Effects of thermal treatments on the structure of two black coral species chitinous exoskeleton

Abstract

Black corals (Antipatharians) are colonial cnidarians whose branched tree-like skeleton is constituted of chitin fibrils inside a lipoproteic matrix. The arrangement of the constituents of these materials provides a structure with outstanding physical properties. In this study, the structural properties of black coral skeletons of Antipathes caribbeana and Antipathes pennacea species are explored by means of thermal treatments in the range from room temperature up to 400 °C and the subsequent analysis using X-ray diffraction (XRD), infrared spectroscopy (FTIR), and thermal analysis (DSC/TGA). The effects of thermal treatment from room temperature up to around 210 °C induce the enhancement of the bands in the FTIR spectra and X-ray diffractograms, above that temperature, the FTIR and X-ray peaks become strongly attenuated. These effects are specially observed in the infrared bands associated to chitin at 3298 cm⁻¹ and to the secondary amide stretching around 1663 cm⁻¹, in particular, allowing the identification of the α-chitin in the black coral. XRD shows that the crystallinity index of the black coral chitin at room temperature is 24% and grows when the temperature increases, reaching a maximum value of 37% at 210 °C and decreases for higher temperatures. In addition, DSC and TGA measurements allowed identifying the most important transformation stages during the thermal treatments, namely, evaporation of water and the beginning and progress of degradation, depolymerization, and denaturation processes and finally, the degradation of the main functional groups of coral skeleton and coral chitin, in which the polysaccharide structure of chitin is depolymerized and the protein matrix is denatured.