The Colours of Carotenoids

Nature did however not develop blue carotenoids by increasing the number of double bonds. The longest conjugated systems in natural carotenoids, as known so far, extends to 15 double bonds and corresponds to a red hue (Fig. 7.17). Examples are tetraterpenes such as 3,3 ,4,4 -tetrahydrolycopene (in unripe tomatoes) and 3,3 -dihydroxyisorenieratene (in Brevibacterium linens 

Nevertheless, based on relatively simple carotenoids, virtually a true kaleidoscope of colours, up to blue shades, can be found in Nature. A few oxidation products of natural carotenoids, such as violerythrin - 549 run in acetone) from actinioerythrol (isolated from sea anemones, A, ax = 500 run in acetone) and 3,4-dihydro-4,3-retro-jS-carotene-3,3-dione (A, = 563 nm in acetone) from 3,3 -dihydroxyisorenieratene (A ax = 460 nm in acetone) have a distinguished blue colour (Fig. 7.18 and Fig. 7.19). [39] While it takes just one reaction step, oxidation is not a preferred route how Nature affords blue hues. 

18 Brevibacterium linens is ubiquitously present, and populates also the human skin, where it causes e.g. foot odour. It is contained in the reddish smear coat ofRemoudou and other Belgian cheeses. The oxidation of its red pigment leads to a tremendous bathochromic shift (about 100 nm). 

20 Influence of aggregation structure and particle size on the colour shade of nano-dispersed -carotene hydro-sols. - Thereby, and by using blends of carotenoids, such as carotene, lycopene, S -apo-ji-carotenal and canthaxanthin, the colour of soft drinks can be adjusted from light yellow to red violet. 

A nice hypsochromic shift was also observed with one of the rare water-soluhle carotenoids crocin, the ingredient, which is primarily responsible for the colour of saffron (Fig. 7.21). The critical aggregation concentration in water was found to be 0.8 mg/ml. Upon increasing the concentration, a hypsochromic shift from Ajjjax = 445 to 410 nm was observed due to a change in the equilibrium between monomeric and aggregated forms (Fig. 7.22). The UVAfis spectra of crocin in water at concentrations below 0.8 mg/ml resemble those in organic solvents and represent the monomeric form (A jax = 445 nm). [41] 

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