Getting to know carotenoids

More than 600 different carotenoids have been isolated from natural sources and fully or partly characterized. Fortunately, although they are all different individual compounds, the carotenoids may be considered as a group in terms of many of their basic properties. The aim of this chapter is to help newcomers and others that are not carotenoid specialists to understand the peculiarities of carotenoids, and how best to work with them. The basic principles of structure and nomenclature of carotenoids are described (especially the IUPAC semi-systematic rules) and the chemical and physical properties on which the biological functions depend are discussed. Guidance is given on how to handle and work with carotenoids safely and reliably and reduce the risk of forming artifacts and introducing impurities. Recommendations are made about their isolation, identification by various physical methods, and spectrophotometric quantitative analysis. Structural features that determine the shape of the carotenoid molecule are discussed, especially the conjugated polyene system. This also determines the light absorption properties of carotenoids and their chemical reactivity, especially toward oxidising agents. The properties of carotenoids in situ are strongly influenced by aggregation in the aqueous environment and by interactions with other molecules in their vicinity, notably proteins and membrane lipids. Many substances that look like fragments of carotenoids have been isolated from natural sources. This chapter does not provide comprehensive coverage of these apocarotenoids, but does consider their nomenclature and numbering according to the IUPAC rules.