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Part I Physical and Chemical Properties of Carotenoids
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CHAPTER 1
Structures, Nomenclature and General Chemistry of Carotenoids and Their Esters
Antonio J. MelĂ©ndez-MartĂnez*a , Paula Mapelli-Brahm a , DĂĄmaso Hornero-MĂ©ndez b and Isabel M. Vicario a
a Food Colour & Quality Laboratory, Area of Nutrition & Food Science, Facultad de Farmacia, Universidad de Sevilla, Seville, 41012, Spain
b Instituto de la Grasa â CSIC, Department of Food Phytochemistry, University Campus Pablo de Olavide, Building 46, Seville, 41013, Spain
*E-mail:
[email protected] Carotenoids are widespread isoprenoids that intervene in actions ranging from the collection of light and photoprotection to the regulation of gene expression and communication within or between species, to mention only some examples. They are, therefore, much more than natural pigments, as they are versatile compounds that are eliciting increasing interest in different disciplines, such as plant science, agriculture, food science and technology, nutrition and health, among others. Although carotenoids in humans are found almost exclusively free, in foods they can be associated with other molecules, like sugars, proteins or fatty acids. Such associations can result in substantial changes in their properties. Indeed, food xanthophylls are commonly found in the form of esters, above all in many fruits. This modification markedly influences properties such as solubility and susceptibility to oxidation, which in turn can have impacts on relevant aspects that explain their levels in foods and humans, such as their biosynthesis, deposition, stability and bioavailability, among others. The study of the esterification of carotenoids is undoubtedly gaining popularity. In this chapter, nomenclature and structural aspects related to isoprenoids, carotenoids, fatty acids and, finally, carotenoid esters are presented, with references to some physicalâchemical properties and their importance at different levels.
1.1 Introduction
Carotenoids are widespread isoprenoids in nature that intervene in many actions ranging from the collection of light and photoprotection to the regulation of gene expression and communication within or between species, to mention some examples. 1 Interestingly, they can be chemically or enzymatically converted into other derivatives that can act as compounds with vitamin activity, phytohormones or aromas, among others. 2
Carotenoids are, therefore, much more than natural pigments providing mainly yellow, orange or red colours. Indeed, they are very versatile compounds that are eliciting increasing interest in different disciplines, such as plant science, agriculture, food science and technology, nutrition and health, among others. In relation to these three latter disciplines, although their roles as natural pigments and precursors of retinoids with vitamin A activity have long been known, the renewed interest in these compounds is mainly due to a large body of evidence accumulated in the last 30 years indicating that they may be health-promoting compounds and may be important in the context of functional foods. 3 Thus, carotenoids are thought to contribute to reducing the risk of developing certain types of cancers, as well as cardiovascular, eye, skin or bone diseases, and are even thought to be beneficial to cognitive function. 4,8 Furthermore, carotenoids can provide cosmetic benefits. 9 There is evidence that such cosmetic benefits can lead certain populations to increase their intake of carotenoid-containing products, which can be used in the context of public health as a strategy to promote the consumption of fruits and vegetables. 10 Although the beneficial health-promoting effects of carotenoids are often attributed to their possible role as antioxidants, other underlying mechanisms should be considered (e.g. pro-oxidant or anti-inflammatory actions or modulation of membrane properties, among others). 4,11,17 Interestingly, carotenoids and/or their derivatives can play essential roles in cell signalling pathways, such as by interacting with transcription factors such as nuclear factor erythroid 2-related factor 2 (Nrf2) 18,19 or nuclear factor-ÎșB (NF-ÎșB). 20,21
Although carotenoids in human fluids and tissues are found almost exclusively free (although sometimes carotenoid esters have been reported in plasma and skin 22,23 at levels markedly lower compared to unesterified ones, and recently in the colostrum, but not in mature human milk 24 ), in foods they can be associated with other molecules, such as sugars, proteins or fatty acids. Such associations can result in significant changes in their properties. Indeed, it is very common for food xanthophylls to be in the form of esters, especially in many fruits. Although esterification does not have an impact on the carotenoid chromophore and therefore on its colour, it can markedly modify its solubility and susceptibility to oxidation, which in turn can have an impact on relevant aspects that explain their levels in foods and humans, such as their biosynthesis, deposition, stability and bioavailability, among others. 25 The study of the esterification of carotenoids is undoubtedly gaining popularity and has been greatly facilitated by important advances in analytical techniques that make possible the separation and identification of a great variety of carotenoid esters. 26
In this chapter, nomenclature and structural aspects related to fatty acids, isoprenoids, carotenoids and, finally, carotenoid associations with other molecules, with an emphasis on carotenoid acyl esters, are dealt with, referring to some physicalâchemical prope...
