Role of polyenes in the coloration of cultured freshwater pearls

Abstract

The exact nature of pigments present in cultured freshwater pearls is still not well known. We examined 21 untreated cultured freshwater pearls from Hyriopsis of typical colors by diffuse reflectance UV-Vis-NIR and Raman scattering measurements, at high resolution. The objective was to establish the relation between color and the nature of the pigment mixture in pearls, using strictly non-destructive methods. All natural color samples show the two major Raman resonance features of unmethylated (unsubstituted) polyenes, not carotenoids. Their general chemical formulae are R-(-CH¼CH) N-R 0 with N ¼ 6 to 14 and they give absorptions from violet to yellow-green. Each color is due to a mixture of pigments, not a single pigment. Different colors are explained by different mixtures. Each pigment identified by Raman spectroscopy can be related to a specific absorption with apparent maximum in the range 405-568 nm, thus absorbing from violet to yellow-green. This is the first study of the precise relation between Raman and diffuse reflectance spectra of cultured freshwater pearls.

The effects of environment on <i>Arctica islandica</i> shell formation and architecture

“…The ingestion of pigment-enriched microalgae potentially leads to an accumulation of pigments in mollusk tissues and the shell (Soldatov et al, 2013). On the other hand, it has been argued that polyenes do not generate from food sources like other pigments (i.e., carotenoids), but they are locally synthesized (Karampelas et al, 2009). In accordance to Stemmer and Nehrke (2014), the results presented here support the view that the specific diets on which the animals rely on do not influence shell pigment composition.…”

“…Furthermore, an enrichment in polyenes has been observed in the growth lines, potentially indicating their involvement in the biomineralization process (Stemmer and Nehrke, 2014). However, the specific functions of these organic compounds have not been disclosed yet (Hedegaard et al, 2006;Karampelas et al, 2009). Given the high phenotypic variation in pigmentation among and within mollusk species, it has been proposed that coloration does not have a primary function as an adaptive tool (i.e., camouflage, warning signaling) as in other animals (Seilacher, 1972;Evans et al, 2009).…”

Conjugated polyenes as chemical probes of life signature: use of Raman spectroscopy to differentiate polyenic pigments

“…[21,25]. Previous works from Maia et al [27,29] and comparison with the literature data [13] have demonstrated that the red octocoral L. punicea may contain 11 conjugated double bonds due to the presence of ν(C=C) modes at ca 1509 cm −1 .…”

“…In this way, under resonance or pre-resonance conditions (λ 0 = 632.8 nm), there is clearly a preferred signal enhancement for the bands at 1504 and 1150 cm −1 . This effect, known as the resonance Raman effect, can also be used for the study of several conjugated polyenes present in the most different samples, such as corals [25,49], the shells of molluscs [24], pearls [21,50], plants [41], etc. It is also clear that the presence of the band at 1008 cm −1 , related to the δ(C−CH 3 ) mode, occurs as one of the enhanced modes in the Raman spectra of carotenoids, according to figure 4, in which it can be seen in the spectrum of pure astaxanthin.…”

“…Non-carotenoid linear conjugated polyenes, such as polyenals (or unsubstituted polyacetylenes) and laetiporic acids, are not as diverse as carotenoids, and they are restricted to a few taxonomic groups. For example, polyenals containing from six to nine conjugated double bonds, named psittacofulvins, were identified from parrots [12]; polyenals containing from six to 14 double bonds were described for pearls [20,21], mollusc shells [22][23][24] and octocorals [25][26][27][28][29][30]; and laetiporic acids [13] containing 10-12 conjugated double bonds were found in fungi. Synthetic polyenals present antioxidant, antitumoural and anti-inflammatory activity [31], and laetiporic acids are potential food colorants [13].…”