Spectroscopic studies for identifying the chemical states of the periostracum of the Corbicula species in Lake Biwa

• Published in: Journal of structural biology 2023-03, Vol.215 (1), p.107944-107944, Article 107944
• Main Authors: Takemoto, Kuniko, Murakami, Masataka, Tabuchi, Masao, Ohta, Toshiaki
• Format: Article
• Language: English
• Subjects: Animals; Chemical Phenomena; Corbicula; Corbicula clam; DOPA-Fe complex; Lake Biwa; Lakes; Raman spectroscopy; Spectrum Analysis, Raman; XAFS
• ISSN: 1047-8477; 1095-8657
• DOI: 10.1016/j.jsb.2023.107944

[Display omitted] •Fe K-edge XAFS and Raman spectroscopies were used to elucidate the chemical state of the periostracum of the Corbicula species in Lake Biwa.•The native periostracum consists of quinone-based polymers and is responsible for developing the yellow color.•Tris-DOPA-Fe3+ complex is responsible for developing the periostracum’s black color due to the superposition of complementary colors. Corbicula clam shells consist of thin periostracum and calcareous layers made of calcium carbonate (CaCO3). Depending on habitat conditions, the shell exhibits various colorations, such as yellow, brown, and black. The chemical state of the periostracum of the Corbicula species in Lake Biwa was studied by X-ray absorption fine structure (XAFS) and Raman scattering spectroscopies. Fe K-edge X-ray absorption near edge structure (XANES) revealed that the Fe3+ intensity increases as the color of the shell changes from yellow to black. Raman spectra suggested that quinone-based polymers cover the yellow shell, and the black shell is further covered by dihydroxyphenylalanine (DOPA) rings of amino acid derivatives. From Fe K-edge extended X-ray absorption fine structure (EXAFS), it was found that Fe3+ in the periostracum was surrounded by five to six oxygen atoms with an average Fe-O ligand distance of 2.0 Å. Accordingly, a tris-DOPA-Fe3+ complex is formed, which is responsible for the periostracum’s black color.