Abnormal (hydroxy)prolines deuterium content redefines hydrogen chemical mass

ABSTRACT Analysing the δ2H in individual amino acids of proteins extracted from vertebrates, we unexpectedly found in some samples, notably bone collagen from seals, more than twice as much deuterium in proline and hydroxyproline residues than in seawater. This corresponds to at least four times higher δ2H than in any previously reported biogenic sample. We ruled out diet as a plausible mechanism for such anomalous enrichment. This finding puts into question the old adage that you are what you eat. SUMMARY The chemical mass of hydrogen is defined as an interval from the lowest to the highest content of deuterium 2 H, hydrogen’s heavy stable isotope. Measurements of the deviations δ 2 H in the deuterium content from the standard (ocean water, δ 2 H = 0‰) are used to characterise biological samples, such as animal bone collagen. The results are often interpreted in terms of the trophic level and diet of the animal as well as prevailing climate during its lifetime. The majority of the published bone collagen δ 2 H data fall into a narrow δ 2 H range limited to ±100‰. Using novel analysis method, we unexpectedly found greatly higher δ 2 H values, up to 1500‰, in seal bone collagen. Such anomalous deuterium enrichment is detected only in two amino acid residues, proline and its derivative hydroxyproline, while other residues show much smaller δ 2 H values. Anomalously high δ 2 H values, albeit of lower magnitudes, are also found for these residues in other biological sources. This finding substantially expands the upper bound of the hydrogen chemical mass for biogenic sources. Since neither diet nor environment explain these mysteriously high enrichment levels amounting to more than twice deuterium content in sea water, our understanding of stable isotopes in nature, as well as the old adage “you are what you eat”, are put in question.