An antioxidant is a substance that at low concentrations delays or prevents oxidation of a substrate. Antioxidant compounds act through several chemical mechanisms: hydrogen atom transfer (HAT), single electron transfer (SET), and the ability to chelate transition metals. The importance of antioxidant mechanisms is to understand the biological meaning of antioxidants, their possible uses, their production by organic synthesis or biotechnological methods, or for the standardization of the determination of antioxidant activity. In general, antioxidant molecules can react either by multiple mechanisms or by a predominant mechanism. The chemical structure of the antioxidant substance allows understanding of the antioxidant reaction mechanism. This chapter reviews the in vitro antioxidant reaction mechanisms of organic compounds polyphenols, carotenoids, and vitamins C against free radicals (FR) and prooxidant compounds under diverse conditions, as well as the most commonly used methods to evaluate the antioxidant activity of these compounds according to the mechanism involved in the reaction with free radicals and the methods of in vitro antioxidant evaluation that are used frequently depending on the reaction mechanism of the antioxidant.

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Phenolic compounds are secondary metabolites found most abundantly in plants. These aromatic molecules have important roles, as pigments, antioxidants, signaling agents, the structural element lignan, and as a defense mechanism. The expression of phenolic compounds is promoted by biotic and abiotic stresses (e.g., herbivores, pathogens, unfavorable temperature and pH, saline stress, heavy metal stress, and UVB and UVA radiation). These compounds are formed via the shikimate pathway in higher plants and microorganisms. The enzymes responsible for the regulation of phenolic metabolism are known, and shikimic acid is a central metabolite. The shikimate pathway consists of seven reaction steps, beginning with an aldol-type condensation of phosphoenolpyruvic acid (PEP) from the glycolytic pathway, and D-erythrose-4-phosphate, from the pentose phosphate cycle, to produce 3-deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP). A key branchpoint compound is chorismic acid, the final product of the shikimate pathway. The shikimate pathway is described in this chapter, as well as factors that induce the synthesis of phenolic compounds in plants. Some representative examples that show the effect of biotic and abiotic stress on the production of phenolic compounds in plants are discussed.

Mexican Jalap roots, a prehispanic medicinal plant complex still considered to be a useful laxative, can be found as an ingredient in some over-the-counter products sold by herbalists in contemporary Mexico. The drug is prepared from the dried roots of several morning glories, all of which have been identified as members of the genus Ipomoea. Analysis of several commercial samples was assessed by generating HPLC and 13C NMR spectroscopic profiles of the glycosidic acids obtained through saponification of the resin glycoside contents. These profiles distinguish the three Mexican jalaps currently in frequent use and can serve as analytical tools for the authentication and quality control of these purgative herbal drugs. Ipomoea purga, the authentic "jalap root", yielded two new hexasaccharides of convolvulinic and jalapinolic acids, purgic acids A (1) and B (2), respectively. Scammonic acid A (3), a tetrasaccharide, was produced from Ipomoea orizabensis, the Mexican scammony or false jalap. Operculinic acid B (4), a pentasaccharide, was identified in Ipomoea stans. Semipreparative HPLC was performed to obtain pure samples of new compounds 1 and 2 in sufficient quantity to elucidate their structure by high-field NMR spectroscopy. Purgic acid A (1) was identified as (11S)-hydroxytetradecanoic acid 11-O-beta-D-quinovopyranosyl-(1-->2)-O-beta-D-glucopyranosyl-(1-->3)-O-[beta-D-fucopyranosyl-(1-->4)]-O-alpha-L-rhamnopyranosyl-(1-->2)-O-beta-D-glucopyranosyl-(1-->2)-O-beta-D-quinovopyranoside, while purgic acid B (2) was characterized with (11S)-hydroxyhexadecanoic acid as its aglycon but having the same glycosidation sequence in the oligosaccharide core.

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An extensive investigation of the so-called jalapin resinoid obtained from roots of the Mexican scammony, Ipomoea orizabensis, using high field NMR spectroscopy led to the characterization of six glycosides, including the known scammonins I (1) and II (2) and four new tetrasaccharides of jalapinolic acid, orizabins V-VIII (3-6). All the isolates (1-6) were found to be weakly cytotoxic toward human oral epidermoid carcinoma (KB).

Microorganisms are recognized worldwide as the major source of secondary metabolites with mega diverse structures and promissory biological activities. However, as yet many of them remain little or under-explored like the microbiota from freshwater aquatic ecosystems. In the present review, we undertook a recompilation of metabolites reported with pesticidal properties from microalgae (cyanobacteria and green algae) and fungi, specifically from freshwater aquatic habitats.

Despite microalgae recently receiving enormous attention as a potential source of biodiesel, their use is still not feasible as an alternative to fossil fuels. Recently, interest in microalgae has focused on the production of bioactive compounds such as polyunsaturated fatty acids (PUFA), which provide microalgae a high added value. Several considerations need to be assessed for optimizing PUFA production from microalgae. Firstly, a microalgae species that produces high PUFA concentrations should be selected, such as Nannochloropsis gaditana, Isochrysis galbana, Phaeodactylum tricornutum, and Crypthecodinium cohnii, with marine species gaining more attention than do freshwater species. Closed cultivation processes, e.g., photobioreactors, are the most appropriate since temperature, pH, and nutrients can be controlled. An airlift column with LEDs or optical fibers to distribute photons into the culture media can be used at small scale to produce inoculum, while tubular and flat panels are used at commercial scale. Depending on the microalgae, a temperature range from 15 to 28 °C and a pH from 7 to 8 can be employed. Relevant conditions for PUFA production are medium light irradiances (50-300 μmol photons m(-2) s(-1)), air enriched with (0-1 % (v/v) CO2, as well as nitrogen and phosphorous limitation. For research purposes, the most appropriate medium for PUFA production is Bold's Basal, whereas mixotrophic cultivation using sucrose or glucose as the carbon source has been reported for industrial processes. For cell harvesting, the use of tangential flow membrane filtration or disk stack centrifugation is advisable at commercial scale. Current researches on PUFA extraction have focused on the use of organic solvents assisted with ultrasound or microwaves, supercritical fluids, and electroporation or are enzyme assisted. Commercial-scale extraction involves mainly physical methods such as bead mills and expeller presses. All these factors should be taken into account when choosing a PUFA production system, as discussed in this review.