Infrared spectroscopy and X ray diffraction study on the morphological variations of carbonate and phosphate compounds in giant prawn ( Macrobrachium rosenbergii ) skeletons during its moulting period

Abstract

A systematic study has been carried out on the quantitative and morphological variations of carbonate and phosphate compounds in giant prawn (Macrobrachium rosenbergii) skeletons during the moulting period on the basis of infrared spectroscopy and X ray diffraction (XRD) analyses. Skeletons samples were prepared from adult giant prawns, extracted from the intact skeletons of the prawns at the ages of 4, 8, 12, 16, 20, 24, 30 days after moulting, as well as from the exuviae skeletons. Their phosphate bands were compared to those of synthetic hydroxyapatite (HAP) and human enamel, while their carbonate bands were compared to those of coral (oculina sp) and sea urchin (psammechinus miliaris) skeletons. It is well known that mineral compounds in human enamel consist mainly of calcium phosphates with a small amount of carbonates, while those found in coral and sea urchin skeletons consist mainly of calcium carbonates, coexisting with significant amount of magnesium. In contrast to those compositions, the spectroscopic data presented in this work display a strong indication that comparable amounts of calcium carbonates and calcium phosphates do exist in giant prawn skeletons during most of their moulting period. Based on the infrared analysis of the carbonate bands it is further suggested that calcium carbonates experience partial conversion from the amorphous to the crystal phase toward the end of the moulting cycle, as confirmed by similar trend exhibited in XRD data. On the other hand, the phosphate bands in giant prawn skeletons were found to be attributed to a mixture of amorphous and microcrystal phases without a clear contribution from apatite phase throughout the moulting period. This is also consistent with the pattern displayed by the XRD profiles. The lack of evidence for the presence of apatites could be understood on the basis of interfering and competing effects induced by the presence of various ions other than calcium phosphate ions, as well as the relatively high susceptibility of calcium phosphates to the associated substitutional effects.