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Composition of tomatoes and tomato products in antioxidants (WG1) page 63
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80% of total lycopene is found as cis isomers in prostate tissue. 7,9,7’,9’- cis-lycopene is the
naturally occuring form of lycopene in fresh tangerine-type tomatoes.
The trans-cis interconversion is thought to occur with the absorption of light, exposure
to heat, or by participation in specific chemical reactions, for instance during metabolisation
in vivo. Very little is known about formation and specific roles of cis isomers in the vertebrate
biology.
7.1.3.3 Physico-chemical properties
Cis isomers of lycopene have distinct physical characteristics and chemical behaviours
from their all-trans counterpart. Some differences observed as a result of a trans to cis
isomerization reaction include :
- lower melting points
- decreased color intensity
- lower extinction coefficient (hypochromic effect)
- a displacement of λmax to shorter wavelength (hypsochromic effect)
- a reduction in vibrational fine structure
- the appearance of a new absorption band in the ultraviolet region so-called
the « cis-peak ». It is at a characteristic position about 142 nm below the
longest-wavelength absorption maximum (III) in the spectrum of the alltrans
compound, when measured in hexane.
For di-cis and poly-cis carotenoids, a large hypsochromic shift in the main absorption
band may be seen. Prolycopene (7,9,7’,9’-cis-lycopene, see fig 8), for example, has λmax at
414, 436, 463 nm, in contrast to 444, 470, 502 nm for all-trans-lycopene, and the spectral fine
structure is also greatly reduced.
Among the observed cis-isomers, some have been characterized
spectrophotometrically ( Table 23).