Assessing Potential Oligomerization Reaction Mechanisms of Isoprene Epoxydiols on Secondary Organic Aerosol.

Extensive studies of secondary organic aerosol (SOA) formation have identified isoprene epoxydiol (IEPOX) intermediates as key species in the formation of isoprene-derived SOA. Recent work has suggested that isoprene-derived dimers and oligomers may constitute a significant fraction of SOA, but a mechanism for the formation of such abundant SOA components has yet to be established. The potential for dimer formation from the nucleophilic addition of 2-methyltetrol to trans-ß-IEPOX was assessed through a series of model epoxide-nucleophile experiments using nuclear magnetic resonance (NMR) spectroscopy. These experiments helped establish a rigorous understanding of structural, stereochemical, and NMR chemical shift trends, which were used along with nucleophilic strength calculations to interpret the results of the trans-ß-IEPOX + 2-methyltetrol reaction and evaluate its relevance in the atmosphere. A preference for less sterically hindered nucleophiles was observed in all model systems. In all addition products, a significant increase in NMR chemical shift was observed directly adjacent to the epoxide-nucleophile linkage, with smaller decreases in chemical shift at all other sites. A partial NMR assignment of a single trans-ß-IEPOX + 2-methyltetrol nucleophilic addition product was obtained, but nucleophilic strength calculations suggest that 2-methyltetrol is a poor nucleophile. Therefore,  this reaction is unlikely to significantly contribute to dimer and oligomer formation on SOA. Nevertheless, the structural and stereochemical considerations, NMR assignments, and NMR chemical shift trends reported here will prove useful in future attempts to synthesize dimer and oligomer analytical standards.

Assessing the Potential for Oligomer Formation from the Reactions of Lactones in Secondary Organic Aerosols.

Laboratory and field measurements have demonstrated that 2-methyl glyceric acid (2-MG) is the base component of a wide range of chemical species found in methacrolein-derived secondary organic aerosol (SOA). In order to explore the recently proposed hypothesis that a lactone oxidation intermediate is the origin of 2-MG and its derivatives in SOA, nuclear magnetic resonance techniques were used to study kinetics and reaction products of the aqueous phase reactions of a model lactone, ß-propiolactone (BPL). BPL was found to react with a lifetime of 4-10 h (depending on solution conditions) via a general acid catalyzed mechanism, which suggests that lactones similar to BPL are reactive on an atmospherically relevant time scale. BPL was also shown to form a variety of nucleophilic addition products (organosulfates and nitrates and oligomers) similar to the 2-MG-based species observed in previous experiments involving the photooxidation and SOA processing of methacrolein. While many of the BPL reaction products could be rationalized via an epoxide-like nucleophilic addition mechanism, evidence for ester-like nucleophilic addition was suggested through the observation of inorganic ion-catalyzed oligomer formation. The formation of oligomers was found to depend strongly on the proportion of organic acid nucleophile present in its deprotonated form. Therefore, due to the nature of the general acid catalysis and importance of deprotonated acids for efficient BPL oligomerization, it is suggested that oligomerization from lactone intermediates will be more efficient at higher SOA pH values. This result may help explain why overall isoprene-derived SOA formation has been observed to be largely pH-independent. Overall, the results strongly support the previous conclusion that a lactone intermediate is responsible for the formation of 2-MG-related species found in methacrolein-derived SOA.

Isolation and analysis of a baculovirus vector that supports recombinant glycoprotein sialylation by SfSWT-1 cells cultured in serum-free medium.

The inability to sialylate recombinant glycoproteins is a critical limitation of the baculovirus-insect cell expression system. This limitation is due, at least in part, to the absence of detectable sialyltransferase activities and CMP-sialic acids in the insect cell lines routinely used as hosts in this system. SfSWT-1 is a transgenic insect cell line encoding five mammalian glycosyltransferases, including sialyltransferases, which can contribute to sialylation of recombinant glycoproteins expressed by baculovirus vectors. However, sialylation of recombinant glycoproteins requires culturing SfSWT-1 cells in the presence of fetal bovine serum or another exogenous source of sialic acid. To eliminate this requirement and extend the utility of SfSWT-1 cells, we have isolated a new baculovirus vector, AcSWT-7B, designed to express two mammalian enzymes that can convert N-acetylmannosamine to CMP-sialic acid during the early phase of infection. AcSWT-7B was also designed to express a model recombinant glycoprotein during the very late phase of infection. Characterization of this new baculovirus vector showed that it induced high levels of intracellular CMP-sialic acid and sialylation of the recombinant N-glycoprotein upon infection of SfSWT-1 cells cultured in serum-free medium supplemented with N-acetylmannosamine. In addition, co-infection of SfSWT-1 cells with AcSWT-7B plus a conventional baculovirus vector encoding human tissue plasminogen activator resulted in sialylation of this recombinant N-glycoprotein under the same culture conditions. These results demonstrate that AcSWT-7B can be used in two different ways to support recombinant N-glycoprotein sialylation by SfSWT-1 cells in serum-free medium. Thus, AcSWT-7B can be used to extend the utility of this previously described transgenic insect cell line for recombinant sialoglycoprotein production.