The nucleophilic displacement reactions F− + CH3Y → CH3F + Y− (Y = Cl, Br, I) and Cl− + OCH3Br → CH3Cl + Br− are studied in a crossed beam experiment where binary clusters produced by supersonic beam expansion interact with a monochromatized electron beam. The nucleophile (F−, Cl−) is generated by resonant dissociative electron attachment (DA) from one component of the cluster and then undergoes an SN2 reaction with the second component (the “substrate” molecule CH3Y). Y− ions carrying the DA resonance profile of the nucleophile are then identified as products of the corresponding SN2 reaction. Although isolated CH3Cl has an unmeasurably low DA (Cl−) cross section (<10−23 cm2 [D.M. Pearl, P.D. Barrow, J. Chem. Phys. 104 (1994) 2940; D.M. Pearl et al., J. Chem. Phys. 102 (1995) 2737]), effective formation of Cl− from CH3Cl is observed when the molecule is coupled to C2F6 in a binary van der Waals cluster via the respective SN2 reactions. In addition, the product ion Y− solvated by CH3Y molecules and also ion-molecule complexes (FCH3Y)− involved in the reaction are observed within the DA resonance of the nucleophile. The relative intensity of the naked product ion with respect to the ion-molecule complexes is a mirror of the exothermicity of the corresponding SN2 reaction. Because in DA the nuclephile can be created with a defined amount of translational energy, the potential use of the present method to derive information on the energy dependence of the reaction efficiency is discussed.

Thyroid carcinoma incidence is increased significantly after ionizing irradiation; however, the possible mechanisms have not yet been identified. To provide clues for an understanding of the radiation-induced transformation of thyroid epithelium, we analyzed the karyotypes of 56 childhood thyroid tumors that appeared in Belarus after the Chernobyl nuclear accident in 1986. We also studied eight secondary thyroid tumors that developed after radiotherapy. Metaphase preparations obtained from primary cultures were analyzed by G-banding. Clonal structural aberrations were found in 13 of 56 Belarussian cases and in 6 of 8 secondary tumors that developed after radiotherapy. Furthermore, we detected multiple chromosomal aberrations as well as complex rearrangements in some of these tumors and performed a detailed analysis of marker chromosomes from a single case using spectral karyotyping and comparative genomic hybridization in a childhood tumor from Belarus with a near-triploid karyotype. Both comparative genomic hybridization and spectral karyotyping analysis revealed structural alterations affecting identical chromosomes 1, 2, 9, and 13, among others. In addition to the known hot spots of alterations in papillary thyroid carcinomas on chromosomes 1q and 10q, a comprehensive breakpoint analysis in the pooled data set revealed novel breakpoints on chromosomes 4q, 5q, 6p, 12q, 13q, and 14q. The chromosomal aberrations in these tumors may provide suitable starting points for the positional cloning of genes involved in radiation-induced tumorigenesis.

Fluorescence in situ hybridization (FISH) to sections of formalin‐fixed paraffin‐embedded archival tissues allows the detection of gene or chromosome copy number changes in interphase cell nuclei within the histological context and thus may be of particular interest in tumor pathology. In this report, we describe the application of FISH to thick (15 μn) paraffin sections of 7 primary cutaneous malignant melanomas. A chromosome 7‐specific centromeric DNA probe was used to detect numerical aberrations of chromosome 7. By optical sectioning using confocal laser scanning microscopy (CLSM) only complete, uncut interphase cell nuclei were scored. The mean percentage (±SEM) of melanoma cell nuclei with three hybridization spots was (20.7±2.8)%; (6.8±1.0)% of nuclei showed one spot and (5.0±1.2) % four or more spots. The frequency distribution of spot numbers among melanoma cell nuclei and normal keratinocyte nuclei was significantly different (χ2= 176.8, df= 5, p<0.001). Trisomy 7 was detected in all 7 cases analyzed, mostly associated with monosomy 7 or polysomy 7. The approach used in our study and the data obtained could be useful for further studies designed to investigate a possible involvement of chromosome 7 in melanocytic tumor progression.

Karyotype analysis of a primary culture from a case of papillary thyroid cancer (PTC) showed an abnormal short arm of one homologue of chromosome 2 as sole abnormality in 4 of 16 metaphases. Based on G-banding analysis, two different aberration types on chromosome 2 could be assumed representing either a del(2)(p22–23) or a pericentric inversion. Further comparative genomic hybridization (CGH) analysis as well as fluorescence in situ hybridization (FISH) analysis were performed to confirm the assumed alterations. While CGH analysis showed no loss of chromosome 2 material, FISH with yeast artificial chromosome (YAC) probes homologous to the region 2p22–23 demonstrated two pericentric inversions of chromosome 2 involving different breakpoints on 2p in 6.8% and 4.2% of the metaphases, respectively. Polymerase chain reaction (PCR) analysis with degenerated oligonucleotide primers that bind within the conserved catalytic domain of tyrosine kinase (tk) genes resulted in amplification products with DNA of YAC 851D11 suggesting the presence of such genes at or near the translocation breakpoint.

We report on the S N 2 reaction F − + CH 3 Br → FCH 3 + Br − in binary clusters composed of CH 3 Br and C 2 F 6 in a crossed electron/molecular beam experiment. The reaction is induced by resonance dissociative electron attachment to the C 2 F 6 component where the nucleophile F − is generated at an energy (around 3.8 eV) where CH 3 Br does not capture electrons to form Br − . The results indicate that the cross section for the S N 2 reaction is independent of the total excess energy which can be varied between 1.7 and 5 eV in the present system.

Free electron attachment in the energy range 0-10 eV and Rydberg electron transfer RET) to NF3 molecules, NF3 clusters, and NF3/Ar clusters are studied by means of negative ion mass spectrometry. The experimental results are compared with density functional theory (DFT) calculations. In the free electron experiment gasphase NF3 generates the dissociative attachment (DA) products F-, F-2(-), and NF2- appearing from a broad resonance around 2 eV, in agreement with an earlier beam experiment. The most abundant fragment, F-, exhibits a small "threshold" signal at incident energies close to 0 eV. This signal increases with the target gas temperature and is due to transitions from the neutral to the anion involving vibrationally excited states close to the intersection between the two potential energy surfaces. From the temperature dependence of this threshold signal the activation energy for the exothermic DA reaction e(-) + NF3 --> F- + NF2 is derived as E-a = 0.1 +/- 0.05 eV. Time-of-flight (TOF) analysis reveals that F- arises from a repulsive electronic state releasing the fragments with appreciable kinetic energy. Free electron attachment and RET to NF3 clusters produce undissociated cluster ions (NF3)(n-) including the monomer. In the case of free electrons they are still dominantly formed via the 2 eV resonance followed by intracluster relaxation processes. DFT calculations predict a lowering of the symmetry from C-3v to C-s on going from the neutral to the anion with one F atom substantially enlarged from the NF2 plane. The calculated (adiabatic) electron affinities are EA(NF3) = 1.71 eV and EA(NF2) = 1.18 eV. From the appearance energy in the DA experiment we derive EA(NF2) = 1.1 +/- 0.1 eV.

Mutations in the p53 tumour-suppressor gene (exons 5-8) were investigated in 31 Belarussian childhood thyroid tumours (24 cases of papillary thyroid carcinoma, 3 benign tumours and 2 cases each of thyroiditis and goiter); 33 thyroid tumours from juveniles and adults without radiation exposures (25 carcinomas of various histological types, including 11 papillary carcinomas and 8 adenomas) and 6 tumours from adults (4 papillary carcinomas, 1 adenoma, 1 goiter) served as controls. The mutational spectrum of p53 differed greatly between the childhood thyroid carcinomas from Belarus and the control groups. In the control groups of 29 malignant thyroid tumours, 7 different mutations were detected on exons 5-8, none of which occurred among the 15 papillary carcinomas in this group. Five mutations were found in tissue samples of the 24 childhood papillary carcinomas, and they were all the same p53 point mutation (CGA --> CGG) on codon 213 of exon 6. To determine whether this mutation is simply a polymorphism or whether it is specific to the tumour cells, laser-assisted microdissection was applied to collect various areas of tumorous and non-tumorous cells (10-20 cells per sample) from each paraffin-embedded tissue section of 8 of the papillary thyroid carcinomas. Using PCR-SSCP and sequence analysis on these cells, the very same p53 mutation on codon 213 was detected in various microdissected tumour samples of 2 cases, but it was not found in any microdissected non-tumorous sample. The exclusive occurrence of this p53 mutation in selective microdissected samples of tumour cells, even as homozygous mutation in 1 case, reflects a distinct tumour heterogeneity within papillary childhood thyroid carcinomas.

Chromosome painting of chromosomes 1, 4 and 12 was performed on metaphase preparations of cultured thyroid cells to analyse the frequency of radiation-induced stable chromosome translocations in papillary thyroid carcinomas from 40 Belarussian children exposed to radioiodine from the Chernobyl accident, and from 31 reference case. As expected, we found the highest translocation frequencies in secondary thyroid tumours after radiotherapy, but there were also high frequencies in tumour tissues as well as in non-tumourous tissues from childhood papillary carcinoma samples from Belarus. Among the Belarussian tumours the cases from the Gomel region exhibited the highest frequency of translocations and five cases lie within the range of frequencies observed in secondary thyroid tumours after radiotherapy. The findings support the assumption that radiation was the principal cause of the tumours in Belarus, but they indicate also that only a minority of the Belarus cases, which have developed papillary carcinomas, were exposed to very high doses of radioiodine.

The reactivity of the molecules C(6)F(5)X (X = Cl, Br, I) following low energy (0-15 eV) electron attachment is studied at different stages of aggregation, namely in single molecules under collision free conditions and in homogeneous clusters. The method used is a crossed electron/molecular beam experiment combined with a mass spectrometric detection system.All three molecules exhibit a very prominent resonance below 1 eV (attachment cross section > 10(-14) cm(2)). Under single collision conditions this resonance decomposes into the complementary channels X(-) + C6F5 (1) and C6F5- + X (2). The branching ratio between (1) and (2) changes by 4 orders of magnitude in favour of (2) in the order X = Cl, Br, I. In both C6F5Cl and C6F5Br a metastable parent anion is observed within a very narrow energy range close to 0 eV. In contrast, C6F5I-* formed by free electron capture completely decays within the mass spectrometric time scale. All three molecules also exhibit higher energy resonances which decompose into the fragments X(-), F- and C6F5-.In the C(6)F(5)X clusters the chemical reactivity is completely changed in that the dissociation channels from the low energy resonance are quantitatively quenched in favour of associative processes yielding M(n)(-) (M = C(6)F(5)X).In clusters associative attachment is also observed at energies above 3 eV. The likely mechanism is autoscavenging, i.e., inelastic scattering of the fast electron from one molecule and transfer of the slow electron to another molecule of the same cluster.