Oxidation biogenic compounds

Precursors of membrane components, carbohydrates, amino acids, and other biogenic compounds formed either directly or indirectly from those chiral and non-chiral molecules delivered to Earth. Benzene, for instance, is one of the aromatic compounds of interstellar dust from which a precursor of the membrane lipid inositol-1-phosphate apparently derived by oxidation in a long series of chemical reactions. The structure of the important lipid, a derivative of inositol-phosphate in Fig. 6.1, displays two different fatty acid side chains (marked in blue and green) that frequently occur in membranes. 

A substantial amount of oxygen is expended for oxidizing sulphur. A larger portion of sulphur comes from rock minerals as native sulphur, sulphides and abiogenic H S and smaller part, as biogenic S or H S. Oxidizing these compounds is accompanied by loss of electrons up to the formation of sulphates ... 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

These compounds have an aldehyde group attached to an alkane ring. Their reactions were studied by D Anna et al. (2005b) in order to understand the reactions of similar compounds formed in the oxidation of biogenic compounds e.g., norpinonaldehyde, formed in the oxidation of a-pinene. They used chamber experiments, coupled with FTIR and PTRMS to study the products and infer the mechanism, obtaining carbon balances in the range 89-97%. 

Carbonyl compounds are key intermediates in the oxidation of biogenic compounds. For example, pinonaldehyde is a first generation intermediate following addition of OH to a-pinene. The oxy radical formed following reaction with O2 and then NO rapidly... 

IV-F-5. Aldehydes Formed in the Oxidation of Biogenic Compounds Pinonaldeyde... 

In this chapter, we discuss the rate coefficients and the mechanisms of oxidation of ketones. The classes covered include alkanones, hydroxyketones, diketones, unsaturated ketones, ketenes, cyclic ketones, ketones derived from biogenic compounds, and halogen-substituted ketones. Photolysis is a major atmospheric process for many ketones, and will be discussed in chapter IX. The major bimolecular reactions removing ketones from the atmosphere are with OH. Although less important than the OH... 

A number of ketones are formed in the oxidation of biogenic compounds, and especially those terpenes with an exocyclic double bond, as illustrated by the formation of nopinone from /3-pinene following reaction with OH ... 

Psilocybin (Figure 3.5a) and psilocin (Figure 3.5b) are indole derivatives substituted in position 4 by a hydroxyl group, where psilocybin is phosphory-lated. Due to its ionic properties, psilocybin is soluble in water. In addition, phosphorylation protects psilocybin from oxidative degradation. Both compounds are found to affect laboratory animals, but there is evidence that only the dephosphorylated form, psilocin, is the active species. In their structure the toxins resemble serotonine, a biogenic amine known to be a neurotransmitter. 

Biogenic processes, however, emit reduced forms of sulfur, including dimethyl sulfide and hydrogen sulfide, with lesser amounts of carbon disulfide (CS2), dimethyl disulfide (CH3SSCH3), carbonyl sulfide (COS), and methyl mercaptan (Cl I3SH). These reduced sulfur compounds are then oxidized in the atmosphere as described in detail in Chapter 8.E. 

Although some of the biogenic VOCs are relatively simple compounds such as ethene, most are quite complex in structure (e.g., Figs. 6.22 and 6.26). Furthermore, they tend to be unsaturated, often with multiple double bonds. As a result, they are very reactive (see Chapter 16.B) with OH, 03, NO, and Cl atoms (e.g., Atkinson et al., 1995a). In addition, because they are quite large and of relatively low volatility, their polar oxidation products are even less volatile. This makes elucidating reaction mechanisms and quantifying product yields quite difficult. For a review of this area, see Atkinson and Arey (1998). 

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