On the survival of peptide cations after electron capture: Role of internal hydrogen bonding and microsolvation

Electron capture by both bare and microsolvated small peptide dications was investigated by colliding these ions with sodium vapor in an accelerator mass spectrometer to provide insight into processes that occur on the microsecond time frame. Survival of the intact peptide monocation after electron capture depends strongly on molecular size. For dipeptides, no intact reduced species were observed; the predominant ions correspond to loss of hydrogen and ammonia. In contrast, the intact reduced species was observed for larger peptides. Calculated structures indicate that the diprotonated dipeptide ions form largely extended structures with low probability of internal ionic hydrogen bonding (i.e., charge solvation) whereas internal ionic H-bonding occurs extensively for larger peptide dications. Solvation of the peptide ions with between one to seven methanol molecules reduces the total extent of H loss even for dipeptides where intact reduced species can survive more than a microsecond after electron capture. The yield of ions corresponding to cleavage of NCalpha bonds (c+ and z+* ions) does not depend strongly on peptide size but decreases with the extent of microsolvation for the dipeptide dications. H-bonding appears to play an important role for the survival of the intact reduced ions but less so for the formation of c+ and z+* ions. Our results indicate that electron capture predominantly occurs at the ammonium groups (at least 70 to 80%), and provides important new insights into the electron capture dissociation process.     

A Mössbauer spectroscopy study of chemically prepared ultrafine particles of amorphous (Fe1-xCox)60B40

Ultrafine amorphous alloy particles of (Fe_1-x Co_x)₆₀B₄₀ with x=0.1, 0.3, 0.5, 0.7, and 0.9 have been prepared by reduction of the metal ions using KBH₄ in aqueous solution. electron microscopy shows that the particle size is of the order of 20 nm. Mössbauer spectroscopy has been used to elucidate the magnetic properties of the particles.     

The ferric/ferrous ratio in basalt melts at different oxygen pressures

Samples of two types of natural basalt were equilibrated over a range of oxygen pressures at four different temperatures, and then quenched to room temperature. Except at the lowest temperature, where magnetic crystals formed under the most oxidizing conditions, totally amorphous samples were obtained. The Mössbauer spectra of 45 samples of the quenched basaltic melts were measured at room temperature. The relationship obtained between the oxidation state and oxygen fugacity differs to some extent from those relations previously described in the literature, in not yielding a linear relationship between log(farric/ferrous) and log(fo₂). This might indicate a more involved redox process than that described by a simple reaction between oxides and/or the influence of the cation composition in the process of glass formation. An investigation was made of the kinetics of the redox process. For the experimental setup used, redox equilibrium would be reached within three hours. Finally, three naturally quenched basalt glasses were analyzed for comparison; two showed lower oxidation states than previously found in Icelandic rocks.     

Degradation of blood in the human digestive tract

Samples of human faeces from patients suffering from intestinal bleeding have been studied by use of Mössbauer spectroscopy. It is shown that it is possible to follow the degradation of blood in the digestive tract.     

A study of the sump beaver's dental enamel

The nature of the iron-containing phase in the dental enamel of sump beaver incisors has been studied by use of Mössbauer spectroscopy. All iron atoms are found to be trivalent and to be present in particles exhibiting superparamagnetic relaxation. The iron-containing phase has Mössbauer parameters similar to those of ferritin.     

Immobilization of aryl and alkynyl groups onto glassy carbon surfaces by electrochemical reduction of iodonium salts

A new versatile method has been developed for the electrochemically assisted grafting of carbon materials. The approach is based on the reduction of iodonium salts and allows the immobilization not only of aryl groups, such as phenyl or nitrophenyl, but also of alkynyl groups under mild conditions. In particular, the immobilization of alkynyl groups is important because such grafting cannot be accomplished using any other known reductive procedure. The electrochemical properties of the grafted surfaces with estimated coverages of (4-6) x 10(-)(10) mol cm(-)(2) are investigated against the ferrocene and Fe(CN)(6)(3)(-) solution probes. The analysis of the surfaces is carried out by means of cyclic voltammetry and X-ray photoelectron spectroscopy.     

Site of gas-phase methylation of l-phenyl-2-aminopropane

The regioselectivity of methyl cation transfer from (CH₃)₂F⁺, (CH₃)₂Cl⁺ and (CH₃)₃O⁺ to 1-phenyl-2-aminopropane was studied by Fourier transform ion cyclotron resonance in combination with collision-induced dissociation and neutralization-reionization mass spectrometry of the stable [M + CH₃]⁺ ions formed in a chemical ionization source. The (CH₃)₂F⁺ ion transfers a methyl cation to the NH₂ group and the phenyl ring with almost equal probability. Predominant CH₃⁺ transfer to the NH₂ group is observed for the (CH₃)₂Cl⁺ ion whereas the (CH₃)₃O⁺ ion reacts almost exclusively at the amino group. The preference for methylation at NH₂ is discussed in terms of a lower methyl cation affinity of the phenyl ring than of the amino group and the existence of an energy barrier for methylation of the phenyl moiety.     

A chemical ionization study of deuteron transfer initiated propene loss from propoxypyridines

The mechanism of propene loss from the metastable [M + D](+) ions of isomeric 2-, 3-, and 4-n-propoxypyridines and the related isopropoxypyridines has been examined by chemical ionization (CI) and tandem mass spectrometry in combination with deuterium labeling. The [M + D](+) ions were generated with CD(3)OD, CD(3)CN, (CD(3))(2)CO, or pyrrole-D(5) (listed in order of increasing proton affinity) as the CI reagent. The results reveal that the deuteron added in the CI process is not interchanged with the hydrogen atoms of the propyl group prior to propene loss from the metastable [M + D](+) ions of the propoxypyridines. The site selective labeling of the alpha-, beta-, or gamma-position of the propyl group indicates that the [M + D](+) ions of 2-n-propoxypyridine expel propene with formation of an ion-neutral complex composed of a propyl carbenium ion and 2-pyridone. By contrast, the [M + D](+) ions of 3-n-propoxypyridine expel propene by: (1) Formation of ion-neutral complexes, and (2) a conventional 1,5-hydride shift from the beta-position of the n-propyl group to the ring and/or a 1,2-elimination type process. For the 4-isomer, the results suggest the occurrence of propene loss by a 1,2-elimination in addition to the intermediate formation of ion-neutral complexes. Loss of propene with one deuterium atom is the only reaction of the [M + D](+) ions of the isopropoxypyridines labeled at the alpha-position of the isopropyl group. The results for the isopropoxypyridines labeled with three deuterium atoms at the beta-position are consistent with: (1) The loss of propene by ion-neutral complex formation and the occurrence of a substantial isotope effect in the subsequent proton/deuteron transfer within the complex, and/or (2) the loss of propene by a 1,2-elimination type reaction.