Determination by Raman scattering of the nature of pigments in cultured freshwater pearls from the mollusk Hyriopsis cumingi

Abstract

The nature of pigments in naturally colored pearls is still under discussion. For this study, Raman scattering measurements were obtained for 30 untreated freshwater cultured pearls from the mollusk Hyriopsis cumingi covering their typical range of colors. The originality of this work is that seven different excitation wavelengths (1064 nm, 676.44 nm, 647.14 nm, 514.53 nm, 487.98 nm, 457.94 nm, 363.80 nm) are used for the same samples at the highest possible resolution. All colored pearls show the two major Raman features of polyenic compounds assigned to double carbon-carbon (C C) -at about 1500 cm −1 -and single carbon-carbon (C-C) -at about 1130 cm −1 -bond stretching mode, regardless of their specific hue. These peaks are not detected in the corresponding white pearls, and therefore seem directly related to the major cause of body color. Additionally, the exact position of C C stretching vibration shows that these compounds are not members of the carotenoid family. Moreover, some changes are observed in intensities, shape and positions of the two main characteristic polyenic peaks from one sample to the next. Similar changes are observed also using several excitation wavelengths for the same point of the same pearl. The exact position of C-C stretching vibration of polyenic molecules depends strongly on the number of double bonds (N) contained in their polyenic chain. Hence, using a constrained decomposition of this band for different excitation wavelengths, up to nine different pigments may be detected in the same pearl. Their general chemical formula is R-(-CH CH-) N -R with N = 6-14. All our colored samples contained at least four pigments (N = 8-11). Different colors are explained by different mixtures, not by a simple change of pigment. The chemical nature of the chain ends is still unknown, because it cannot be detected with Raman scattering. However, it is possible that these polyenes are complexed with carbonate molecules of the nacre. Similar coloration mechanisms are found in products from other living organisms (e.g. parrots feathers). Moreover, it seems that a similar series of pigments is found in other pearls also, as well as in some marine animals living in similar environments (e.g. corals).

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

“…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.…”

Micro‐Raman spectroscopy of pigments contained in different calcium carbonate polymorphs from freshwater cultured pearls

“…psittacolfulvins, carotenoids or porphyrins. 2 -13 Numerous studies 3,6 investigated the pigments associated with aragonitic pearls, and the recent discovery of the metastable CaCO 3 polymorph vaterite in freshwater cultured pearls 14,15 now offers the first opportunity for an in-depth investigation of the pigments related to a different calcium carbonate polymorph.…”

Colour or no colour in the juvenile shell of the black lip pearl oyster, Pinctada margaritifera?

“…More recently, Xinnong et al (1997) report "that the CaCO 3 crystal orientation is responsible for the colour of nacreous layer or pearl" in Hyriopsis cumingii. Karampelas et al (2007) show that the various colours of the pearls built by the same species are due to different ratios of organic pigments. It is probable, that both components, mineral and organic are involved in colours.…”