Abstract
Tortoiseshell is a proteinaceous material derived from the scutes of marine turtles, and was shaped into an abundance of objects, especially luxurious items, at its peak in the seventeenth and eighteenth century. It has continued to be used even after the advent of plastics and remains one of the main causes of illegal poaching of marine turtles, in particular the hawksbill turtle Eretmochelys imbricata. Tortoiseshell is made of structural proteins, of which the most abundant are known as β-keratins, or ‘corneous beta-proteins' (CBPs), a family of short proteins containing a central structure in β-sheets. There are, however, few CBP sequences of marine turtles in protein databases. The scutes of the five main species of marine turtles (Chelonia mydas, Caretta caretta, Eretmochelys imbricata, Lepidochelys olivacea and Lepidochelys kempii) were analysed by proteomics, using nano-liquid chromatography-Orbitrap-mass spectrometry to generate peptidic markers for species identification. A total of 187 marker sequences were identified, the large majority of them obtained from automated de novo sequencing. The sequences were classified into peptides A to F: A to D at the N-terminus and central region that forms the β-pleated sheets, E1–4 for a variable region of glycine-repeats region and F at the C-terminus. The markers were tested against a set of combs discovered in various archaeological sites of modern period in France, successfully identifying hawksbill turtle and highlighting patterns of degradation in archaeological tortoiseshell.
Highlights
IntroductionMarine turtles have been exploited for their bones, meat and their most prized appendage, the epithelial layer of the scutes that is commonly referred to as tortoiseshell [1]
Recent research has shown that biomolecular methods had potential to identify the turtle species after tortoiseshell was made into an object [12], and even the geographical origin of the turtle based on mitochondrial DNA haplotypes (Atlantic and Indo-Pacific stocks in hawksbill turtle [38])
Most of peptide D belongs to the portion of the sequence containing the β-sheet; for lack of lysine or arginine in this long part of the corneous betaproteins’ (CBPs), it is arbitrarily stopped before the glycine-rich region
Summary
Marine turtles have been exploited for their bones, meat and their most prized appendage, the epithelial layer of the scutes that is commonly referred to as tortoiseshell [1]. It is obtained predominantly from the hawksbill turtle, for which the high demand has contributed to the major decline of the species [2]. Sought after as an exotic material, tortoiseshell was used in a variety of small items such as combs, knife handles, fans, cases and other boxes, in bookbinding, and as inlays for furniture [5,6], and transformed into luxurious objects in the seventeenth and eighteenth century [6,10]. Less frequently recovered from archaeological excavations, in part due to the fast decay of the tissue in the soil and possibly lack of proper identification, it has been found in the form of scraps or ancient comb fragments in modern periods [11,12]
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