Carotenoids

Dear Colleagues,

Carotenoids are, by definition, polyenes of 6-14 conjugated double bonds, stabilized by methyl groups and cyclohexene endrings. Straight chain polyenes without these stabilizations only reach 5 double bonds. The major part of the 750 natural carotenoids occurs with 9-11 conjugated double bonds, which may be the effective range regarding stability and activity. Such carotenoids are given as prime examples in textbooks to explain UV-VIS spectroscopy. Carotenoids with a considerably increased number of double bonds have been synthesized to approach λ_∞, the magic absorption limit.

Carotenoids participate in photosynthesis and are, therefore, biosynthesized in plants and alga. Although photosynthesis may appear a long procedure taking place during hours of sunlight exposition the photophysical examination of light absorbing by carotenoids has reached the incredible short span of femtoseconds. In water based environments carotenoids aggregate to micelles and vesicles; increasing effort is directed to study these nano sized structures. Carotenoid chemists have traditionally focused on structure elucidation and on synthesis and various natural occurring carotenoids are also available by synthesis. Eight carotenoids are offered industrially in considerable amounts and are marketed as food and feed colors. Since most carotenoids are hydrophobic, they add to proteins for passing aqueous environments. Hydrophobicity also obstructs an all-purpose assignment of carotenoids and they need extensive formulation before introduced in water-based applications, e.g. as food colors. The pharmacological importance of carotenoids has been limited; a few modified carotenoids for the treatment of cardiovascular and ophthalmological diseases pass now clinical or pharmacokinetic studies. Carotenoids function as molecular wires and modified carotenoids have been developed for electrochemical and spectroscopic research.

Carotenoids are best known as antioxidants. Nevertheless, it is still not clear whether the intact carotenoid is the active agent or one of its oxidative degradation products. A practical impact of carotenoids to the development of chemistry cannot be underestimated: the purification method “chromatography” was invented with these colorful compounds. Carotenoids are the subject of substantial interdisciplinary research. It is hoped that the Special Issue of Molecules attract researchers who present the various aspects of carotenoid research.

Hans-Richard Sliwka
Guest Editor