Solid-State 13C NMR of Chicken Eggshells

¹³C CP/MAS and direct single pulse ¹³C MAS NMR with high power decoupling can give very different results for a wide variety of materials.  ¹³C CP/MAS NMR relies on the transfer of magnetization from protons to ¹³C via the dipolar coupling mechanism whereas the direct single pulse method does not.  An interesting material to demonstrate this principle is the shell of a chicken egg.  Chicken eggshell is a complex bio-mineral consisting largely (~ 95%) of the calcite polymorph of calcium carbonate as well as proteins and lipids.  The ¹³C CP/MAS and ¹³C single pulse MAS NMR spectra of a sample of dry ground eggshell, from which the membranes had been removed, are shown in the lower and upper traces in the figure below, respectively.

The ¹³C CP/MAS spectrum in the lower trace has very broad resonances in the aliphatic region of the spectrum due to the proteins and lipids.  The carbonyl region of the spectrum  consists of two resonances; one at ~173 ppm due to the carbonyl carbons of the amino acid residues of the proteins and a resonance at  ~169 ppm which has been shown^1,2 to originate from bicarbonate ions (HCO₃)^-.  Although ~95% the eggshell consists of CaCO₃, the carbonate resonance is not present in the ¹³C CP/MAS spectrum as there are no proximate protons for cross polarization.  In contrast, the single pulse ¹³C MAS spectrum in the top trace shows only the ¹³C resonance from the carbonate ions which has a coincident chemical shift with that of the bicarbonate ions.  All of the other ¹³C resonances are buried in the noise as they are in much lower concentration. These spectra are an excellent example of how one can obtain different information from ¹³C CP/MAS and single pulse ¹³C MAS spectra.

1. D.M. Pisklak, L. Szcleszczuk, I. Wawer, Journal of Agricultural and Food Chemistry, 60, 12254 (2012).
2. J. Feng, Y.J. Lee, R.J. Reeder, B.L. Phillips, American Mineralogist, 91, 957 (2006).