Low-energy (0-10 eV) electron transmission spectra of multilayer tryptophan films

Electron Spin-Polarization Dependent Damage to Chiral Amino Acid l-Histidine

The damage of approximately monolayer films of l-histidine by low-energy spin-polarized electrons (SPE) ejected from a magnetized cobalt substrate has been probed using X-ray photoelectron spectroscopy (XPS). Average damage cross sections for N-containing motifs of l-histidine are 25-30 and 2-5 Mb for zwitterions and neutral molecules, respectively. The magnetization direction of the substrate, which controls the ejected SPE helicity, was reversed in situ, and statistically significant differences in the damage cross sections of 10-30% were measured between positive and negative electron helicities. This is the first measurement of spin dichroism (SD) in an amino acid. The differential cross sections suggest that inelastic scattering of SPE with chiral molecules could contribute to the persistence of one enantiomer vs the other under certain irradiation conditions, particularly for the zwitterionic species.

Low energy secondary electron induced damage of condensed nucleotides

Radiation damage and stimulated desorption of nucleotides 2'-deoxyadenosine 5'-monophosphate (dAMP), adenosine 5'-monophosphate (rAMP), 2'-deoxycytidine 5'-monophosphate (dCMP), and cytidine 5'-monophosphate (rCMP) deposited on Au have been measured using x-rays as both the probe and source of low energy secondary electrons. The fluence dependent behavior of the O-1s, C-1s, and N-1s photoelectron transitions was analyzed to obtain phosphate, sugar, and nucleobase damage cross sections. Although x-ray induced reactions in nucleotides involve both direct ionization and excitation, the observed bonding changes were likely dominated by the inelastic energy-loss channels associated with secondary electron capture and transient negative ion decay. Growth of the integrated peak area for the O-1s component at 531.3 eV, corresponding to cleavage of the C-O-P phosphodiester bond, yielded effective damage cross sections of about 23 Mb and 32 Mb (1 Mb = 10^-18 cm²) for AMP and CMP molecules, respectively. The cross sections for sugar damage, as determined from the decay of the C-1s component at 286.4 eV and the glycosidic carbon at 289.0 eV, were slightly lower (about 20 Mb) and statistically similar for the r- and d- forms of the nucleotides. The C-1s component at 287.6 eV, corresponding to carbons in the nucleobase ring, showed a small initial increase and then decayed slowly, yielding a low damage cross section (∼5 Mb). Although there is no statistical difference between the sugar forms, changing the nucleobase from adenine to cytidine has a slight effect on the damage cross section, possibly due to differing electron capture and transfer probabilities.

Low-energy electron-induced damage in a trinucleotide containing 5-bromouracil

The reaction of low-energy electrons (LEEs; 10 eV) with 5'-TpXpT-3' (TXT), where X is uracil (U), thymine (T), and 5-bromouracil (5BrU), was examined by HPLC-UV analysis. The presence of 5BrU increased total damage by >50%. The radiation products of T5BrUT included TUT (40%), free U, T, 5BrU (23%), and fragments (13%). These products may be explained by initial capture of LEEs by the nucleobase to form a transient anion, followed by transfer of the electron within the molecule and cleavage of susceptible bonds by dissociative electron attachment (C-Br, C-N, or C-O bonds). In addition, these products may arise from the uracilyl-5-yl (U-5-yl) radicals that undergo H-atom abstraction from the sugar moiety. Interestingly, several products contained two sites of cleavage (U, pUT, and TUp). The formation of these products was linear with dose, and thus, they arise from the single-electron reactions. To explain these products, we propose that the reaction of LEEs (10 eV) involves the coupling of two dissociative processes in the same molecule (for example, dissociative excitation and dissociative electron attachment). The latter reactions may contribute to the formation of clustered damage, which is the most deleterious damage induced by ionizing radiation.

Low-energy electron-induced DNA damage: effect of base sequence in oligonucleotide trimers

DNA damage induced by low-energy electrons (LEEs) has attracted considerable attention in recent years because LEEs represent a large percentage of the total energy deposited by ionizing radiation and because LEEs have been shown to damage DNA components. In this article, we have studied the effect of base sequences in a series of oligonucleotide trimers by the analysis of damage remaining within the nonvolatile condensed phase after LEE irradiation. The model compounds include TXT, where X represents one of the four normal bases of DNA (thymine (T), cytosine (C), adenine (A), and guanine (G)). Using HPLC-UV analysis, several known fragments were quantified from the release of nonmodified nucleobases (T and X) as well as from phosphodiester C-O bond cleavage (pT, pXT, Tp, and TXp). The total damage was estimated by the disappearance of the parent peaks in the chromatogram of nonirradiated and irradiated samples. When trimers were irradiated with LEE (10 eV), the total damage decreased 2-fold in the following order: TTT > TCT > TAT > TGT. The release of nonmodified nuclobases (giving from 17 to 24% of the total products) mainly occurred from the terminal sites of trimers (i.e., T) whereas the release of central nucleobases was minor (C) or not at all detected (A and G). In comparison, the formation of products arising from phosphodiester bond cleavage accounted for 9 to 20% of the total damage and it partitioned to the four possible sites of cleavage present in trimers. This study indicates that the initial LEE capture and subsequent bond breaking within the intermediate anion depend on the sequence and electron affinity of the bases, with the most damage attributed to the most electronegative base, T.

Phosphodiester and N-glycosidic bond cleavage in DNA induced by 4-15 eV electrons

Thin molecular films of the short single strand of DNA, GCAT, were bombarded under vacuum by electrons with energies between 4 and 15 eV. Ex vacuo analysis by high-pressure liquid chromatography of the samples exposed to the electron beam revealed the formation of a multitude of products. Among these, 12 fragments of GCAT were identified by comparison with reference compounds and their yields were measured as a function of electron energy. For all energies, scission of the backbone gave nonmodified fragments containing a terminal phosphate, with negligible amounts of fragments without the phosphate group. This indicates that phosphodiester bond cleavage by 4-15 eV electrons involves cleavage of the C-O bond rather than the P-O bond. The yield functions exhibit maxima at 6 and 10-12 eV, which are interpreted as due to the formation of transient anions leading to fragmentation. Below 15 eV, these resonances dominate bond dissociation processes. All four nonmodified bases are released from the tetramer, by cleavage of the N-glycosidic bond, which occurs principally via the formation of core-excited resonances located around 6 and 10 eV. The formation of the other nonmodified products leading to cleavage of the phosphodiester bond is suggested to occur principally via two different mechanisms: (1) the formation of a core-excited resonance on the phosphate unit followed by dissociation of the transient anion and (2) dissociation of the CO bond of the phosphate group formed by resonance electron transfer from the bases. In each case, phosphodiester bond cleavage leads chiefly to the formation of stable phosphate anions and sugar radicals with minimal amounts of alkoxyl anions and phosphoryl radicals.

Degradation of N-Acetyl Tryptophan by Low-Energy (<12 eV) Electrons

Secondary low-energy electrons are abundantly created during the early moments following the deposition of energy by radiation into cells. Here we show the ability of slow (<12 eV) electrons to effectively decompose gas-phase N-acetyl tryptophan (NAT) which can model a simple protein. The fragmentation of NAT, initiated via a resonant electron-molecule interaction exclusively at the peptide bridge, produces a large variety of negative species. The present findings contribute to the molecular description of the initial step in the radiation-induced damage.

Alteration of protein constituents induced by low-energy (<35 eV) electrons: II. Dissociative electron attachment to amino acids containing cyclic groups

We report measurements of the desorption of anions from thin condensed films of tryptophan (Trp), histidine (His) and proline (Pro) stimulated by 5-35 eV electron impact. H-, O-, OH- and CN- desorb from Trp, His and Pro, whereas CH2- is observed only from Pro fragmentation. Below 12 eV, the anion yield functions exhibit resonant structures indicative of dissociative electron attachment. For all three amino acids, this process is likely to be initiated by the resonant capture of the incident electron at the NH3(+)-CH-.....-COO- and/or NH2-CH-.....-COOH group of the molecule. Temporary electron attachment to the ring leads to anion desorption only for tryptophan and proline. The energy-averaged yields measured at the detector of the mass spectrometer are (4.9, 0.3 and 54.0) x 10(-8) H-/incident electron and (3.4, 2.9, 1.8) x 10(-11) O-/incident electron, respectively, from Trp, His and Pro dissociation. Fragmentation of amino acids is found to be as intense as that of the nucleic acid bases. These results are discussed within the context of radiobiological damage induced by secondary electrons.

Sequence-specific damage induced by the impact of 3-30 eV electrons on oligonucleotides

The ability of low-energy electrons to induce single- and double-strand breaks in DNA has recently been demonstrated. Here we show the propensity of 3-30 eV electrons to initiate base sequence-dependent damage to a short single DNA strand. Solid monolayer films of homogeneous thymidine (T(9)) and deoxycytidine (dCy(9)) and heterogeneous oligomers (T(6)dCy(3)) are bombarded with 1-30 eV electrons in an ultrahigh-vacuum system. CN, OCN and/or H(2)NCN are detected by a mass spectrometer as the most intense neutral fragments desorbing in vacuum. A weaker signal of CH(3)CCO is also detected, but only from oligonucleotides containing thymine. Below 17 eV, the energy dependence of the yields of CN, OCN and CH(3)CCO exhibits resonance-like structures, attributed to dissociative electron attachment (DEA). Above 17 eV, the monotonic increase in the fragment yields indicates that nonresonant processes (i.e. dipolar dissociation) control the fragmentation of these molecules. Within the energy range investigated, comparison of the magnitude of the total fragment yields produced by electron attack on dCy(9), T(6)-dCy(3) and T(9) suggests the following order in the sensitivity of single-strand DNA: dCy(9) > T(6)-dCy(3) > T(9). At 12 eV, the total fragment yields are found to be 5.8, 5.0 and 3.9 x 10(-3) fragment/electron, respectively. From the yields obtained with the two homo-oligonucleotides, we differentiate between contributions arising from the chemical nature of the base and the effect of environment (i.e. the sequence) when a thymidine unit in T(9) is replaced by dCy. The base sequence-dependent damage is found to vary with incident electron energy. These results reinforce the idea that genomic sensitivity to ionizing radiation depends on local genetic information. Furthermore, they underscore the possible role of low-energy electrons in the pathways responsible for the induction of specific genomic lesions.

Damage induced by 1-30 eV electrons on thymine- and bromouracil-substituted oligonucleotides

The impact of low-energy (1-30 eV) electrons on self-assembled monolayers of heterogeneous oligonucleotides chemisorbed on a gold surface has been investigated by mass spectrometry of desorbed neutral species in an attempt to understand the consequences of secondary electron damage in a short sequence of a DNA single strand. We demonstrate that the most intense observable neutral species (CN, OCN and/or H(2)NCN) desorbed from Cy(6)-Th(3) and Cy(6)-(BrdU)(3) oligos are related to primary fragmentation of the bases induced by electron impact. The dependence of the neutral species desorption on electron energy shows typical signatures of dissociative electron attachment initiated by the formation of shape- and core-excited resonances (i.e. single-electron and two-electron- one-hole transitory anions, respectively). Substitution of dTh by BrdU increases the production of neutral fragments by as much as a factor of about 3 for the entire electron energy range. When the distribution of secondary electrons along radiation tracks in H(2)O is taken into account, we show that the probability for electron damage to heterogeneous oligonucleotides is enhanced by a factor of 2.5-3 for electron energies below 20 eV for both sensitized and unsensitized strands.

Low-energy electron microscopy (LEEM) and mirror electron microscopy (MEM) of biological specimens: preliminary results with a novel beam separating system

Low-energy electron microscopy (LEEM) and mirror electron microscopy (MEM) utilize a parallel beam of slow-moving electrons backscattered from the specimen surface to form an image. If the electrons strike the surface an LEEM image is produced and if they are turned back just before reaching the surface an MEM image results. The applications thus far have been in surface physics. In the present study, applications of LEEM and MEM in the biological sciences are discussed. The preliminary results demonstrate the feasibility of forming images of uncoated cultured cells and cellular components using electrons in the threshold region (i.e. 0-10 V). The results also constitute a successful test of a novel beam-separating system for LEEM and MEM.