Edward C.M. Chen

The determination of absolute electron affinities of the purines and pyrimidines in DNA and RNA from reversible reduction potentials.

We report the reversible reduction potentials of the purines and pyrimidines of DNA and RNA. These were determined in dimethylsulfoxide using cyclic voltammetry. The absolute electron affinities have been determined from these reduction potentials by calibration with the absolute electron affinities for acridine and anthracene measured in the gas phase. These are the first experimentally determined values of the electron affinities of these purines and pyrimidines and are: Guanine = 1.51 eV, Adenine = 0.95 eV, Uracil = 0.80 eV, Thymine = 0.79 eV and Cytosine = 0.56 eV.

The negative ion states of sulfur hexafluoride

Summary talk at the Lepton-Photon Symposium, Cornell University, Aug. 10-15, 1993.The reaction of SF6 with thermal electrons has been studied in a Ni‐63 atmospheric pressure ionization source for a quadrupole mass spectrometer (API/MS). The major ions that are observed are the parent negative ion (SF−6) and the parent minus a fluorine atom (SF−5). The ratio of [SF−5]/[SF−6] is highly temperature dependent above 500 K. The dissociation energy of the ground state negative ion into SF−5 and F has been determined to be 1.35±0.1 eV. This gives values of 3.8±0.15 eV for the electron affinity of SF5 and 1.15±0.15 eV for the electron affinity of SF6. The negative ion states of sulfur hexafluoride have been described by ‘‘pseudo‐two‐dimensional’’ Morse potentials calculated using experimental data.

Changes in the free amino acid pool during environmental stress in the gill tissue of the oyster, Crassostrea virginica

Abstract 1. 1. Oysters were exposed for 2- and 5-day periods to increased salinity (26%.–38%.), anoxia, turbidity and drilling effluents. 2. 2. After two days, the FAA pool in the gill tissue of oysters exposed to 38%. salinity had elevated glycine, alanine and β-alanine levels; oysters exposed to anoxia showed elevated glycine and alanine and decreased aspartic acid levels. 3. 3. After 2 days, both oysters exposed to turbidity and to drilling effluents had increased cysteic acid levels. Glutamic acid and alanine levels were also elevated in oysters exposed to drilling effluents. 4. 4. After 5 days, glycine, alanine and β-alanine remained above control levels in oysters exposed to increased salinity whereas in those exposed to anoxia, turbidity and drilling effluents, a significant decrease in most amino acids occurred with the total FAA pool decreasing by 50%. 5. 5. The FAA pools response was unique for each stress studied suggesting that the FAA pool may prove to be a useful diagnostic tool for determining a posteriori the causative agent responsible for a given stress response.

Structural analysis of leuconostoc dextrans containing 3-O-α-d-glucosylated α-d-glucosyl residues in both linear-chain and branch-point positions, or only in branch-point positions, by methylation and by 13C-N.M.R. spectroscopy

Abstract It had been established by methylation-structural analysis that dextran fraction S from Leuconostoc mesenteroides NRRL B-1355 has two types of α- d -glucopyranosyl residues that are linked through O-3, i.e. , 35% of the residues carry a (1→3)-bond, and ∼10% carry a (1→6)-bond in addition to a (1→3)-bond. Two similarly constituted dextrans have now been identified by methylation-structural analysis, namely, the S-type fractions from L. mesenteroides strains NRRL B-1498 and B-1501. The S-type fractions from L. mesenteroides strains B-1355, B-1498, and B-1501 are structurally differentiated from the α- d -glucans (characteristically insoluble) of certain cariogenic Streptococci which also contain both 3- O - and 3,6-di- O -substituted α- d -glucopyranosyl residues. 13 C-N.m.r. spectra have been recorded at 90° for both the S- and L-type fractions of strains B-1355, b-1498, and B-1501. The L-type fractions have a low degree of branching through 3,6-di- O -substituted α d -glucopyranosyl residues, but no 3-mono- O -substituted residues. (Dextran fraction S of Streptococcus 5000 g.l.c. instrument equipped with hydrogen-flame detectors. On-column injection of glass columns (2 mm i.d. x 1.23 m) was employed for all such chromatography. The 13 C-n.m.r. conditions and methods for preparation of dextran samples have been described(su4). In general, a Varian XL-100-15 spectrometer equipped with a Nicolet TT-100 system was employed in the Fourier-transform mode. Chemical shifts are expressed in p.p.m. relative to external tetramethylsilane, but were actually calculated by reference to the lock signal.

Methylation structural analysis of unusual dextrans by combined gas-liquid chromatography-mass spectrometry☆

Abstract Eight bacterial dextrans from NRRL strainsLeuconostoc mesenteroides B-742, B-1299, B-1355, B-1399, and B-1402, and from Streptobacterium dextranicum B-1254 were examined by methylation structural analysis. Methyl ethers of d -glucose that were present in hydrolyzates of permethylated dextrans were analyzed by combined g.l.c.−m.s. as the peracetylated aldononitriles. The various dextrans differed significantly in frequency and type of chain branching.

Identification Of aldoses by use of their peracetylated aldononitrile derivatives: A G.L.C-M.S. approach☆

Abstract The g.l.c. retention-times and detector responses have been examined for peracetylated aldononitrile derivatives from aldoses. Correlations have been made between changes in g.l.c. retention-times and changes in the stereochemistry and functional groups of the parent aldose. The mass spectra [electron impact (e.i.), ammonia chemical ionization (c.i.), and methane c.i.] for these g.l.c. peaks were recorded. C.i.-mass spectrometry (m.s.) indicated the molecular weights of the derivatives, and the number of aldehyde and alcohol groups in the parent aldose. E.i.-m.s. indicated the nature and position of functional groups present in the parent aldose. Aldoses containing acetamido, amino, deoxy, and thio substituents were studied.

The role of electron donors and acceptors in base stacking in DNA and RNA

The role of donor-acceptor interactions in base pair stacking in DNA and RNA has been minimized because of the perceived low or negative electron affinities of the purines and pyrimidines. The use of the electron capture detector was among the first methods for measuring electron affinities in the gas phase. Recently, the experimental determination of electron affinities has been extended and improved. Now, there are data for similar compounds in the literature which enable us to estimate electron affinities for purines and pyrimidines. These values are significant, and positive, such that donor-acceptor interactions can, and indeed should play a role in the stacking of bases in nucleic acids.

Classification of organic molecules to obtain electron affinities from half wave reduction potentials: The aromatic hydrocarbons

Adiabatic absolute electron affinities (EA) of about 80 aromatic hydrocarbons are calculated from literature values of half wave reduction potentials in aprotic solvents. Solution energy differences are estimated by grouping the molecules based on experimental and theoretical values, chemical logic and statistical analysis. The electronegativity values, (IP+EA)/2, calculated using literature data for ionization potentials, are constant for many of the compounds. Others vary in a systematic manner, such as benzene, 4.26±0.05; naphthalene, 4.14±0.02; anthracene, 4.06±0.02; tetracene, 4.03±0.03; pentacene, 3.98±0.03, in eV. Theoretical values, recalculated from the beginning, compare favorably with the literature values. Three independent methods for obtaining absolute EA’s are summarized and verified; the calibration of reduction potentials, the combination of ionization potentials with electronegativities and the use of semiempirical methods.

Windowless pulsed-discharge photoionization detector application to qualitative analysis of volatile organic compounds

Abstract The effluent from a gas chromatograph was split and directed to four identical windowless photoionization detectors. These detectors use pure helium, helium with Ar (0.64%), Ar (4.15%) and Kr (0.58%) as the discharge gas. The relative ionization cross-sections can be obtained from the ratio of the normalized response using an internal standard. This value is characteristic of the compound and could be used for qualitative analysis. These ratios for 47 compounds encompassing 13 functional groups have been determined and are reported. The values have an average relative standard deviation of 2% for the majority of the compounds. The long-term reproducibility of a smaller set of compounds has been determined and was 0.78–2% relative standard deviation for the Ar detector and 5.6–11.3% (worse cases) where the response of the krypton detector is low.

CLASSIFICATION OF ORGANIC MOLECULES TO OBTAIN ELECTRON AFFINITIES FROM HALF-WAVE REDUCTION POTENTIALS : CYTOSINE, URACIL, THYMINE, GUANINE AND ADENINE

A procedure for obtaining the adiabatic electron affinities (AEA) of organic molecules from half-wave reduction potentials in aprotic solvents is presented. Molecules are placed into groups according to their structure. Each group has a different solution energy difference. Calculations of AEA and charge distributions with AM1-multiconfiguration configuration interaction are used to support the intuitive classification of the molecules. The procedure is illustrated for Vitamins A and E, riboflavin, the azines, polyenes, hydroxy-pyrimidine, oxo-guanine, the hydrogen bonded cytosine-oxo-guanine as well as the AEA, and vertical EA (VEA) of Cytosine (C), Uracil (U), Thymine (T), Guanine (G) and Adenine (A). The latter values are: (VEA) G, 0.10; A, -0.49; U, 0.33; T, 0.31; C, -1.48 and (AEA) G, 1.51 +/- 0.05; A, 0.95 +/- 0.05; U, 0.80 +/- 0.05; T, 0.79 +/- 0.05; C, 0.56 +/- 0.05 in eV.