Precision and accuracy of quantitative emission spectrometry with particular reference to gold alloys

An emission spectrometric method is described for the determination of major constituents in metal samples. The metal sample is made the hollow-cathode of a demountable emission source. Use of the hollow-cathode discharge permits intensity measurements of the analytical lines (resonance lines) to be made with high signal-to-noise ratio. For ultimate precision and accuracy, such as is required in the determination of gold in gold alloys, internal standardization is also necessary. The precision is approximately 0.03% (w/w) (standard deviation) anal the accuracy is 0.03% (w/w).     

Performance and application of controlled temperature-gradient lamps in atomic absorption spectrometry

An improved design of controlled temperature-gradient lamp (CTGL) is suitable for arsenic, cadmium, phosphorus, potassium, rubidium, selenium, sodium, sulphur and zinc. Intensity and linewidth measurements indicate that the CTGL is significantly more intense than an electrodeless discharge lamp (EDL) at the same linewidth. CTGL's also compare favourably with EDL's when used as light sources for a.a.s. Arsenic and selenium can be determined at very low concentrations (ng ml^-1) by the hydride generation technique. Sulphur and phosphorus can be detected in the vacuum ultra-violet region using nitrogen-separated flames; the limits of detection are 13 and 10 μg ml^-1, respectively.     

A rare example of three abundant conformers in one retro model of the cisplatin-DNA d(GpG) intrastrand cross link. Unambiguous evidence that guanine O6 to carrier amine ligand hydrogen bonding is not important. possible effect of the Lippard base pair step adjacent to the lesion on carrier ligand hydrogen bonding in DNA adducts

Guanine O6 to carrier ligand hydrogen bonding is a central feature of many hypotheses advanced to explain the anticancer activity of cis-type anticancer drugs, cis-PtA(2)X(2) (A(2) = diamine or two amines). Early structural evidence suggested that cis-Pt(NH(3))(2)(d(GpG)) (the cross-link model for the key cisplatin-DNA adduct) and other cis-PtA(2)(d(GpG)) adducts exist exclusively or mainly as the HH1 conformer with head-to-head (HH) bases. The dynamic motion of the d(GpG) in these adducts is too rapid to permit definitive characterization of both the conformation and the H-bonding. Hence, we use retro models having A(2) ligands designed to slow the motion. Here, we employ Me(2)ppz (N,N'-dimethylpiperazine), which lacks NH groups. Me(2)ppz is unique in having sp(3) N-methyl groups directly in the coordination plane, allowing the coexistence of multiple conformers but hindering dynamic motion in Me(2)ppzPt(d(GpG)) and Me(2)ppzPt(GpG) retro models. Dynamic processes are decreased enough in Me(2)ppzPt(d(GpG)) to permit HPLC separation of three abundant forms. After HPLC separation, the three re-equilibrate, proving that the three forms must be conformers and that Me(2)ppz has little influence on conformer distribution. This marks the first reported characterization of three abundant conformers for one cis-PtA(2)(d(GpG)) adduct. From NMR evidence, the Me(2)ppzPt(d(GpG)) HH1 conformer has uncanted bases. Another conformer, one of two recently discovered conformer types, has head-to-tail (HT) bases with Delta chirality. For this Delta HT1 form, several lines of evidence establish that the dinucleotide moieties have essentially identical structures in d(GpG) (and GpG) adducts of Me(2)ppzPt and other cis-PtA(2) complexes. For example, the shifts of the highly structure-sensitive G H8 NMR signals are almost identical for the Delta HT1 form of all adducts. In previous models, the stabilization of the Delta HT1 form could be attributed to G O6 H-bonding to A(2) NH groups. Such H-bonds are not possible for Me(2)ppz. The unambiguous conclusions are that G O6 H-bonding is weak and that neither canting nor H-bonding is essential in HH forms. These two features are present in almost all other small models but are essentially absent in the cross-link base pair (bp) step in duplexes. We conclude from our work that the forces favoring canting and H-bonding are weak, and we hypothesize that steric effects within the Lippard bp step adjacent to this cross-link bp step easily overcome these forces.     

DNA EXCISION REPAIR IN THE VERY UV-SENSITIVE XERODERMA PIGMENTOSUM COMPLEMENTATION GROUP A STRAIN XP4LO

Abstract— XP4L0, a xeroderma pigmentosum complementation group A strain, exhibits very limited DNA repair activity. It has extreme sensitivity to UV (254 nm) as determined by colony forming ability. The rate of loss of UV (1 J/m²)-induced pyrimidine dimers from populations of quiescent, nondividing XP4LO cells was determined and found to be slower than that observed for other group A strains (XP25R0, XP12BE, XP8LO). The extreme UV-sensitivity is also exhibited by the nondividing cells in a survival assay that employs nondividing cell populations and does not involve cell reproduction. This result suggests that the extreme sensitivity measured previously by colony-forming ability (a cell-reproduction assay) is due to the excision repair defect alone and not to an additional post-replication repair defect. The very limited excision allows for an accurate definition of target size for inactivation of nondividing cells, about 1 pyrimidine dimer per 10⁵ base pairs, and when compared to results observed for other XP-A strains, provides further evidence that even though excision repair in group A is severely limited, it has biological significance.     

Kinetics and mechanism of the beta- to alpha-CuAlCl(4) phase transition: a time-resolved (63)Cu MAS NMR and powder X-ray diffraction study

The beta and alpha phases of CuAlCl(4) have been characterized by solid-state (27)Al and (63)Cu magic angle spinning nuclear magnetic resonance. The very short spin--lattice relaxation times of the copper spins, and the sensitivity of the I = 3/2 (63)Cu nucleus to the small differences in the local structure of Cu in the two phases, allowed (63)Cu spectra to be acquired in very short time periods (1 min), in which the beta and alpha phases were clearly resolved. This time resolution was exploited to follow the phase transition from the pseudohexagonal close-packed beta-CuAlCl(4) into the pseudocubic close-packed alpha-CuAlCl(4), which occurs above 100 degrees C. In situ time-resolved (63)Cu MAS NMR and synchrotron X-ray diffraction experiments were used to measure the kinetics of this phase transition as a function of temperature. The transformation was shown to be a first-order phase transition involving no intermediate phases with an activation energy of 138 kJ/mol. The kinetic data obey a first-order Avrami--Erofe'ev rate law. A one-dimensional growth mechanism is proposed that involves a combination of Cu(+) ion self-diffusion and a translational reorganization of the close-packed anion layers imposed by the periodic rotations of [AlCl(4)](-) tetrahedra. This beta to alpha phase transformation can be induced at ambient temperatures by low partial pressures of ethylene.     

Unusual, bifurcated photoreactivity of a rhenium(I) carbonyl complex of triethynylphosphine

Preparations of the first metal complexes of triethynylphosphine (TEP) are described. They are of the type fac-Re(bpy)(CO)(3)(TEP)(+) (1) and cis,trans-[Re(bpy)(CO)(2)(TEP)L](n)(+) (CH(3)CN, n = 1, complex 2; Cl, n = 0, complex 3), where bpy is 2,2'-bipyridine. Complex 1 displays unusual photochemical behavior compared to analogous fac-[Re(bpy)(CO)(3)(PR(3))](+) complexes in that it emits from a state that has pi-pi* character but undergoes competitive photosubstitution of both TEP and CO. Density functional theory (DFT)/time-dependent DFT calculations predict that the lowest emitting state should, in fact, have pi-pi* character.     

Synthesis of aryl- and heteroaryl[a]pyrrolo[3,4-c]carbazoles

Synthesis of aryl- and hetero[a]pyrrolo[3,4-c]carbazoles by photochemical oxidation and Heck cyclization are described. Photochemical oxidation of 2-naphthyl indolyl maleimide affords two different carbazole regioisomers, depending on the reaction conditions. The regiochemistry of the cyclization can be controlled using the Heck reaction.     

Thermal expansion of methanol + cyclohexane near the critical solution point

We report precise measurement of volume changes as function of temperature in the one phase region of methanol + cyclohexane near the critical solution point. We find a value for the critical exponent α of 0.11 ± 0.07 in the region 11 Mk < T – T_c < 6.4 K. This value agrees with the Ising value of $\text{1}\text{8}$. For temperatures closer to Tc we observe a stronger divergence. In addition we observe unusual behavior over a small region of temperature centered at T – T_c = 15 mK.     

Synthesis, structure, and catalytic oxidation chemistry from the first oxo-imido Schiff base metal complexes

The molybdenum oxo-imido complex, [Mo(O)(NtBu)Cl2(dme)] (1), was obtained from the reaction between [MoO2Cl2(dme)] and [Mo(NtBu)2Cl2(dme)]. Reactions between [Mo(O)(NR)Cl2(dme)] (where R = tBu or 2,6-iPr2C6H3) and the disodium Schiff base compounds Na(2)(3,5-tBu2)2salen, Na(2)(3,5-tBu2)2salpen, and Na(2)(7-Me)2salen afforded the first oxo-imido transition metal Schiff base complexes: [Mo(O)(NtBu)[(3,5-tBu2)2salen]] (2), [Mo(O)(NtBu)[(3,5-tBu2)2salpen]] (3), and [Mo(O)(N-2,6-iPr2C6H3)[(7-Me)2salen]] (4), respectively. The compounds [Mo(NtBu)2[(3,5-tBu2)2salpen]] (5) from [Mo(NtBu)2(NHtBu)2] and [Mo(N-2,6-iPr2C6H3)(2)[(7-Me)2salen]](6) from [Mo(N-2,6-iPr2C6H3)(2)(NHtBu)2] (7) are also reported. Compounds 1-7 were characterized by NMR, IR, and FAB mass spectroscopy while compounds 3, 4, and 5 were additionally characterized by X-ray crystallography. In conjunction with tBuOOH as oxidant, compound 3 is a catalyst for the oxidation of benzyl alcohol to benzaldehyde and cis-cyclooctene and 1-octene to the corresponding epoxides.