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).

13C NMR of a Delicious Christmas Treat

As my Santa Claus-like belly may indicate, I love holiday treats. The solid-state ¹³C NMR spectra below were collected from a special sample of a holiday treat prepared by my wife, Patty from her Great Grandma Jennings lab book. The sample was prepared from only four ingredients as follows:

To 227 g of softened butter, 65 g of fructose was added while stirring with a spatula. Slowly, 199 g of flour and 1.26 g of sodium chloride were stirred into the mixture until it became difficult to mix with a spatula. The mixture was kneaded gently until cracks in the surface began to appear after which it was rolled to a thickness of 38 mm and cut into round samples of approximately 51 mm in size. The samples were heated in an oven at 436 K - 450 K for approximately 600 seconds until gold in color.

The bottom trace is a ¹³C CPMAS spectrum and the top trace is a ¹³C MAS spectrum. Both spectra were acquired with high power ¹H decoupling. This pair of spectra serves to illustrate the different types of information available from each of these techniques. The sample is a mixture of rigid and mobile components. The ¹³C CPMAS technique detects mainly the more rigid components as it relies on the dipolar coupling between protons and ¹³C for the cross polarization. The dipolar coupling is averaged to nearly zero for the mobile constituents and therefore they do not appear in the spectrum. The ¹³C CPMAS spectrum therefore, shows primarily all of the rigid constituents (mainly flour and sugar). The ¹³C MAS spectrum with high power ¹H decoupling shows both rigid and mobile constituents. The resonances from the mobile constituents (mainly butter) have sharp lines while the broader lines from the rigid constituents show up at very low intensity as the sensitivity is not enhanced by cross polarization.

Now, you too have enjoyed Patty's delicious shortbread.