Detailed spectroscopic study of the role of Br and Sr in coloured parts of the Callinectes sapidus crab claw

Abstract

The exoskeleton of crustaceans consists mainly of calcium carbonate (CaCO3) minerals and in many cases exhibits vivid colouration due to the presence of proteins rich in carotenoid chromophores. The exposure of aquatic animals in sea water results often in the incorporation of trace elements in their exoskeleton. The bonding configuration of Br and Sr trace elements in regions with different staining (white, orange and blue) of the exoskeleton of the Callinectes sapidus in crab claw are systematically investigated by a number of complementary spectroscopic techniques, including X-ray absorption fine structure spectroscopy (EXAFS), X-ray fluorescence, Raman and visible light reflectivity spectroscopies. It is found that Sr substitutes for Ca and the Sr/Ca ratio is constant along the claw. In the orange region that includes the claw fingers, CaCO3 adopts a calcite-like structure, whereas in the blue and white regions, located in the palm of the claw, an aragonite-like structure dominates. On the other hand, Br, present only in the blue and orange stained parts of the claw, is bound to phenyl and/or phenol rings of amino acid residues, most probably to phenylalanine and/or tyrosine, of the chromophore protein.

Color-specific porosity in double pigmented natural 3d-nanoarchitectures of blue crab shell

“…In line scan of elemental composition, EDX data constantly showed higher content of C associated to corresponding lower content of Ca, Mg and P, particularly in epicuticle, which mostly contains amorphous minerals and waxy lipoproteins 45 . None of the collected EDX spectra showed any trace of Br or Sr to support the previous consideration 33 on the role of these elements in blue crab coloration ( Supplementary Table S2). FWHM values of the main peak are inserted.…”

“…6A), which also follows the helicoid path orientation. This structure, which has nanometer-scale arrangements at subwavelength optics, might be essential for selective light waveguiding through the canals, making the blue shell absorbance peaking between 550-650 nm, red between 400-500 nm, while the white one above 600 nm, which corresponds with the diffuse reflectance data previously reported 9,33 . To further check any optical correlation, we measured a series of selected cuticle morphological features (Table 3).…”

“…2). All Raman bands recorded from the blue shell with 5 laser lines are listed in Table 1 and their assignments are provided according to our computational chemistry results and literature 11,12,15,17,24,33 .…”

Color-specific porosity in double pigmented natural 3d-nanoarchitectures of blue crab shell

“…This structure, which has nanometer-scale arrangements at subwavelength optics, might be essential for selective light waveguiding through the canals, making the blue shell absorbance peaking between 550-650 nm, red between 400-500 nm, while the white one above 600 nm, which corresponds with the diffuse reflectance data previously reported 9,33 . To further check any optical correlation, we measured a series of selected cuticle morphological features (Table 3).…”

“…We hypothesize an additional photonic color component to the overall blue cuticle appearance, sustained by its optical properties 9,33 (high reflectance between 380-480 nm, selective absorption 33 between 500-700 nm, weak light transmission of about 9.6% of red C. sapidus shell 33 ) and, yet limited, RR scattering data of one blue shell 33 . Exoskeleton photonic crystal would be sustained, provided that the ultrastructure periodicity influences the propagation of certain light wavelengths and its dielectric properties are tuned by its morphological biochemistry.…”