On the Structure and Role of Avian Eggshells: A 31P, 1H, and 13C Solid-State NMR Study

Abstract

The eggshell is a composite and highly ordered structure formed by biomineralization. Besides other functions, it has a vital and intricate role in the protection of an embryo from various potentially harsh environmental conditions. Solid-state nuclear magnetic resonance (SSNMR) has been used for detailed structural investigations of the chicken, tinamou, and flamingo eggshell materials. ³¹P NMR spectra reveal that hydroxyapatite and β-tricalcium phosphate in the ratio 3:2 represent major constituents of phosphate species in the eggshells. All three eggshells exhibit similar spectra, except for the line widths, which implies different structural order of phosphate species in the chicken, tinamou, and flamingo eggshells. ¹H NMR spectra for these materials are comparable, differentiating overlapped peaks in three spectral regions at around 7, 4–5, and 1–2 ppm. These spectral regions have been attributed to protons from NH or CaHCO₃, water, and possibly isolated monomeric water molecules or hydroxyl groups in calcium-deficient hydroxyapatite. ¹H–¹³C CP MAS NMR revealed the presence of organic matter in the form of lipids and proteins. Two overlapped resonances in the carbonyl region at around 173 and 169 ppm are assigned to the carbonyls of the peptide bonds and the bicarbonate unit in calcite, respectively. Fourier-transform infrared spectroscopy (FTIR) spectra confirmed the presence of structural units detected in the NMR spectra.