were employed to obtain insights into some of the reaction mechanisms.     

Palladium(II) complexes, as synthetic peptidases, regioselectively cleave the second peptide bond "upstream" from methionine and histidine side chains

Palladium(II) complexes promote hydrolysis of natural and synthetic oligopeptides with unprecedented regioselectivity; the only cleavage site is the second peptide bond upstream from a methionine or a histidine side chain, that is, the bond involving the amino group of the residue that precedes this side chain. We investigate this regioselectivity with four N-acetylated peptides as substrates: neurotransmitter methionine enkephalin (Ac-Tyr-Gly-Gly-Phe-Met) and synthetic peptides termed Met-peptide (Ac-Ala-Lys-Tyr-Gly-Gly-Met-Ala-Ala-Arg-Ala), His-peptide (Ac-Val-Lys-Gly-Gly-His-Ala-Lys-Tyr-Gly-Gly-Met(OX)-Ala-Ala-Arg-Ala), in which a Met is oxidized to sulfone, and HisMet-peptide (Ac-Val-Lys-Gly-Gly-His-Ala-Lys-Tyr-Gly-Gly-Met-Ala-Ala-Arg-Ala). While maintaining protein-like properties, these substrates are suitable for quantitative study since their coordination to Pd(II) ion can be determined (by NMR spectroscopy), and the cleavage fragments can be separated (by HPLC methods) and identified (by MALDI mass spectrometry). The only peptide bonds cleaved were the Gly3-Phe4 bond in methionine enkephalin, Gly4-Gly5 bond in Met-peptide, Gly3-Gly4 in His-peptide, and Gly3-Gly4 and Gly9-Gly10 bonds in HisMet-peptide. We explain this consistent regioselectivity of cleavage by studying the modes of Met-peptide coordination to the Pd(II) ion in [Pd(H(2)O)(4)](2+) complex. In acidic solution, the rapid attachment of the Pd(II) complex to the methionine side chain is followed by the interaction of the Pd(II) ion with the peptide backbone upstream from the anchor. In the hydrolytically active complex, Met-peptide is coordinated to Pd(II) ion as a bidentate ligand - via sulfur atom in the methionine side chain and the first peptide nitrogen upstream from this anchor - so that the Pd(II) complex approaches the scissile peptide bond. Because the increased acidity favors this hydrolytically active complex, the rate of cleavage guided by either histidine or methionine anchor increased as pH was lowered from 4.5 to 0.5. The unwanted additional cleavage of the first peptide bond upstream from the anchor is suppressed if pH is kept above 1.2. Four Pd(II) complexes cleave Met-peptide with the same regioselectivity but at somewhat different rates. Complexes in which Pd(II) ion carries labile ligands, such as [Pd(H(2)O)(4)](2+) and [Pd(NH(3))(4)](2+), are more reactive than those containing anionic ligands, such as [PdCl(4)](2)(-), or a bidentate ligand, such as cis-[Pd(en)(H(2)O)(2)](2+). When both methionine and histidine residues are present in the same substrate, as in HisMet-peptide, 1 molar equivalent of the Pd(II) complex distributes itself evenly at both anchors and provides partial cleavage, whereas 2 molar equivalents of the promoter completely cleave the second peptide bond upstream from each of the anchors. The results of this study bode well for growing use of palladium(II) reagents in biochemical and bioanalytical practice.     

1,2-dithiin annelated with bicyclo[2.2.2]octene frameworks. One-electron and two-electron oxidations and formation of a novel 2,3,5,6-tetrathiabicyclo[2.2.2]oct-7-ene radical cation with remarkable stability owing to a strong transannular interaction

A stable derivative of 1,2-dithiin annelated with bicyclo[2.2.2]octene frameworks 4 was synthesized as red crystals by the reaction of a dilithiated dimer of bicyclo[2.2.2]octene with elemental sulfur in 59% yield. The cyclic voltammetry of 4 in CH(2)Cl(2) at -78 degrees C showed two reversible oxidation waves at E(1/2) +0.18 V and +0.72 V versus Fc/Fc(+), indicating that the radical cation and dication of 4 are stable under these conditions. Upon chemical one-electron oxidation of 4 in a rather low concentration (4.0 x 10(-4) M) with a 1.5 equiv of SbCl(5) in CH(2)Cl(2), a radical cation 4.+ was formed, whose spin distribution was determined by ESR spectroscopy and by the results of theoretical calculations (UB3LYP/6-31G). The electronic absorption spectrum of 4.+ in CH(2)Cl(2) exhibited a maximum absorption at 428 nm (epsilon = 2.3 x 10(3)), which was hypsochromically shifted from that of neutral 4 (469 nm). When the radical cation 4.+ was produced in higher concentration (0.06 M) in CH(2)Cl(2), a disproportionation was found to take place to give a SbCl(6)(-) salt of remarkably stable radical cation 5.+ having a novel 2,3,5,6-tetrathiabicyclo[2.2.2]oct-7-ene structure. In the X-ray structure of 5.+SbCl(6)(-), the transannular distance (2.794(3) A) between the sulfur atoms was found to be less than the sum of the van der Waals radii of a sulfur atom (3.70 A), suggesting the existence of a bonding interaction between the two disulfide linkages. The theoretical calculations (UB3LYP/6-31G) suggested that this transannular interaction could be described as the resonance between the limiting structures, each of them having a two-center three-electron bond between two sulfur atoms belonging to two different disulfide linkages: thus, both the spin and positive charge are equally delocalized to the four sulfur atoms, causing a great stabilization of 5.+. On the other hand, the 1,2-dithiin radical cation 4.+ was found to readily react with triplet oxygen with subsequent rearrangement to give the 1,2-dithiolium derivative 6+ having a carboxyl group. Finally, the reaction of 4 with an excess amount of SbF(5) gave the corresponding dication 4(2+), which was found to be a 6pi aromatic system on the basis of the results of NMR measurement and theoretical calculations.     

Diketone radical cations: ketonic and enolic forms as revealed by matrix EPR studies and DFT calculations

Radical cations of 2,3-butanedione, 2,4-pentanedione, 3-methylpentane-2,4-dione, 2,5-hexanedione, and 2,3-pentanedione were investigated by electron paramagnetic resonance (EPR) spectroscopy in a solid Freon matrix and density functional theory (DFT) quantum chemical calculations. All the diketone radical cations in ketonic form show small proton hyperfine couplings (typically unresolved in the EPR spectra). In the cases of 2,4-pentanedione and 3-methylpentane-2,4-dione, enolic forms of the radical cations (pi-type species with main spin population at carbon atom) were characterized. Preferential stabilization of the enolic form of 3-methylpentane-2,4-dione radical cation was explained by trap-to-trap positive hole migration rather than monomolecular relaxation of the ionized ketonic form through H atom transfer.     

The nature of sulfur bonding in thiopyrylium cation

Comparison of the PMR spectrum ofthiopyrylium cation with those of the oxygen (pyrylium cation) and nitrogen (pyridinium cation) analogs has suggested the unique electronic structure of the thiopyrylium cation. To investigate this structure the extended Hückel MO calculations have been carried out using two basis sets, one with and another without sulfur 3d orbitals. The electronic structure of thiopyrylium cation can be rationalized by the 3d orbital involvement of the S atom in the basis set. The primary effect of the involvement of 3d orbitals on the S atom is shown to be the electron transfer from the ring carbon fragment, in particular from the β ring carbons, to the S atom, with an accompanying increase in sulfur-α-carbon bond order.     

A new approach to the synthesis of strained cyclic systems: II. Mass spectrometric study of 2-dialkylamino-3-phenylthiophenes and their structural isomers, iminothietanes and iminocyclobutenes

Triads of isomeric N-alkyl-N-methyl-3-phenylthiophen-2-amines, N-methyl-3-alkyl-4-methylidene-3-phenylthietan-2-imines, and N-methyl-4-alkylsulfanyl-2-phenylcyclobut-2-en-1-imines (Alk = Me, Et, Bu) were synthesized from 1,3-dilithio-3-phenylpropyne, methyl isothiocyanate, and alkyl halides, and their fragmentation under electron impact was studied. Primary decomposition of the molecular ions of 2-aminothiophenes is determined by the localization of a radical cation center on the nitrogen atom, and it follows a path typical of alkyl(aryl)amines with elimination of hydrogen atom or methyl or propyl radical from the α-carbon atom in the N-alkyl substituent. Fragmentation of the iminothietanes involves cleavage of the four-membered ring in half to give neutral MeNCS molecule and 1-alkyl-1-phenylallene radical cation. Alkylsulfanyl(imino)-cyclobutenes undergo cleavage at the sulfur-containing side chain according to general relations holding in the fragmentation of alkyl sulfides.     

Conformational influence on the type of stabilization of sulfur radical cations in cyclic peptides.

The free-radical chemistry of two oxidized cyclic dipeptides is investigated using time-resolved optical and conductivity detection. Two cyclic dipeptides, cyclo-Gly-L-Met and cyclo-D-Met-L-Met, are synthesized and irradiated with nanosecond pulses of electrons, which initiate the oxidation of the methionine side chains with hydroxyl radicals from the radiolysis of water. The cyclic peptides are taken to be models for the interior of proteins where there are no terminal groups. This opens up the possibility that neighboring-group effects can be studied directly between the initially formed sulfur radical cations and the heteroatoms associated with the peptide bonds. Such complexation of the sulfur radical cations is observed with the amide nitrogen atoms. In addition, intermolecular stabilization with the unoxidized sulfur atoms on separate cyclic dipeptide molecules is observed. Little or no intramolecular stabilization by the unoxidized sulfur in the neighboring methionine occurs in cyclo-D-Met-L-Met, in contrast to the previously observed intramolecular sulfur stabilization of the sulfur radical cation in the isomer cyclo-L-Met-L-Met. This contrasting behavior is rationalized by conformational differences in the two isomers as seen through molecular-modeling simulations. The implications for the oxidation of the protein calmodulin, which contains multiple residues of methionine, are discussed as having analogous determining factors.     

Structure,bonding, and spectra of cyclic dithia radical cations: a theoretical study

Ab initio molecular orbital and hybrid density functional theory methods are employed to characterize the structure, bonding and properties of several cyclic dithia radical cation systems, particularly in the context of intra molecular two-center three-electron (2c-3e) bonding between two sulfur atoms. The calculated results are able to interpret the time-resolved transient optical spectra obtained from pulse radiolysis technique for these positively charged dithia systems in aqueous solution. Visualization of the appropriate molecular orbital (MO) in the systems is able to depict the presence of a 2c-3e bond between two sulfur atoms and its sigma character. Geometry optimizations of these doublet systems are carried out at restricted open shell Becke's half-and-half (BHH) nonlocal exchange and Lee-Yang-Parr (LYP) nonlocal correlation functionals (BHHLYP) with 6-311+G(d,p) basis set including solvent effects adopting Onsager's reaction field model. Hessian calculations are done at the same level to check the nature of the equilibrium geometry. Energy data are further improved by performing MP2/6-311+G(d,p) calculations on these radical cation systems. Excited-state calculations are done following configuration interaction with single-electron excitation (CIS) method and the optical transition wavelength from the highest doubly occupied molecular orbital (HDOMO) to the lowest singly occupied molecular orbital (LSOMO) is seen to correspond and match to the position of the absorption maxima (lambda(max)) obtained from the experimental spectra for all these radical cation systems in aqueous solution. These calculations are able to resolve a long-standing ambiguity in the assignment of intra molecular 2c-3e bonding in the case of the 3-methyl-2,4-dithiapentane radical cation system and to provide new insights into bonding features of this odd electron system as well as of other cyclic dithia systems studied.     

Kinetic and thermochemical study of the antioxidant activity of sulfur-containing analogues of vitamin E

Sulfur-containing analogues of vitamin E (thiachromanols), either linked or not to a catechol moiety, were synthesized and their hydrogen-atom donating ability evaluated. The determination of the O--H bond dissociation enthalpy (BDE) of the alpha-tocopherol analogue 4 by the electron paramagnetic resonance (EPR) equilibration technique provided a value of 78.9 kcal mol(-1), that is, approximately 1.8 kcal mol(-1) higher than that of alpha-tocopherol. The kinetic rate constants for the reaction with peroxyl radicals (kinh), measured by inhibited autoxidation studies, showed that thiachromanols react 2.5 times slower than the corresponding tocopherols, in agreement with the higher BDE value. This behavior was explained, on the basis of crystallographic analyses and DFT calculations, in terms of a change in the molecular geometry caused by insertion of a sulfur atom into the framework of vitamin E. This behavior implies a greater deviation of the condensed ring from coplanarity with the aromatic ring, thus giving rise to a decrease in the conjugative stabilization of the phenoxyl radical and consequently to an increase in the O--H bond strength. Although less reactive than tocopherols, thiachromanols may, however, act as bimodal antioxidants as a result of the hydroperoxide decomposing ability of the sulfur atom.     

Anodic oxidation of methyl alpha-dimethylsilyldihydrocinnamate. A novel silicon gamma-aryl effect

The anodic oxidation of methyl 3-phenyl-2-dimethylsilylpropionate occurs at a potential almost 1 V positive of that required to oxidize other alpha-silyl esters. Semiempirical and ab initio calculations on the model compound 1-phenyl-2-trimethylsilylethane indicate that electron removal from these two compounds is highly stereoelectronically dependent. Both molecules exist almost exclusively in a conformation in which the phenyl group and silicon atom are anti and the side chain is perpendicular to the aromatic ring. This conformation has a higher energy HOMO orbital and lower computed ionization potential than the only other significantly populated conformation of 1-phenyl-2-trimethylsilylethane. Finally, the ab initio calculations show that in the cation radical of this model compound the ipso carbon of the aromatic ring and the side chain carbon bound to silicon draw significantly closer together than in the neutral species; an electrostatic potential map of the cation radical shows that the ipso carbon bears the highest degree of positive charge of any of the benzenoid carbons. We interpret these data, taken together, as an indication that this cation radical is stabilized by overlap of the rear lobe of the carbon-silicon bond with the p-orbital of the ipso carbon.     

Time-Resolved Electron Paramagnetic Resonance Study of Photoionization of Tyrosine Anion in Aqueous Solution

Abstract— Photoionization of the amino acid tyrosine in basic water was studied by time-resolved electron paramagnetic resonance (TREPR) at X-band (9.5 GHz). Photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive chemically induced dynamic electron polarization (CIDEP) spectrum produced by the radical pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction, J, is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of photoionization of deprotonated tyrosine is seen only upon addition of the electron acceptor 2-bro-mo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic acid). This channel is also found to be monophotonic.     

On the mechanism of intramolecular nitrogen‐atom hopping in the carbon chain of C6N radical: A Plausible 3c−4e crossover Long‐Bond

Linear isomers of C₆N radical differ in the position of the nitrogen atom in the carbon chain of C₆N. Reaction routes, involving intramolecular nitrogen atom insertion at varying position in the carbon chain of C₆N, are analyzed for the isomerisation between linear isomers of C₆N. Through an automated and systematic search performed with global reaction route mapping of the potential energy surface, thermal isomerisation pathways for C₆N radical are proposed based on the computations carried out at CASSCF/aug‐cc‐pVTZ, and CCSD(T)/6‐311++G(d,p)//B3LYP/6‐311++G(d,p) levels of the theory. Notably, a high lying linear isomer, centrosymmetric with respect to the nitrogen atom, is observed to be stabilized by a unique crossover three center‐four electron π long bond between the carbon atoms that are spatially separated by a nitrogen atom in a natural bond orbital. This long bond is concluded to be responsible for the predicted thermal isomerisation to be more feasible than the dissociation during the isomerisation pathway of a linear isomer of C₆N. © 2014 Wiley Periodicals, Inc.