Structures of α-type ions formed in the 157 nm photodissociation of singly-charged peptide ions

One hundred fifty-seven nm photodissociation of singly-charged peptide ions induces the cleavage of alpha-carbon to carbonyl-carbon bonds along the backbone. a(n) + 1 radical ions are observed as the primary photolysis products of peptides with N-terminal arginines in a linear ion trap mass spectrometer. The radical elimination pathways undertaken by the a(n) + 1 radical ions to form more stable even-electron species are studied in hydrogen-deuterium (H/D) exchange experiments. Two types of a(n) ions along with d-type ions are observed as secondary elimination products. The relative abundance of each depends on the C-terminal residue of the radical fragment ion.     

How do P-type ATPases transport ions?

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins, the energy-providing ATP hydrolysis is coupled to ion transport of one or two ion species across the respective membrane. The pump function of the investigated pumps is described by a so-called Post-Albers cycle. Main features of the pumping process are (1) a Ping-Pong mechanism, i.e. both transported ion species are transferred successively and in opposite direction across the membrane, (2) the transport process for each ion species consists of a sequence of reaction steps, which are ion binding, ion occlusion, conformational transition of the protein, successive deocclusion of the ions and release to the other side of the membrane. (3) Recent experimental evidence shows that the ion-binding sites are placed in the transmembrane section of the proteins and that ion movements occur preferentially during the ion binding and release processes. The main features of the mechanism include narrow access channels from both sides, one gate per access channel, and an ion-binding moiety that is adapted specifically to the ions that are transported, and differently in both principal conformations.     

“Binding” of water by ions

Summary We confirmed the inadequacy of the correction for, binding of water by ions for explaining the discrepancy between the electrostatic theory in [2] and experiment.     

Theoretical investigations of β-trimethylsilyl-substituted iminium ions

Calculated reaction energies, NBO analyses, and natural charge distributions show that a trimethylsilyl group at the β carbon atom stabilizes the positive charge of an iminium cation more effectively than a tert-butyl group in the same position. Different from the classical β-silicon effect, where theoretical studies suggest that the positive charge is stabilized hyperconjugatively by interaction of the Si-C σ-bond with a formally empty 2p-orbital of suitable symmetry at the carbonium carbon, the stabilization in the case of iminium ions occurs by interaction of the Si-C σ-bond with the π^∗ orbital of the CN double bond. Stabilization of the silyl-substituted derivatives by hyperconjugation might be enhanced by interaction of the positive charge with a neighboring induced dipole.     

Tetracoordinate carbon as a nucleophile? Interconversion of carbenium ions with carbonium ions possessing nearly square-pyramidal pentacoordinate carbons

[structure: see text] Quantum chemical calculations (B3LYP and Moeller-Plesset second-order perturbation theory) on carbonium ions containing hypercoordinated carbon atoms in distorted square-pyramidal geometries are described. The importance of overall charge (in addition to the delocalization of a certain number of electrons) for the stability of such structures is explored through calculations on isoelectronic systems containing boron atoms. The effects of alkyl substitution as well as variations in the structure of the linker attached to the C(5) core substructure are explored systematically for a variety of structures that do not have severe geometric constraints that rigidly enforce unusual geometries. In addition, transition structures for reactions involving the intramolecular attack of tetracoordinate carbons on carbenium ion centers were located; in some cases, such carbenium-to-carbonium rearrangement processes (in which tetracoordinate carbon centers act as nucleophiles) are actually both exothermic and accompanied by low activation barriers.     

Novel ions of quaternary ammonium halides in FDMS

Field desorption mass spectrometry exhibited novel ions [ Y-n H+n R ]+and [ Y-nH+nX]+as well as cluster ions [nY+(n-1)X-mH+mR]+and [nY+( n-1)X-mH+mX]+(n,m=0-3; X and Y represent the cation and anion moieties of quaternary ammonium halides),whose formation mechanism was discussed.     

Hyperaromatic stabilization of arenium ions: a remarkable cis stereoselectivity of nucleophilic trapping of β-hydroxyarenium ions by water

Cis- and trans-1,2-dihydrodiol isomers of benzene undergo acid-catalyzed dehydration to form phenol. In principle the isomeric substrates react through a common β-hydroxybenzenium (cyclohexadienyl) carbocation. Notwithstanding, the isomers show a large difference in reactivity, k(cis)/k(trans) = 4500. This difference is reduced to k(cis)/k(trans) = 440 and 50 for the 1,2-dihydrodiols of naphthalene and 9,10-dihydrodiols of phenanthrene, respectively, and to 6.9 for the dihydrodiols of the nonaromatic 7,8-double bond of acenaphthylene. Because the difference in stabilities of cis- and trans-dihydrodiols should be no more than 2-3-fold, these results imply a high cis stereoselectivity for nucleophilic trapping of a β-hydroxyarenium cation by water in the reverse of the carbocation-forming reaction. This is confirmed by studies of the 10-hydroxy-9-phenanthrenium ion generated from aqueous solvolyses of the trans-9,10-bromohydrin derivative of phenanthrene and the monotrichloroacetate ester of the phenanthrene cis-9,10-dihydrodiol. The cis stereoselectivity of forward and reverse reactions is explained by the formation (in the "forward" reaction) of different conformations of carbocation from cis- and trans-dihydrodiol reactants with respectively β-C-H and β-C-OH bonds in pseudoaxial positions with respect to the charge center of the carbocation optimal for hyperconjugation. Formation of different conformations is constrained by departure of the (protonated) OH leaving group from a pseudoaxial position. The difference in stability of the carbocations is suggested to stem (a) from the greater hyperconjugative ability of a C-H than a C-OH bond and (b) from enhanced conjugation arising from the stabilizing influence of an aromatic ring in the no-bond resonance structures representing the hyperconjugation (C(6)H(6)OH(+) ? C(6)H(5)OH H(+)). This is consistent with an earlier suggestion by Mulliken and a demonstration by Schleyer that the benzenium ion is subject to hyperconjugative aromatic stabilization. It is proposed that, in analogy with the terms homoconjugation and homoaromaticity, arenium ions should be considered as "hyperaromatic".     

Electron-capture—Induced dissociation of protoporphyrin IX ions

Electron-capture induced dissociation of protoporphyrin cations and anions has been studied. The cations captured two electrons in two successive collisions and were converted to the corresponding even-electron anions. About one fifth of the ions lost a hydrogen atom to become radical anions but otherwise very little fragmentation was observed. The anions captured an electron to become dianions. No hydrogen loss occurred, and the only fragmentation channel observed was loss of CO2H, to give a doubly charged carbanion. Our results indicate that protoporphyrin ions are very efficient in accommodating one or even two electrons in the lowest unoccupied molecular orbital of the porphyrin macrocycle, and that electron capture induces only limited dissociation.     

Why are a 3 ions rarely observed?

It has been determined experimentally that a(3) ions are generally not observed in the tandem mass spectroscopic (MS/MS) spectra of b(3) ions. This is in contrast to other b(n) ions, which often have the corresponding a(n) ion as the base peak in their MS/MS spectra. Although this might suggest a different structure for b(3) ions compared to that of other b(n) ions, theoretical calculations indicate the conventional oxazolone structure to be the lowest energy structure for the b(3) ion of AAAAR, as it is for other b(n) ions of this peptide. However, it has been determined theoretically that the a(3) ion is lower in energy than other a(n) ions, relative to the corresponding b ions. Furthermore, the a(3) --> b(2) transition structure (TS) is lower in energy than other a(n) --> b(n-1) TSs of AAAAR, compared with the corresponding b ions. Consequently, it is suggested that the b(3) ion does fragment to the a(3) ion, but that the a(3) ion then immediately fragments (to b(2) and a(3)) because of the excess internal energy arising from its relatively low energy and the facile a(3) --> b(2) reaction. That is why a(3) ions are not observed in the MS/MS spectra of b(3) ions.     

Interaction of sodium and potassium ions withκ-carrageenan

Summary NMR and potentiometric methods revealed at least two types of interactions of Na⁺ and K⁺ ions with -carrageenan, viz., Coulombic interaction with polysaccharide sulfate groups, and a coordination one, leading, in the case of the Na⁺ ions, to formation of nonstoichiometric complexes. The absence of any correlation between the coordination binding density of the cations and their promoting effect on gelation process was demonstrated.     

Chelation of somebisazophenyl-β-diketone derivatives with lanthanide metal ions

Summary The chelation behaviour ofbisazo--diketone compounds with La, Ce, Nd, Sm, Gd, Er, and Lu ions was investigated. Composition and stability constants of the chelates have been determined by conductometric and potentiometric techniques. Electronic absorption, IR, and¹H NMR spectra as well as molar conductance and elemental and thermal analyses were used to characterize the complexes.

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Komplexierung einiger Lanthaniden mitBisazophenyl--diketon-Derivaten

Zusammenfassung Das Komplexierungsverhalten einigerBisazophenyl--diketone mit La, Ce, Nd, Sm, Gd, Er und Lu wurde untersucht. Zusammensetzung und Stabilitätskonstanten der Chelate wurden mit konduktometrischen und potentiometrischen Methoden bestimmt. Zur Charakterisierung der Komplexe wurden UV/Vis-, IR- und¹H-NMR-Spektroskopie sowie molare Leitfähigkeit, Elementaranalyse und thermische Analyse herangezogen.

     

Why Are B ions stable species in peptide spectra?

Protonated amino acids and derivatives RCH(NH₂)C(+O)X · H⁺ (X = OH, NH₂, OCH₃) do not form stable acylium ions on loss of HX, but rather the acylium ion eliminates CO to form the immonium ion RCH = NH ₂ ⁺ . By contrast, protonated dipeptide derivatives H₂NCH(R)C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B₂ ions by elimination of HX. These B₂ ions fragment on the metastable ion time scale by elimination of CO with substantial kinetic energy release (T _1/2 = 0.3–0.5 eV). Similarly, protonated N-acetyl amino acid derivatives CH₃C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B ions by loss of HX. These B ions also fragment unimolecularly by loss of CO with T _1/2 values of ～ 0.5 eV. These large kinetic energy releases indicate that a stable configuration of the B ions fragments by way of activation to a reacting configuration that is higher in energy than the products, and some of the fragmentation exothermicity of the final step is partitioned into kinetic energy of the separating fragments. We conclude that the stable configuration is a protonated oxazolone, which is formed by interaction of the developing charge (as HX is lost) with the N-terminus carbonyl group and that the reacting configuration is the acyclic acylium ion. This conclusion is supported by the similar fragmentation behavior of protonated 2-phenyl-5-oxazolone and the B ion derived by loss of H-Gly-OH from protonated C₆H₅C(+O)-Gly-Gly-OH. In addition, ab initio calculations on the simplest B ion, nominally HC(+O)NHCH₂CO⁺, show that the lowest energy structure is the protonated oxazolone. The acyclic acylium isomer is 1.49 eV higher in energy than the protonated oxazolone and 0.88 eV higher in energy than the fragmentation products, HC(+O)N⁺H = CH₂ + CO, which is consistent with the kinetic energy releases measured.     

Crystalline alignment of metal ions templated by β-chitin ester

The highly crystalline β-chitin from diatom Thalassiosira weissflogii was esterified via intercalation with succinic anhydride followed by simple heating, maintaining the original crystalline order. Due to the introduced free carboxyl groups, the chitin ester crystal showed ion exchange ability for metal cations in aq. solution. Heavy metal cations such as Pb²⁺ bound to the β-chitin succinate gave characteristic X-ray diffraction patterns, indicating regular alignment of metal ions. Such materials represent a new type of organometallic architecture, possibly leading to novel functionalities.     

Generation of multiple charged ions: Photoemission electron impact ionization

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Competing binding of metal ions with protein studied by microdialysis

A method has been established to study the competing binding of metal ions with protein by a combined technique of microdialysis with high performance liquid chromatography (HPLC). Ni2+, Cd2+, Zn2+, Cu2+ and human serum albumin (HSA) were chosen as model metal ions and protein. The experimental results show that Ni2+ and Cu2+ share a common primary binding site on HSA, and Zn2+ and Cd2+ share a different common primary binding site from them, but there is a common multi-metal binding site for all of those four metal ions. This method show advantages of fast sampling, easily to be operated and especially to be useful when ideal spectroscopic probes are not available for the study of interaction between protein and metal ions.     

Tautomerism of uracil and 5-bromouracil in a microcosmic environment with water and metal ions. What roles do metal ions play?

The base tautomerization processes of uracil/5-bromouracil were investigated in a microcosmic environment with both H2O and Na+ (W-M environment). It was found that uracil was more stable in the W-M environment than in the microcosmic environment with only water, which suggested that the metal ions and water work cooperated to maintain the classical nucleic acid bases. However, 5-bromouracil, a chemical mutagen, was found to be less stable than uracil in the W-M environment. Why the 5-bromouracil is easier to tautomerize and therefore induce gene mutation was explained to some extent. Further research revealed that the water molecule would assist the tautomerization in the W-M environment. However, the metal ions in different regions play absolutely opposite roles: in one region, the metal ions can prevent the base from tautomerizing, whereas in another region, the metal ion can assist the tautomerization process. Furthermore, from the viewpoint of ionization of the base, it seems BrU has a stronger tendency to lose the proton at N3, which is an intrinsic consequence of the bromine atom and is not affected by the metal cation.     

Polarographic immunoassay coupled with catalytic amplification of labeled copper ions

A new approach of polarographlc immunoassay based on the catalytic amplification of the labeled metal ions and the polarographlc detection of the catalytic product was developed.In this approach,the copper ions used as the catalyst for substrate conversion instead of natural enzyme were labeled to model antigen diphtheria toxoid (DT) through the bifunctional chelating reagent diethylenetriamine pentaacetic acid (DTPA).After heterogeneous competitive imrnunoreaction,the oxidation of substrate o-phenylenediatnine (OPD) was catalyzed by the labeled copper ions to generate an electroactive product 2,3-diammophenazine (DAP); subsequently,the product DAP was detected with linear-sweep polarography The proposed assay can determine the concentration in the range of 10-1 000 ng/mL of DT,two orders of magnitude more sensitive than those based on the direct detection of the metal ion labels The proposed immunoassay can be applied to detecting various proteins of interest.     

Microfluidic chip for electrochemically-modulated liquid∣liquid extraction of ions

A microfluidic device with integrated electrodes for the electrochemically-modulated extraction of ions across immiscible aqueous–organic liquid–liquid interfaces is presented. Using a Y-shaped microfluidic channel with in situ electrodes and co-flowing aqueous and organic immiscible electrolyte solutions, the manipulation of the applied interfacial potential enabled the extraction of ions from the aqueous phase into the organic phase. Data for the extraction of tetraethylammonium cations from aqueous electrolyte into 1,2-dichloroethane electrolyte are presented. The device demonstrates the benefits of combination of microfluidics and liquid–liquid electrochemistry.     

Développement par déplacement sur échangeurs d'ions : Cas de la séparation de deux ions

In the case of ion exchange Chromatography it is possible to forecast, from the equilibrium constants, the phenomena that occur in frontal analysis of two ions.With the results obtained the separation of two compounds by displacement development can be predicted. In particular it is possible to calculate the minimum distance that a band must travel to obtain a separation of the solutes in zones.The method has been applied to the separation of sodium and potassium ions with the resin Dowex-50, using calcium ions as the developer.