NEaar uses a multiple amino acid approach to estimate the extent of degradation in intra-crystalline biomineral proteins, enabling an determination of relative age.
Independent geochronology and / or protein degradation kinetics can then be linked with thermal models to provide an estimate of absolute age (e.g. Penkman et al., 2011).
Conventional racemization analysis tends to report a D-alloisoleucine / L-isoleucine (A/I or D/L ratio). This amino acid ratio has the advantage of being relative easy to measure and also sufficiently slow to be used to “date” sediments in the European Quaternary. Our new approach utilises multiple amino acids, thanks to advances made in chromatography (Kaufman & Manley, 1998). As each amino acid has its own particular characteristics, only in a well behaved system will the extent of racemization of the multiple amino acids agree with each other. If an amino acid has an unusually low ratio (due to modern contamination) or unusually high racemization (due to inclusion of bacterial cell wall contaminants) either some or all of the amino acids will no longer fit to the idealized degradation model. Indeed we can use elevation of D/LAsx, D/LGlu and D/LAla to detect evidence of bacterial contamination (Jones et al., 2005; Willerslev et al., 2007).
However, amino acid geochronology requires a closed system – we believe that you can’t date an open system of protein, as environmental factors have a major influence on overall protein breakdown. We pre-treat our samples with bleach, removing contamination and any exposed (open system) amino acids, leaving behind an "intra-crystalline" fraction of closed-system protein (Towe, 1980; Sykes et al., 1998; Penkman et al., 2008). We also routinely analyse both the free amino acids (FAA) and total hydrolysable amino acids (THAA) within each sample, as these show different levels of protein breakdown that are highly correlated (Preece & Penkman, 2005). This combination of bleach pre-treatment and analysis of multiple chiral amino acid analysis results in a measure of "Intra-crystalline Protein Decomposition" (IcPD) of a fossil sample, which is affected only by the time and temperature of its burial conditions.
As temperature is an important driver of protein breakdown, samples can only be directly compared within a region that has experienced a similar effective diagenetic history.
The protein sequence and structure is very important in the mechanisms and rates of protein breakdown, and differes significantly between genera, so it is important to analyse mono-generic samples for direct comparison.
- Kirsty Penkman
Department of Chemistry
University of York
York YO10 5DD
(+44) 01904 322574
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