Photoisomerization by Hula-twist. Photoactive biopigments*
Robert S. H. Liu
Department of Chemistry, University of Hawaii, Honolulu,
HI 96822, USA
Abstract: A review of literature on photoisomerization of bilirubin
and photoactive yellow protein suggests possible involvement of the
volume-conserving Hula-twist (HT) reaction mechanism in their primary
photochemical processes. Additional definitive experiments to verify
their involvement are proposed. Information related to photoproducts
of bacteriorhodopsin, rhodopsin, and phytochrome are reviewed. For carotenoids,
because of participation of the longer lived triplet state in, for example,
the photosynthetic apparatus, the involvement of HT is probably less
likely.
With the aim of extending the color fan of nature-identical food colorants
offered by Roche and therefore offering a less critical colorant to
the food industry, a project was initiated at Roche. The goal was to
find a safer, naturally occurring pigment with a color hue similar to
tartrazine.
This paper discusses the process of how such a project is addressed
in industry, as well as how promising candidates were selected from
the wide variety of the naturally occurring carotenoids. The syntheses
of some of these carotenoids will also be described.
*Lecture presented at the 13 th International Symposium
on Carotenoids, Honolulu, Hawaii, USA, 6-11 January 2002.
Other lectures are published in this issue, pp. 1369-1477.
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