Formation of Secondary Organic Aerosol by Reactive Condensation of Furandiones, Aldehydes, and Water Vapor Onto Inorganic Aerosol Seed Particles

Volatile furandiones and aldehydes are significant atmospheric oxidation products of aromatic compounds. The mechanism of secondary organic aerosol formation by these compounds was probed using particle chamber observations and macroscale simulations of condensed phases. Growth of inorganic seed aerosol was monitored in the presence of humidity and high concentrations of 2,5-furandione (maleic anhydride), 3-methyl-2,5-furandione (citraconic anhydride), benzaldehyde, and trans-cinnamaldehyde. Particle growth commenced when the gas-phase saturation level of each organic compound and water vapor (relative to its pure liquid), when summed together, reached a threshold near one, implying the formation of a nearly ideal mixed organic/aqueous phase. However, these organics are immiscible with water at the high mole fractions that would be expected in such a phase. Highly acidic dicarboxylic acids produced by the reactions between furandiones and water were shown to rapidly acidify an aqueous phase, resulting in greatly increased benzaldehyde solubility. Thus, the uptake of these organics onto particles in the presence of humidity appears to be reaction-dependent. Finally, it is shown that dicarboxylic acids produced in these reactions recyclize back to furandiones when subjected to normal GC injector temperatures, which could cause large artifacts in gas/particle phase distribution measurements

Simulating Secondary Organic Aerosol Activation by Condensation of Multiple Organics on Seed Particles

The conditions under which semivolatile organic gases condense were studied in a Teflon particle chamber by scanning mobility particle sizing (SMPS) of the resultant particles. Benzaldehyde, maleic and citraconic anhydrides, n-decane, trans-cinnamaldehyde, and citral were introduced in various combinations into a particle chamber containing either particle-free nitrogen or nitrogen with dry seed particles made out of sodium chloride, d-tartartic acid, ammonium sulfate, or 1,10-decanediol. No organic gas was allowed to reach its saturation point relative to the vapor pressure of its pure liquid in any experiment. In the absence of seed particles, organic aerosol particles formed by ternary nucleation when the sum of the individual organic saturation levels reached a threshold between 1.17 and 1.86. With seed particles present, particle sizes began to increase when the sum of organic saturation levels reached 1.0. This size increase corresponds to the establishment and activation of ternary organic layers on the “clean” seed particles, as predicted by absorption partitioning theory. The observed increases in particle volume depended on initial seed particle volume, indicating that either gas diffusion rates or chemical reactions were controlling the rate of uptake

Efforts Toward Expansion of the Genetic Alphabet: Structure and Replication of Unnatural Base Pairs

Expansion of the genetic alphabet has been a long time goal of chemical biology. A third DNA base pair that is stable and replicable would have a great number of practical applications and would also lay the foundation for a semi-synthetic organism. We have reported that DNA base pairs formed between deoxyribonucleotides with large aromatic, predominantly hydrophobic nucleobase analogs, such as propinyl isocarbostyril (d PICS ), are stable and efficiently synthesized by DNA polymerases. However, once incorporated into the primer, these analogs inhibit continued primer elongation. More recently, we have found that DNA base pairs formed between nucleobase analogs that have minimal aromatic surface area in addition to little or no hydrogen-bonding potential, such as 3-fluoro benzene (d 3FB ), are synthesized and extended by DNA polymerases with greatly increased efficiency. Here we show that the rate of synthesis and extension of the self pair formed between two d 3FB analogs is sufficient for in vitro DNA replication. To better understand the origins of efficient replication, we examined the structure of DNA duplexes containing either the d 3FB or d PICS self pairs. We find that the large aromatic rings of d PICS pair in an intercalative manner within duplex DNA, while the d 3FB nucleobases interact in an edge-on manner, much closer in structure to natural base pairs. We also synthesized duplexes containing the 5-methyl substituted derivatives of d 3FB (d 5Me3FB ) paired opposite d 3FB or the unsubstituted analog (d BEN ). In all, the data suggest that structure, electrostatics and dynamics can all contribute to the extension of unnatural primer termini. The results also help explain the replication properties of many previously examined unnatural base pairs and should help design unnatural base pairs that are better replicated