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Wet degradation of keratin proteins: Linking amino acid, elemental and isotopic composition

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JournalRapid Communications in Mass Spectrometry
DateE-pub ahead of print - 26 Aug 2014
DatePublished (current) - 15 Oct 2014
Issue number19
Volume28
Number of pages13
Pages (from-to)2121-2133
Early online date26/08/14
Original languageEnglish

Abstract

RATIONALE:  Archaeological keratin samples are increasingly the subject of palaeodietary, provenancing and dating studies. Keratin samples from wet archaeological contexts are microbiologically and chemically degraded, causing differential diagenesis of protein structures in hair fibres. The effects of these processes on the analytical parameters of interest are currently unknown. 

METHODS:  This study examined the impact of degradation of wool fibres on isotopic (δ13C, δ15N, un-exchangeable δ2H and δ18O values) composition. It compared two models of archaeological protein degradation in wet burial environments: (1) short term (up to 8-years) experimental burial in three contrasting soil environments; and (2) laboratory wet conditions, in which elevated temperature (80-°C, 110-°C, and 140-°C) and pressure simulated longer exposure. Elemental and amino acid (AA) composition were also measured. 

RESULTS In experimentally soil-buried samples, AA, elemental and isotopic composition changes were small, despite extensive macroscopic alteration. Isothermally heated samples showed preferential loss of hydrophilic AAs (Asx, Glx, Ser, Gly) from wool residues, with depletion in 2H and 18O at higher temperatures (up to -73‰ change in δ2H and -2.6‰ in δ18O values). The δ13C and δ15N values showed little change except in densely pigmented samples at low temperatures only. Samples dyed with madder/alum were better preserved than undyed samples.

CONCLUSIONS Diagenesis in experimentally soil-buried wool textiles was consistent with microbiological, non-protein-selective activity, in contrast to highly AA-selective hydrolytic behaviour under laboratory wet conditions. Changes in δ2H and δ18O values were correlated with degree of AA change, but the δ13C and δ15N values were not. The results contribute to a baseline for interpreting analytical data from archaeological hair samples preserved by burial in wet environments.

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