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Research Profile

NMR’s got sole

Figures

Jennifer Griffiths

Anal. Chem., 2009, 81 (5), p 1725

DOI: 10.1021/ac9000223

Publication Date (Web): January 21, 2009

Garbage is like gold to an archaeologist. For example, in Lyon, France, near the Sane River, an excavation is under way on a giant, 800-year-old trash dump (discovered during the construction of a parking garage) where people used to discard all manner of waste. In a recent paper published in AC (DOI 10.1021/ac802052a), Michel Bardet and colleagues of the Commissariat l’nergie Atomique (France) report findings from NMR and electron paramagnetic resonance (EPR) studies on the leather soles of shoes found in this trash heap.

The new analyses, according to Bardet, will help archaeologists understand the preservation of leather in anaerobic, wet environments and will assist in the restoration of those artifacts. “We looked at the research and found that very little had been done in this field,” says Bardet, an NMR spectroscopist. The archaeologists “found these soles dating from the 13th through the 18th century. It was very interesting to have all these sets of samples available.”

A leather sole recovered from an archaeological trash dump in Lyon, France.

PIERRE ALAIN BAYLE

Bardet and his group usually work on archaeological wood samples, so analyzing leather was new to them. “One thing that was interesting for us...is that both [wood and leather] are what we call ‘waterlogged’ materials. It’s organic matter full of water,” he says. “Generally, when we are working on wood found in similar conditions, the wood is in very poor condition.... Most of the cellulose has been destroyed. In the case of leather, the material seems to be in better preservation.”

Leather is made up of collagen, a robust protein that can survive hundreds of thousands of years—maybe even tens of millions—under the correct conditions. But the wet environment where these samples were found isn’t ideal for preservation.

The researchers used two types of solid-state ¹³C-NMR techniques: cross-polarization transfer magic-angle spinning (CP-MAS) and single-pulse excitation magic-angle spinning (SP-MAS). MAS is a solid-state NMR technique in which the sample is tilted to the “magic angle” of 54.74° with respect to the external magnetic field and spun at a high rate. This sharpens the signals and makes the spectra much more useful. CP-MAS takes advantage of the strong dipolar interactions between ¹³C and protons and is good for looking at the relatively rigid domains of collagen and tannin moieties. SP-MAS instead directly excites ¹³C and is used to observe the more “mobile” part of the material—for instance, the lubricants used to soften the leather.

“You can only observe mobile molecules with direct excitation,” explains Bardet. “That’s the case if you have lubricant in your sample—for example, oil or wax. You can see them very easily with direct excitation. By doing both experiments, you are sure to see all types of carbons that you have in your samples.”

The investigators subjected both modern leathers and archaeological soles to the NMR analysis. The modern leather showed clear signals for tannins and lubricants, as well as for collagen. The soles generated only collagen signals—no tannins or lubricant remained. “That was a moment that was very surprising,” says Bardet. “Probably the tannins have been leached out of the material because of the water.”

So why were the soles so well preserved if their typical leather-preserving agents had been washed away? Luckily, Bardet and colleagues almost always subject their samples to EPR analysis to look for paramagnetic compounds. “It’s important to check for their presence in the sample because it’s important for interpreting the NMR spectra,” says Bardet. “For instance, if you have a high concentration of metallic impurities, it can lead to an enlargement of the NMR peaks and lose the resolution in your spectra.”

In this case, EPR analysis of the soles showed that they had accumulated large amounts of iron oxides—compounds not found in the modern leather samples. The presence of iron compounds may explain the surprisingly good condition of the artifacts. “We think that iron more or less took the place [of the tannins] and acted as a post-tannin effect,” says Bardet. The iron likely leached from the environment where the soles were found.

Bardet says that their work continues. They plan to look at “historical leather dated from the same periods but not preserved in water surroundings in order to check and to look at the tannins,” he says. “We also continue the work on archaeological wood.”