Introduction biological targets, availability

Although only three carotenoids are commercially synthsised for use as permitted food colourants, namely p-carotene, apocarotenal and canthaxanthin, the availability of nature-identical counterparts of the natural products has always attracted interest. For the study of biological properties and for chromatographic studies access to synthetic versions is highly desirable. In the course of the isolation of the natural product its total synthesis finally always remained historically as a target. Such chemical work had invariably been carried out well before the introduction of the permitted range concept focussed attention on certain natural products... 

Y nuclei such as Si, and by cross-polarization (a process called indirect DNP). DNP-NMR has first been successfully applied to biologically relevant molecules and more recently in materials science to an increasingly large range of nuclei [8-13]. Efforts have been targeted to the introduction of new radicals [14,15], especially biradicals, to reach higher DNP enhancement factors in terms of sensitivity. Commercial equipment is now available. Examples illustrating the potentialities of DNP experiments to characterize sol-gel materials are presented in Section 20.4. 

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