PYRIMIDINE DIMERS INDUCED IN ESCHERICHIA COLI DNA BY ULTRAVIOLET RADIATION PRESENT IN SUNLIGHT

Abstract— Escherichia coli DNA was irradiated with various wavelengths of monochromatic UV light from 254 to 320 nm, and the relative yields of the different cyclobutane pyrimidine dimers determined. Cytosine–thymine dimers (C < > T) were more frequent than thymine dimers (T < > T) at low fluences of 300 and 313 nm light, whereas the reverse was true at either longer or shorter wavelengths. Thus, in the solar UV range deemed responsible for skin cancer (i.e. 295–315 nm), C < > T are probably more important than T < > T.     

Variable pi-bonding in iron(II) porphyrinates with nitrite, CO, and tert-butyl isocyanide: characterization of [Fe(TpivPP)(NO2)(CO)]-

The addition of the strongly pi-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)]- (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)]- and [Fe(TpivPP)(NO2)(t-BuNC)]-. These species have been characterized by single-crystal structure determinations and by UV-vis, IR, and M?ssbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)]-, the two trans, pi-accepting ligands CO and nitrite compete for pi density. The CO ligand however dominates the bonding. The Fe-N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) A are lengthened compared to other nitrite species with either no trans ligands or non-pi-accepting trans ligands to nitrite. The Fe-C(CO) bond lengths are 1.782(4) A and 1.789(5) A for the two anions. The two Fe-C-O angles at 175.5(4) and 177.5(4) degrees are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)]- was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the M?ssbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the M?ssbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm(-1) is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)].1/2C6H5Cl: monoclinic, space group P2(1)/c, Z = 8, a = 33.548(6) A, b = 18.8172(15) A, c = 27.187(2) A, beta = 95.240(7) degrees, V = 17091(4) A3.     

Negative-ion photoelectron spectroscopy, gas-phase acidity, and thermochemistry of the peroxyl radicals CH(3)OO and CH(3)CH(2)OO

Methyl, methyl-d(3), and ethyl hydroperoxide anions (CH(3)OO(-), CD(3)OO(-), and CH(3)CH(2)OO(-)) have been prepared by deprotonation of their respective hydroperoxides in a stream of helium buffer gas. Photodetachment with 364 nm (3.408 eV) radiation was used to measure the adiabatic electron affinities: EA[CH(3)OO, X(2)A' '] = 1.161 +/- 0.005 eV, EA[CD(3)OO, X(2)A' '] = 1.154 +/- 0.004 eV, and EA[CH(3)CH(2)OO, X(2)A' '] = 1.186 +/- 0.004 eV. The photoelectron spectra yield values for the term energies: Delta E(X(2)A' '-A (2)A')[CH(3)OO] = 0.914 +/- 0.005 eV, Delta E(X(2)A' '-A (2)A')[CD(3)OO] = 0.913 +/- 0.004 eV, and Delta E(X(2)A' '-A (2)A')[CH(3)CH(2)OO] = 0.938 +/- 0.004 eV. A localized RO-O stretching mode was observed near 1100 cm(-1) for the ground state of all three radicals, and low-frequency R-O-O bending modes are also reported. Proton-transfer kinetics of the hydroperoxides have been measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to determine the gas-phase acidity of the parent hydroperoxides: Delta(acid)G(298)(CH(3)OOH) = 367.6 +/- 0.7 kcal mol(-1), Delta(acid)G(298)(CD(3)OOH) = 367.9 +/- 0.9 kcal mol(-1), and Delta(acid)G(298)(CH(3)CH(2)OOH) = 363.9 +/- 2.0 kcal mol(-1). From these acidities we have derived the enthalpies of deprotonation: Delta(acid)H(298)(CH(3)OOH) = 374.6 +/- 1.0 kcal mol(-1), Delta(acid)H(298)(CD(3)OOH) = 374.9 +/- 1.1 kcal mol(-1), and Delta(acid)H(298)(CH(3)CH(2)OOH) = 371.0 +/- 2.2 kcal mol(-1). Use of the negative-ion acidity/EA cycle provides the ROO-H bond enthalpies: DH(298)(CH(3)OO-H) = 87.8 +/- 1.0 kcal mol(-1), DH(298)(CD(3)OO-H) = 87.9 +/- 1.1 kcal mol(-1), and DH(298)(CH(3)CH(2)OO-H) = 84.8 +/- 2.2 kcal mol(-1). We review the thermochemistry of the peroxyl radicals, CH(3)OO and CH(3)CH(2)OO. Using experimental bond enthalpies, DH(298)(ROO-H), and CBS/APNO ab initio electronic structure calculations for the energies of the corresponding hydroperoxides, we derive the heats of formation of the peroxyl radicals. The "electron affinity/acidity/CBS" cycle yields Delta(f)H(298)[CH(3)OO] = 4.8 +/- 1.2 kcal mol(-1) and Delta(f)H(298)[CH(3)CH(2)OO] = -6.8 +/- 2.3 kcal mol(-1).     

XAS study of lanthanum coordination environments in glasses of the system K2O---SiO2---La2O3

The La L₁ and L₃ XANES and L₃ EXAFS have been investigated for the series of glasses 10K₂O---50SiO₂---x La₂O₃ (x = 1, 5, 10) and (10 − x)K₂O---40SiO₂−(x/3)La₂O₃ (x = 7.5, 5, 2.5) and model compounds La₂O₃, LaAlO₃, LaPO₄, La₂NiO₄, La₂CuO₄ and La(OH)₃. An edge resonance at 25 eV above the L₁ edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s → nd forbidden transition increases with La₂O₃ concentration, indicating a reduction in the ‘average’ site symmetry of the first coordination shell of La. Mapping _X(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete _X(k) space a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 Å (± 0.03 Å) for La---O. The coordination numbers (N ≤ 7 ± 1.5) were derived by mapping and comparison to the published structures for other La compounds. _X(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.     

Performance of uncertainty evaluation strategies in a food proficiency scheme

A study of the performance of different uncertainty evaluation strategies among 163 voluntary respondents from food proficiency schemes is presented. Strategies included use of: single-laboratory validation data, quality control data, past proficiency testing data, reproducibility data, a measurement equation and the dispersion of replicate observations on the test material. Most performed reasonably well, but the dispersion of replicate observations underestimated uncertainty by a factor of approximately 3. Intended compliance with accreditation requirements was associated with significantly improved uncertainty evaluation performance, while intended compliance with the ISO “Guide to the expression of uncertainty in measurement” had no significant effect. Substituting estimates based on the Horwitz or Horwitz–Thompson models or on PT target standard deviation for the respondents’ own estimates of uncertainty led to a marked reduction in poor zeta scores and significant improvement in dispersion of zeta scores.     

Fluorescence studies of confinement in polymer films and nanocomposites: Glass transition temperature, plasticizer effects, and sensitivity to stress relaxation and local polarity

Confinement effects in polystyrene and poly(methyl methacrylate) films and nanocomposites are studied by fluorescence. The ability to employ an intensive measurable, the excited-state fluorescence lifetime, in defining the glass transition temperature, T_g, of polymers is demonstrated and compared to the use of an extensive measurable, fluorescence intensity. In addition, intrinsic fluorescence from the phenyl groups in polystyrene is used to determine the T_g-confinement effect in films as thin as ~15 nm. The decrease in T_g with decreasing film thickness (below ∼60 nm) agrees well with results obtained by extrinsic pyrene fluorescence. Dye label fluorescence is used to quantify the enhancement in T_g observed with decreasing thickness (below ~90 nm) in poly(methyl methacrylate) films; addition of 2–4 wt% dioctyl phthalate plasticizer reduces or eliminates the T_g-confinement effect in films down to 20 nm thickness. Intrinsic polystyrene fluorescence, which is sensitive to local conformation, is used to quantify the time scales (some tens of minutes) associated with stress relaxation in thin and ultrathin spin-coated films at T_g + 10 K. Finally, the shape of the fluorescence spectrum of pyrene doped at trace levels in polystyrene films and polystyrene-silica nanocomposites is used to determine effects of confinement on microenvironment polarity.     

Measurement uncertainty and its implications for collaborative study method validation and method performance parameters

ISO principles of measurement uncertainty estimation are compared with protocols for method development and validation by collaborative trial and concomitant "top-down" estimation of uncertainty. It is shown that there is substantial commonality between the two procedures. In particular, both require a careful consideration and study of the main effects on the result. Most of the information required to evaluate measurement uncertainty is therefore gathered during the method development and validation process. However, the information is not generally published in sufficient detail at present; recommendations are accordingly made for future reporting of the data.     

Confinement and processing effects on glass transition temperature and physical aging in ultrathin polymer films: Novel fluorescence measurements

Fluorescence intensity measurements of chromophore-doped or -labeled polymers have been used for the first time to determine the effects of decreasing film thickness on glass transition temperature, T _g, the relative strength of the glass transition, and the relative rate of physical aging below T _g in supported, ultrathin polymer films. The temperature dependence of fluorescence intensity measured in the glassy state of thin and ultrathin films of pyrene-doped polystyrene (PS), poly(isobutyl methacrylate) (PiBMA), and poly(2-vinylpyridine) (P2VP) differs from that in the rubbery state with a transition at T _g. Positive deviations from bulk T _g are observed in ultrathin PiBMA and P2VP films on silica substrates while substantial negative deviations from bulk T _g are observed in ultrathin PS films on silica substrates. The relative difference in the temperature dependences of fluorescence intensity in the rubbery and glassy states is usually reduced with decreasing film thickness, indicating that the strength of the glass transition is reduced in thinner films. The temperature dependence of fluorescence intensity also provides useful information on effects of processing history as well as on the degree of polymer-substrate interaction. In addition, when used as a polymer label, a mobility-sensitive rotor chromophore is demonstrated to be useful in measuring relative rates of physical aging in films as thin as 10 nm. Received 21 August 2001     

Direct evidence for base-mediated decomposition of alkyl hydroperoxides (ROOH) in the gas phase

The reaction of F(-) with CH(3)OOH has been studied in the gas phase using a tandem flowing afterglow-selected ion flow tube apparatus. The reaction is rapid (k = 1.23 x 10(-9) cm(3) s(-1), 49% efficiency), and formation of HO(-) + CH(2)O + HF is the major reaction channel observed (85%). Isotopic labeling, reactions of F(-) with larger alkyl hydroperoxides, and computational studies demonstrate that the major product ion, HO(-), is formed via a concerted elimination mechanism that appears to be general to all alkyl hydroperoxides possessing an alpha-hydrogen. This mechanism represents a base-mediated decomposition of alkyl hydroperoxides in the gas phase that may have important implications for solution and biochemical reactions. The reverse reaction, CH(3)OO(-) + HF is also efficient (k = 2.43 x 10(-9) cm(3) s(-1)). The major product ensemble HO(-) + CH(2)O + HF (81%) is identical to that of the forward reaction, and represents a novel neutral-catalyzed decomposition of the anion.