No abstract

Using a crossed electron/molecule beam technique the dissociative electron attachment (DEA) to gas phase L-valine, (CH(3))(2)CHCH(NH(2))COOH, is studied by means of mass spectrometric detection of the product anions. Additionally, ab initio calculations of the structures and energies of the anions and neutral fragments have been carried out at G2MP2 and B3LYP levels. Valine and the previously studied aliphatic amino acids glycine and alanine exhibit several common features due to the fact that at low electron energies the formation of the precursor ion can be characterized by occupation of the pi* orbital of the carboxyl group. The dominant negative ion (M-H)(-) (m/Z=116) is observed at electron energies of 1.12 eV. This ion is the dominant reaction product at electron energies below 5 eV. Additional fragment ions with m/Z=100, 72, 56, 45, 26, and 17 are observed both through the low lying pi* and through higher lying resonances at about 5.5 and 8.0-9.0 eV, which are characterized as core excited resonances. According to the threshold energies calculated here, rearrangements play a significant role in the formation of DEA fragments observed from valine at subexcitation energies.

Fragmentation of protonated peptides with a single N‐methylated residue was studied by low‐energy collision‐induced dissociation (CID) and the effects of the N‐alkylation on the fragmentation were evaluated. Peptides with an N‐terminal N‐alkylated amino acid behave similarly to regular peptides except for an increased stability of the protonated molecular ion due to the increased proton affinity. On the other hand, the N‐alkylation of an internal amino acid residue has very distinct effect on fragmentation. It causes activation of the amide bond on the C‐terminal side of the N‐alkylated residue resulting in preferential cleavage of this bond and formation of corresponding b ions. This is in a sharp contrast to the effect of a natural N‐alkylated amino acid, proline, which activates its N‐terminal amide bond providing abundant y ions upon CID. Both peptides with Ac‐N‐methylamino acid N‐terminus and with an internal N‐methylated amino acid exhibit the former type of fragmentation. A fragmentation mechanism is proposed that explains the observed effects based on fragmentation of model peptide analogs and isotopically labeled peptides. It was found that an interaction between carbonyl groups of neighboring amide moieties is an important factor in the formation of the b ion with a structure of a protonated N‐methyloxazolone. AM1 calculations suggest that the stability of this ion favors its formation in the CID of N‐methylated peptides. It was also shown that an incorporation of a basic site outside the peptide backbone does not affect the course of fragmentation of the peptides with an internal N‐alkylamino acid. These observations provide support for the ‘mobile proton’ model for the initial stage of protonated peptide fragmentation in the low‐energy CID. The observed phenomenon is analogous to the solution‐phase acid‐catalyzed hydrolysis of these peptides. © 1998 John Wiley & Sons, Ltd.

No abstract

We report the generation of deoxyriboadenosine dinucleotide cation radicals by gas-phase electron transfer to dinucleotide dications and their noncovalent complexes with crown ether ligands. Stable dinucleotide cation radicals of a novel hydrogen-rich type were generated and characterized by tandem mass spectrometry and UV-vis photodissociation (UVPD) action spectroscopy. Electron structure theory analysis indicated that upon electron attachment the dinucleotide dications underwent a conformational collapse followed by intramolecular proton migrations between the nucleobases to give species whose calculated UV-vis absorption spectra matched the UVPD action spectra. Hydrogen-rich cation radicals generated from chimeric riboadenosine 5'-diesters gave UVPD action spectra that pointed to novel zwitterionic structures consisting of aromatic π-electron anion radicals intercalated between stacked positively charged adenine rings. Analogies with DNA ionization are discussed.

No abstract

No abstract

Abstract. The role that climate and environmental history may have played in influencing human evolution has been the focus of considerable interest and controversy among paleoanthropologists for decades. Prior attempts to understand the environmental history side of this equation have centered around the study of outcrop sediments and fossils adjacent to where fossil hominins (ancestors or close relatives of modern humans) are found, or from the study of deep sea drill cores. However, outcrop sediments are often highly weathered and thus are unsuitable for some types of paleoclimatic records, and deep sea core records come from long distances away from the actual fossil and stone tool remains. The Hominin Sites and Paleolakes Drilling Project (HSPDP) was developed to address these issues. The project has focused its efforts on the eastern African Rift Valley, where much of the evidence for early hominins has been recovered. We have collected about 2 km of sediment drill core from six basins in Kenya and Ethiopia, in lake deposits immediately adjacent to important fossil hominin and archaeological sites. Collectively these cores cover in time many of the key transitions and critical intervals in human evolutionary history over the last 4 Ma, such as the earliest stone tools, the origin of our own genus Homo, and the earliest anatomically modern Homo sapiens. Here we document the initial field, physical property, and core description results of the 2012-2014 HSPDP coring campaign.