Where Does the Electron Go? Stable and Metastable Peptide Cation Radicals Formed by Electron Transfer

Electron transfer to doubly and triply charged heptapeptide ions containing polar residues Arg, Lys, and Asp in combination with nonpolar Gly, Ala, and Pro or Leu generates stable and metastable charge-reduced ions, (M + 2H)^+●, in addition to standard electron-transfer dissociation (ETD) fragment ions. The metastable (M + 2H)^+● ions spontaneously dissociate upon resonant ejection from the linear ion trap, giving irregularly shaped peaks with offset m/z values. The fractions of stable and metastable (M + 2H)^+● ions and their mass shifts depend on the presence of Pro-4 and Leu-4 residues in the peptides, with the Pro-4 sequences giving larger fractions of the stable ions while showing smaller mass shifts for the metastables. Conversion of the Asp and C-terminal carboxyl groups to methyl esters further lowers the charge-reduced ion stability. Collisional activation and photodissociation at 355 nm of mass-selected (M + 2H)^+● results in different dissociations that give sequence specific MS³ spectra. With a single exception of charge-reduced (LKGLADR + 2H)^+●, the MS³ spectra do not produce ETD sequence fragments of the c and z type. Hence, these (M + 2H)^+● ions are covalent radicals, not ion–molecule complexes, undergoing dramatically different dissociations in the ground and excited electronic states. The increased stability of the Pro-4 containing (M + 2H)^+● ions is attributed to radicals formed by opening of the Pro ring and undergoing further stabilization by hydrogen atom migrations. UV–VIS photodissociation action spectroscopy and time-dependent density functional theory calculations are used in a case in point study of the stable (LKGPADR + 2H)^+● ion produced by ETD. In contrast to singly-reduced peptide ions, doubly reduced (M + 3H)⁺ ions are stable only when formed from the Pro-4 precursors and show all characteristics of even electron ions regarding no photon absorption at 355 nm or ion-molecule reactions, and exhibiting proton driven collision induced dissociations.

Open image in new window

Graphical Abstract ?

     

The Effective Hamiltonian and Electron Correlation Energy

An effective,hermitian hamiltonian is derived in amodel space. Its perturbation expressions to third order approximation are given.The correlation energy is also given to the third order approximation.The effective hamiltonian deviates form the actual one by the presence of acorrelation operator.The cprrelation operatop is given in an explicit form.     

Electron ionization mass spectrometry: Quo vadis?

Mass spectrometry (MS) is a fundamental technique to identify compounds by their mass-to-charge ratio. It is known that MS can only detect target compounds when they are converted to ions in the gas phase. The ionization procedure is considered one of the most critical steps, and there are distinct techniques for it. One of them is electron ionization (EI), a widely used hard-ionization technique capable of generating several ions due to the excess energy employed. The existence of distinct ionization mechanisms turns EI capable of producing a fingerprint-like spectrum for each molecule. So, it is an essential technique for obtaining structural information. EI is often combined with chromatography to obtain a practical introduction of pretreated samples despite its excellent performance. EI–MS has been applied coupled with gas chromatography (GC) since the 1960s as both are very compatible. Currently, analytes of interest are more suitable for liquid chromatography (LC) analysis, so there are researchers dedicated to developing suitable interfaces for coupling LC and EI–MS. EI excels, as a reliable technique to fill the gap between GC and LC, possibly allowing them to coexist in a single instrument. In this work, the authors will present the fundamentals of EI–MS, emphasizing the development over the years, coupling with gas and LC, and future trends.     

Influence of the Lone Electron Pairs of the Ether Oxygen Atom on the Electron Density Distribution in α-Chloro Ethers

The axial and equatorial C-Cl bonds in cyclic -chloro ethers are polarized differently because of different orientation of the lone electron pair of oxygen relative to these bonds.     

Does Electron Capture Dissociation Cleave Protein Disulfide Bonds?

Peptide and protein characterization by mass spectrometry (MS) relies on their dissociation in the gas phase into specific fragments whose mass values can be aligned as ‘mass ladders’ to provide sequence information and to localize possible posttranslational modifications. The most common dissociation method involves slow heating of even-electron (M+n H)ⁿ⁺ ions from electrospray ionization by energetic collisions with inert gas, and cleavage of amide backbone bonds. More recently, dissociation methods based on electron capture or transfer were found to provide far more extensive sequence coverage through unselective cleavage of backbone N–Cα bonds. As another important feature of electron capture dissociation (ECD) and electron transfer dissociation (ETD), their unique unimolecular radical ion chemistry generally preserves labile posttranslational modifications such as glycosylation and phosphorylation. Moreover, it was postulated that disulfide bond cleavage is preferred over backbone cleavage, and that capture of a single electron can break both a backbone and a disulfide bond, or even two disulfide bonds between two peptide chains. However, the proposal of preferential disulfide bond cleavage in ECD or ETD has recently been debated. The experimental data presented here reveal that the mechanism of protein disulfide bond cleavage is much more intricate than previously anticipated.     

Electron Ionization Mass Spectrometry of Some Ferrocenyl Thia-crown Ethers

ＥｌｅｃｔｒｏｎＩｏｎｉｚａｔｉｏｎＭａｓＳｐｅｃｔｒｏｍｅｔｒｙｏｆＳｏｍｅＦｅｒｒｏｃｅｎｙｌＴｈｉａ┐ｃｒｏｗｎＥｔｈｅｒｓＳＵＪｉｎｇ－ｚｈｅａｎｄＪＵＹｉｎｇ（ＢｅｉｊｉｎｇＩｎｓｔｉｔｕｔｅｏｆＣｈｅｍｉｃａｌＲｅａｇｅｎｔｓ，Ｂｅｉｊｉ...     

Nucleic Acid–Promoted Electron Transfer to Cytochrome c

Gold electrodes were modified with short oligonucleotides in order to facilitate the electron transfer to the small redox protein cytochrome c. DNA immobilization was followed by impedance spectroscopy. The electron exchange was found to be quasi-reversible for both the protein in solution or adsorbed at the electrode surface. Variations of base sequence or structure (DNA-PNA hybrids) of the nucleic acid promoter layer did not significantly change the electron transfer rate constant which was in the range of 0.2–1×10⁻² cm/s. Cytochrome c adsorbed at low ionic strength was coupled to molecules in solution such as superoxide or laccase. Aspects of the DNA conductivity and possibilities for DNA detection are also discussed.     

Electron–phonon interactions in atomic and molecular devices

Electron–phonon interactions are extremely important for understanding charge transport, inelastic processes, heating, and heat dissipation in nanoscale molecular and atomic devices. In molecular electronics Inelastic Electron Tunneling Spectroscopy (IETS) has recently emerged as one of the premier methods for characterizing molecular-scale junctions and devices. This method provides a distinct chemical fingerprint for identifying molecules within a junction, and has allowed for clear demonstrations of single molecule devices, the effects of electric field on molecular orbitals, the importance of molecular configuration on conductance, as well as information about the charge transport mechanism. In this review we will discuss the use of Point Contact (PC) and IET spectroscopies on molecular and atomic systems, discuss the basic principles involved in inelastic transport for these spectroscopic methods to function, and focus on the experimental techniques involved and the important conclusions drawn from the experiments performed to date.     

Electron Beam Nanolithography of β-Ketoester Modified Aluminium Tri-Sec-Butoxide

Electron beam-sensitive spin-coatable Al₂O₃ resists were prepared by chemically modifying aluminium tri-sec-butoxide with various -ketoesters (R-acetoacetates, R = methyl, ethyl, isopropyl, isobutyl, isoamyl, heptyl, benzyl and 2-[methacryloyloxy] ethyl) in isopropyl alcohol. The reaction product was a chelated complex. With increasing molecular weight of R, there was an increase in electron beam sensitivity of spin-coatable Al₂O₃ resists. This appears to be due to decreased stability of the chelated complex formed by higher molecular weight R-acetoacetates. Fourier transform infrared (FTIR) spectroscopy studies indicate that exposure to an electron beam results in the breakdown of chelate rings, making the exposed area insoluble during development. Electron beam nanolithography produced 8 nm wide lines. These are the smallest oxide lines written using a sol-gel-based resist.     

Studies on Electron Impact Mass Spectra of Some Anhydro-sugars

ＳｔｕｄｉｅｓｏｎＥｌｅｃｔｒｏｎＩｍｐａｃｔＭａｓＳｐｅｃｔｒａｏｆＳｏｍｅＡｎｈｙｄｒｏ┐ｓｕｇａｒｓ＊ＹＵＪｉａｎ－ｘｉｎ＊＊，ＬＩＵＹｕ－ｔｉｎｇ（ＤｅｐａｒｔｍｅｎｔｏｆＣｈｅｍｉｓｔｒｙ，ＸｉｎｊｉａｎｇＵｎｉｖｅｒｓｉｔｙ，Ｕｌｕｍｕｑ...     

Photosynthesis of Tetrahydroprotoberberines with Electron withdrawing Groups on Ring D

Althoughmanymethodshavesofarbeendevelopedforthesynthesisoftetrahydroprotoberberinesl'2,mostofthemareonlysuitableforthepreparationofthecompoundswithhydroxyoralkoxylgroupsonthebenzeneringsandcannotbeusedtosynthesizecompoundswithelectron-withdrawinggroupsonthebenzenemoieties.Infact,thepresenceofanelectron-withdrawinggrouponthearomaticringspreventtheringBorCclosureundertheclassicalreactionconditions3.Lenzanhasreportedthesynthesisofseveraltetrahydroprotoberberineswithmethoxyandmethylenedioxygroupso…     

Photochromic Polyoxoniobates with Photoinduced “D-f-A” Electron Transfer Mechanism

Compounds with redox activities have appealing applications in catalytic, electronic and magnetic properties, but the redox inert of polyoxoniobates (PONbs) significantly limits their applications for a long time. In this work, we are able to integrate organophosphate and lanthanide cluster into PONb to create the first family of inorganic-organic hybrid organophosphate-Ln-PONb composite clusters. These novel species not only present the first family of redox active PONbs that can be reduced to form long-lived “heteropoly blues” under ambient conditions, but also a new photochromic system. More importantly, the analyses of the electronic configurations and photochromic properties for a series of designed proof-of-concept PONbs models allow us to discover a D-f-A electron transfer mechanism, that is, photoinduced electron is transferred from a photosensitive organophosphate electron donor (D) to the Nb^V electron acceptor (A) through the unoccupied 4 f-orbitals of Ln (f). This work paves the way for developing diverse PONb-based redox materials and expanding the possibility of the applications of PONbs in the redox chemistry.     

Electron beam writing of epoxy based sol–gel materials

We report the use of an epoxy based hybrid sol–gel material as negative resist for electron beam lithography (EBL). The matrix has been prepared starting from 3-glycidoxypropyltrimethoxysilane as specific organic–inorganic precursor and the synthesis has been strictly controlled in order to preserve the epoxy ring and to obtain a proper inorganic cross-linking degree. The film has been exposed to an electron beam, inducing the polymerization of the organic part and generating the film hardening. Preliminary results of a resolution test on the synthesized epoxy based sol–gel material, performed with electron beam lithography, are presented. Structures below 300 nm were achieved. The direct nanopatterning of this hybrid sol–gel system simplify the nanofabrication process and can be exploited in the realization of photonic devices. A demonstration has been carried out doping the hybrid films with commercial Rhodamine 6G and reproducing an already tested laser structure.     

Electron emission from photo-excited testosterone in water–ethanol solution

Testosterone (TES; 4-androstene-17β-ol-3-on) is found for the first time to eject electrons from its singlet excited state in water–ethanol solvent mixture. This ability was very recently also observed for 17β-estradiol (17βE2) and progesterone (PRG)/1/. With increasing TES-concentration, the yield of solvated electrons (${\text{e}}_{\text{s}}^{-}$) is decreasing, because of “associate” formation. At higher absorbed UV-doses (λ = 254 nm) the ${\text{e}}_{\text{s}}^{-}$ yield is passing a sharp maximum by formation of TES–ethanol adducts, which are able likewise to emit electrons when excited. At prolonged irradiation the resulting photolytic products of TES–ethanol adducts are also able to emit electrons.The capability of the hormones: 17βE2, PRG and TES to eject electrons and the resulting metabolites, some of which can induce cancer, is discussed.     

Electron Exchange Between Oxidized and Reduerd Forms of Ferrocene Derivatives

The rates of electron exchange between oxidized and reduced forms of sev-eral derivatives of ferrocene(1-3)have been measured in acetonitrile by the NMR linehoadening method at 25℃。It was found that the tes vary with the lengtn of thesubstitutent chain and the potential of E_(Fe(Ⅲ)/Fe(Ⅱ))in the ferrocene derivatives.     

Amperometric Glucose Sensor with Tetrathiafulvalene as an Electron Transfer Mediator

ＡｍｐｅｒｏｍｅｔｒｉｃＧｌｕｃｏｓｅＳｅｎｓｏｒｗｉｔｈＴｅｔｒａｔｈｉａｆｕｌｖａｌｅｎｅａｓａｎＥｌｅｃｔｒｏｎＴｒａｎｓｆｅｒＭｅｄｉａｔｏｒＳＵＮＣｈａｎｇ－ｑｉｎｇ，ＹＡＮＧＸｕ，ＬＵＡＮＣｈａｉ－ｘｉａ，ＧＡＯＱｉａｎａｎｄＸＵＨｏｎｇ...     

Room Temperature Phosphorescence pH Switch Based on Photo-induced Electron Transfer

In this paper, photoinduced electron transfer(PET)phosphoroionophore,N-(1-bromo-2-naphthylmethyl)-dicthanolamine (BND)was synthesized and its phosphorescent characteristics were studied,The experimental results showed that strong phosphorescence could be observed in β-cyclodextrin apueous solution only at low pH value ,This system combined AND and NOT function to produce a three-input inhibit (INH)logic gate .