Mass spectral evidence that small changes in composition caused by oxidative aging processes alter aerosol CCN properties

Oxidative processing (i.e., "aging") of organic aerosol particles in the troposphere affects their cloud condensation nuclei (CCN) activity, yet the chemical mechanisms remain poorly understood. In this study, oleic acid aerosol particles were reacted with ozone while particle chemical composition and CCN activity were simultaneously monitored. The CCN activated fraction at 0.66 +/- 0.06% supersaturation was zero for 200 nm mobility diameter particles exposed to 565 to 8320 ppmv O3 for less than 30 s. For greater exposure times, however, the particles became CCN active. The corresponding chemical change shown in the particle mass spectra was the oxidation of aldehyde groups to form carboxylic acid groups. Specifically, 9-oxononanoic acid was oxidized to azelaic acid, although the azelaic acid remained a minor component, comprising 3-5% of the mass in the CCN-inactive particles compared to 4-6% in the CCN-active particles. Similarly, the aldehyde groups of alpha-acyloxyalkylhydroperoxide (AAHP) products were also oxidized to carboxylic acid groups. On a mass basis, this conversion was at least as important as the increased azelaic acid yield. Analysis of our results with K?hler theory suggests that an increase in the water-soluble material brought about by the aldehyde-to-carboxylic acid conversion is an insufficient explanation for the increased CCN activity. An increased concentration of surface-active species, which decreases the surface tension of the aqueous droplet during activation, is an interpretation consistent with the chemical composition observations and K?hler theory. These results suggest that small changes in particle chemical composition caused by oxidation could increase the CCN activity of tropospheric aerosol particles during their atmospheric residence time.     

Using a homology model of cytochrome P450 2D6 to predict substrate site of metabolism

CYP2D6 is an important enzyme that is involved in first pass metabolism and is responsible for metabolizing ~25% of currently marketed drugs. A homology model of CYP2D6 was built using X-ray structures of ligand-bound CYP2C5 complexes as templates. This homology model was used in docking studies to rationalize and predict the site of metabolism of known CYP2D6 substrates. While the homology model was generally found to be in good agreement with the recently solved apo (ligand-free) X-ray structure of CYP2D6, significant differences between the structures were observed in the B′ and F–G helical region. These structural differences are similar to those observed between ligand-free and ligand-bound structures of other CYPs and suggest that these conformational changes result from induced-fit adaptations upon ligand binding. By docking to the homology model using Glide, it was possible to identify the correct site of metabolism for a set of 16 CYP2D6 substrates 85% of the time when the 5 top scoring poses were examined. On the other hand, docking to the apo CYP2D6 X-ray structure led to a loss in accuracy in predicting the sites of metabolism for many of the CYP2D6 substrates considered in this study. These results demonstrate the importance of describing substrate-induced conformational changes that occur upon binding. The best results were obtained using Glide SP with van der Waals scaling set to 0.8 for both the receptor and ligand atoms. A discussion of putative binding modes that explain the distribution of metabolic sites for substrates, as well as a relationship between the number of metabolic sites and substrate size, are also presented. In addition, analysis of these binding modes enabled us to rationalize the typical hydroxylation and O-demethylation reactions catalyzed by CYP2D6 as well as the less common N-dealkylation.     

Uptake of small oxygenated organic molecules onto ammonium nitrate under upper tropospheric conditions

The uptake of formic (C1), propanoic (C3), butanoic (C4), and pentanoic (C5) acids onto ammonium nitrate (AN) has been investigated as a function of temperature and relative humidity using a Knudsen cell flow reactor coupled with FTIR-reflection absorption spectroscopy (FTIR-RAS). The uptake of acetone and methanol onto AN was also briefly studied. Initial uptake coefficients (gamma) were determined over the temperature range 200-240 K. Formic, propanoic, and butanoic acids exhibited efficient but temperature-dependent uptake on AN, with larger uptake coefficients observed at lower temperatures. Pentanoic acid was not taken up by AN under any of the conditions studied. Uptake of acetone and methanol onto AN was observed, but in insignificant amounts under atmospherically relevant conditions. Infrared spectra revealed that propanoic and butanoic acids ionized on the surface, despite the fact that the AN films were effloresced. Formic acid reacted with the AN film to produce ammonium formate and ionized nitric acid. Adding small amounts of water vapor (4% RH) to the chamber resulted in dramatically increased gamma values for all of the acids. Furthermore, the IR spectra showed the formation of a liquid layer when propanoic and butanoic acids adsorbed on the surface at RH = 20% and greater. Liquid water features were not observed at a similar relative humidity in the absence of the acids. These results show that small organic acids can be efficiently scavenged by AN and lead to enhanced water uptake under upper tropospheric conditions.