Comparison of size-resolved hygroscopic growth factors of urban aerosol by different methods in Tianjin during a haze episode.

Size-resolved hygroscopic growth factors of urban aerosol during a haze episode were measured using a Humidified Tandem Differential Mobility Analyzer (HTDMA) (gm(RH)). These factors were also derived from size-resolved particulate chemical composition combined with the κ-Köhler theory (gκ(RH)) and the thermodynamic model ISORROPIA-II running in forward mode (giso-f(RH)) and reverse mode (giso-r(RH)), respectively. In terms of agreement among these hygroscopic growth factors, gκ(RH) matched gm(RH) best, followed by giso-r(RH). In contrast, giso-f(RH) demonstrated a poorer agreement with gm(RH). The good consistency among gm(RH), gκ(RH), and giso-r(RH) was because they only focus on the physical hygroscopic process, whereas giso-f(RH) contains not only the direct influence of relative humidity (RH) on particle size but also the influence of gaseous precursor on the particle chemical composition, which indirectly affects the hygroscopicity of the particles. In this sense, size-resolved gκ(RH) and giso-r(RH) in a wide size range are more adequate to investigate the impact of RH on light scattering and aerosol radiative forcing. At RH = 80%, gκ(RH) for accumulation mode particles was 1.30-1.45 on polluted days and higher than that on clean days (1.2-1.3). Whereas on both polluted and clean days, gκ(RH) of ultrafine and coarse mode particles were generally lower than 1.25. The strong hygroscopicity of accumulation mode particles observed on polluted days can deteriorate visibility due to their high extinction efficiency.

Tris(8-quinolinolato-N,O)cobalt(III) ethanol solvate.

In the crystal of the title complex, [Co(C(9)H(6)NO)(3)].C(2)H(5)OH, the central Co atom has a distorted octahedral coordination comprised of three N atoms and three O atoms from the three 8-quinolinolato ligands. The three Co-O bond distances are in the range 1.887 (2)-1.910 (2) A, while the three Co-N bond distances range from 1.919 (2) to 1.934 (2) A. The solvent ethanol molecule forms an intermolecular O-H.O hydrogen bonding with a quinolinolato ligand.

Genetic complementation and resistance to 5-fluoro-2'-deoxyuridine in thymidine auxotrophs expressing a highly defective mutant of human thymidylate synthase.

A mutant human thymidylate synthase (TS) has been created in which a glutamine residue at position 214 has been replaced by glutamate. Glutamine at position 214 is postulated to be involved in maintaining the enzyme in a conformation that facilitates the binding of the substrate dUMP. Although the kcat/Km of the mutant protein for the substrate, dUMP, is 10(3) lower than that of wild-type TS, the mutant TS confers thymidine prototrophy on a TS-deficient bacterial strain when expressed at high levels. In the present investigation, a TS-deficient Chinese hamster lung cell line was transfected with DNA encoding the defective protein. Thymidine prototrophs were isolated that expressed the defective protein at levels that were physiologically relevant. The activities of the enzymes expressed endogenously in representative prototrophs were consistent with the activities observed for the purified proteins. At similar levels of TS expression, thymidine prototrophs expressing Glu214 TS were 8-fold more resistant to 5-fluoro-2'-deoxyuridine (FdUrd) cytotoxicity than are prototrophs expressing Gln214 TS. FdUrd is a prodrug of the tight-binding TS inhibitor, 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP). The resistance to FdUrd was associated with a significant decrease in the binding of FdUMP to the purified mutant enzyme. The data are consistent with the interpretation that TSs that are highly defective are capable of sufficient dTMP production for cell survival and optimal growth, yet may confer resistance to TS-directed inhibitors.

A structural role for glutamine 214 in human thymidylate synthase.

Studies of the crystal structures of thymidylate synthase (TS) have revealed that a kink is present in beta-sheets that form the core of the enzyme. The beta-kink is proposed to serve as a "hinge" during conformational changes that occur in the enzyme after ligand binding at the active site. A residue in one of the beta-bulges that form the kink, glutamine at position 214 of human TS, is highly conserved in all TSs and is postulated to interact with nucleotide ligands that bind at the active site. To examine the role of this residue, glutamine at position 214 was replaced by residues that differ in volume, hydrophobicity, electrostatic charge, and hydrogen bonding potential. Genetic complementation studies utilizing a TS-deficient bacterial strain revealed that residues with large side chain volumes or that are prohibited in beta-bulges created loss of function proteins. Kinetic studies indicated that residue hydrophobicity is not correlated with catalytic activity. Residues that are predicted to alter the charge at position 214 created enzymes with kcat/Km values at least 10(3) lower than those of the wild type. Kinetic and ligand binding studies indicated that residue 214 is involved in nucleotide binding; however, hydrogen bonding potential does not contribute significantly to nucleotide binding energy. The data are consistent with the hypothesis that residue 214 is involved in maintaining the enzyme in a conformation that facilitates nucleotide binding and catalysis.