Approaches to the analysis of plant-derived natural products

Abstract

The term plant-derived natural product is extremely broad and the scope of this chapter is determined by the nature of lower-abundance secondary metabolites rather than storage proteins, starch, cell walls, and lipids. In some instances, however, similar techniques can used to measure both groups of compounds. The bioactivity of secondary metabolites underlines their importance in human nutrition, health, pharmacy and plant defence mechanisms, and is the basis for their commercial value. Consequently, these two features are the driving force behind the continuing development of techniques for their analysis. The most important recent advance has been the advent of metabolomics. The metabolome, by analogy to the genome, proteome and transcriptome, is the entire small-molecule complement of the plant. The study of the metabolome, metabolomics, cannot be achieved by any single method and is largely a consequence of recent improvements to technology permitting high throughput analyses and data-handling. Interest in understanding details of natural product biosynthetic pathways as well as a desire to measure the bioactive end-product means, however, that it is necessary to cover methods of sufficient sensitivity to detect low-abundance intermediates as well as those methods that can investigate metabolite structure. Hence this chapter represents an introduction and overview of the fundamentals that underlie the wide range of methods used in the quantification of plant-derived natural products and a brief introduction to metabolomics. It is hoped that a strong understanding of the fundamentals will allow the reader to judge from the plethora of manufacturers' brochures, primary literature, and on-line resources, which technologies and approaches best fit their situation. The process of analysis will be followed through its stages, from extraction of material to detection of analytes. Methods have been chosen not only with the chemistry of the analyte in mind, but also with a firm idea of the biological question that is to be answered. There will be an unashamed bias towards chromatography and mass spectroscopy since plants are complex systems containing many interesting chemicals, often in low amounts. Chromatography can simplify the process of addressing a complex mixture, and mass spectroscopy yields rich information from low-abundance analytes. For a brief list of methods for the main secondary metabolites, consult Table 1.