Collagen proteins exchange O with demineralisation and gelatinisation reagents and also with atmospheric moisture

Mediterranean precipitation isoscape preserved in bone collagen δ2H

The prehistory of the Mediterranean region has long been a subject of considerable interest, particularly the links between human groups and regions of origin. We utilize the spatial variation in the δ²H and δ¹⁸O values of precipitation (isoscapes) to develop proxies for geographic locations of fauna and humans. Bone collagen hydrogen isotope ratios (δ²H) in cattle (and to a lesser extent, ovicaprids) across the Mediterranean reflect the isotopic differences observed in rainfall (but δ¹⁸O values do not). We conclude that δ²H in herbivore bone collagen can be used as a geolocation tracer and for palaeoenvironmental studies such as tracing past isotopic variations in the global hydrological cycle. In contrast, human bone δ²H values are relatively tightly grouped and highly distinct from precipitation δ²H values, likely due to human-specific food practices and environmental modifications. Given the inter-species variability in δ²H, care should be taken in the species selected for study.

Ionisation bias undermines the use of matrix-assisted laser desorption/ionisation for estimating peptide deamidation: Synthetic peptide studies demonstrate electrospray ionisation gives more reliable response ratios

RATIONALE

Although mass spectrometry (MS) is routinely used to determine deamination in peptide mixtures, the effects of the choice of ionisation source have not yet been investigated. In particular, matrix-assisted laser desorption/ionisation (MALDI) has become a popular tool with which to measure levels of glutamine deamidation in ancient proteins. Here we use model synthetic peptides to rigorously compare MALDI and electrospray ionisation (ESI).

METHODS

We used two synthetic peptides, with glutamine (Q) in one substituted for glutamic acid (E) in the other, to investigate the suitability of MALDI and ESI sources for the assessment of deamidation in peptides using MS. We also compared measurements of the same Q- and E-containing peptide mixtures using two different mass analysers (time-of-flight (TOF) and Fourier transform ion cyclotron resonance (FT-ICR)).

RESULTS

When standard mixtures of the Q- and E-containing peptides were analysed using MALDI, under-representation of the E-containing peptide was observed. This observation was consistent between analyses carried out using either TOF or FT-ICR-MS. When the same mixtures were analysed using ESI FT-ICR-MS, no ionisation bias was observed.

CONCLUSIONS

MALDI may not be a suitable ionisation method for the determination of deamidation in peptide mixtures. However, ESI was successfully used to determine the ratio in known mixtures of Q- and E-containing peptides. These preliminary observations warrant further investigation into ionisation bias when measuring deamidation in other peptide sequences.

Stable isotopes of H, C and N in mice bone collagen as a reflection of isotopically controlled food and water intake. ISOTOPES IN ENVIRONMENTAL AND HEALTH STUDIES 2019;55:129-149. [PMID: 30793970 DOI: 10.1080/10256016.2019.1580279]

²H/¹H ratios in animal biomass reflect isotopic input from food and water. A 10-week controlled laboratory study raised 48 mice divided in two generations (8 mothers Mus musculus and their offspring). The mice were divided into four groups based on the combination of ²H, ¹³C, ¹⁵N-enriched and non-enriched food and water. Glycine, the most common amino acid in bone collagen, carried the ²H, ¹³C, ¹⁵N-isotopic spike in food. ANOVA data analysis indicated that hydrogen in food accounted for ∼81 % of the hydrogen isotope inventory in collagen whereas drinking water hydrogen contributed ∼17 %. Air humidity contributed an unspecified amount. Additionally, we monitored ¹³C and ¹⁵N-enrichment in bone collagen and found strong linear correlations with the ²H-enrichment. The experiments with food and water indicate two biosynthetic pathways, namely (i) de novo creation of non-essential amino acids using hydrogen from water, and (ii) the integration of essential and non-essential amino acids from food. The lower rate of isotope uptake in mothers' collagen relative to their offspring indicates incomplete bone collagen turnover after ten weeks. The variance of hydrogen stable isotope ratios within the same cohort may limit its usefulness as a single sample proxy for archaeological or palaeoenvironmental research.

The interconversion of δ2 H values of collagen between thermal conversion reactor configurations

RATIONALE

Different thermal conversion reactor packings result in distinct δ² H values in nitrogen-containing materials, such as bone collagen. An older 'traditional' glassy carbon packing method causes incomplete conversion of N-containing samples into H₂ gas, resulting in altered δ² H values compared with the complete conversion of hydrogen obtained with a chromium-packed reactor. Given that δ² H values from collagen are gaining importance in palaeoecological and archaeological studies, a determination of the relationship between δ² H values produced with a glassy-carbon-packed and a chromium-packed reactor is needed.

METHODS

We obtained δ² H values (normalized on the VSMOW-SLAP scale) from both glassy-carbon-packed (GP) and chromium-packed (Cr) reactor configurations from bone collagen (n = 231) from a variety of archaeological sites, using a High-Temperature Conversion Elemental Analyzer (TC/EA) coupled to a Delta Plus XP isotope ratio mass spectrometer.

RESULTS

δ² H values from both methods are linearly correlated (r²  = 0.934) and yield the following interconversion equation, δ² H(Cr) = 1.054 δ² H(GP) + 11.6‰ (95% conf. slope 1.020-1.090, intercept 10.6-12.6), and a mean difference of δ² H(Cr) - δ² H(GP) = 10.1‰ (1 sd 5.2, 1 se 0.3, n = 231).

CONCLUSIONS

We recommend adopting this interconversion between δ² H values produced with a glassy-carbon-packed and chromium-packed reactor for bone collagen only, with appropriate propagation of uncertainty.