Modeling red coral (Corallium rubrum) and African snail (Helixia aspersa) shell pigments: Raman spectroscopy versus DFT studies

Abstract

Pigments from red coral (Corallium rubrum) and African snail (Helixia aspersa) shell were studied non-invasively using Raman spectroscopy with 1064-nm laser beam. The two observed bands because of organic pigments confined in biomineralized CaCO 3 matrix at about 1500 and 1100 cm À1 were assigned to ν(C¼C) and ν(C-C), respectively. Both signals originate from polyene(s) of largely unknown structure, containing several conjugated C¼C bonds. The small peak at 1016 cm À1 in the Raman spectrum of coral pigment was assigned to in-plane -CH 3 rocking or structural deformation of polyene chain because of spatial confinement in the mineral matrix. The organic pigments in red coral and snail shell were present in inorganic matrix containing aragonite (shell) and calcite (coral). In addition, using Raman spectroscopy, it was observed that aragonite was replaced by calcite as result of healing damaged parts of snail shell. This is an important finding which indicates a great potential of nondestructive Raman spectroscopy instead of X-ray technique, as a diagnostic tool in environmental studies. To support analysis of the observed Raman spectra detailed calculations using density functional theory (DFT with B3LYP and BLYP density functionals) on structure and vibrations of model all-trans polyenes were undertaken. DFT calculated C¼C and C-C stretching frequencies for all-trans polyenes containing from 2 to 14 C¼C units were compared with the observed ν(C¼C) and ν(C-C) band positions of the studied coral and shell. Individual correction factors were used to better match theoretical wavenumbers with observed band positions in red coral and African snail. It was concluded that all-trans polyene pigments of red coral and dark parts of African snail shell contain 11-12 and 14 C¼C double bond units, respectively. However, Raman spectroscopy cannot produce any clear information on the presence and nature of the end-chain substituents in the studied pigments.

Multi-methodological Characterisation of Medieval to Early Modern Coral Beads from a Cesspit of the “Fronerei auf dem Schrangen” in Lübeck (Germany)

“…Raman spectra of all investigated beads and original corals show bands at n2 = 1128 -1130 cm -1 and n1 = 1516 -1521 cm -1 , which are typical for pink and red corals (Hedegaard, Bardeau and Chateigner, 2006;Urmos, Sharma and Mackenzie, 1991;Karampelas, et al, 2009;Kupka, et al, 2010;2016a;2016b;Fan, 2018). The N eff of the pigment polyenes ranging from 10.4 to 10.6 of the beads are in agreement with the data of corallium rubrum.…”

Raman spectroscopy of natural and cultured pearls and pearl producing mollusc shells

“…[30] Similar Raman bands due to demethylated carotenoids occur for natural and cultured pearls found into other molluscs as well as shells of various bivalves and gastropods, corals, parrot feathers, fungi, and bacteria. [3,7,[16][17][18][19][20][21][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] Thirty out of 45 of our natural and cultured pearls (Table S1) and all 11 shells (Table S2) are found to present pigments with analogous polyenic structure. In total, samples from 17 different mollusc species were found to be coloured by demethylated carotenoids.…”

“…[8,21] Raman spectroscopy shows that the vast majority of natural and cultured pearls present more than one polyenic pigments and that different colours are explained by different mixtures and not by a single pigment. [18][19][20][21] These pigments are rarely encountered elsewhere in nature with examples including some mollusc shells, [16,17,43] corals, [7,20,[40][41][42][43][44][45][46][47][48][49][50] parrot feathers, [32,51,52] fungi, [53] and bacteria. [54] The difference between carotenoids and polyenic pigments is established through Raman scattering based on ν 2 vibration, which for carotenoids is shifted by around 25 cm −1 to higher wavenumbers due to methyl group attached to the polyenic chain.…”

Recent advances in linear and non‐linear Raman spectroscopy. Part XI

“…Kupka et al described the modeling of red coral ( Corallium rubrum ) and African snail (Helixia aspersa) shell pigments by means of the comparison of measured Raman versus calculated DFT spectra. It was concluded that all‐trans polyene pigments of red coral and dark parts of African snail shell contain 11–12 and 14 C=C double bond units, respectively; however, Raman spectroscopy cannot produce any clear information on the presence and nature of the end‐chain substituents in the studied pigments . Lofrumento et al described the identification of dyes in toned and tinted XX century cinematographic films by SERS.…”