Preservation of peptide moieties in three Spanish sulfur-rich Tertiary kerogens

Abstract

Thermochemolysis with tetramethylammonium hydroxide (TMAH) and solid-state 15 N NMR were utilized for the characterization of refractory organic nitrogen in Tertiary Spanish kerogens. The samples included sulfur-rich oil shales from the Ribesalbes (Serravallian), Libros (Tortonian) and Cerdanya (Tortonian) basins. Analysis using solid state 15 N NMR showed that part of the refractory nitrogen in the kerogens corresponds to amide groups. Moreover, the release of amino acid derivatives after pyrolysis in the presence of TMAH indicated that this amide-N arose from peptide moieties. The amino acids released from the kerogens were dominated by high amounts of glycine and alanine. Minor amounts of aspartic acid, serine, a-aminobutyric acid and other unidentified amino acids were also detected. Because proteinaceous structures, including small peptides, are generally considered as being highly sensitive to diagenetic degradation, encapsulation of labile peptide material into aliphatic structures in S-rich kerogens (probably via lipid sulfurization) has been proposed to explain the survival of these moieties. Substantial amounts of fatty acids (as methyl esters) were also released from all the kerogens after pyrolysis/TMAH, indicating their highly aliphatic character. The production of both fatty acids and amino acids from the kerogens supports the encapsulation process. !

Pyrolysis-gas chromatography–mass spectrometry of the 20 protein amino acids in the presence of TMAH

“…Phenylalanine and tyrosine contribution is revealed by their deamination products [11,12,16,17,19,30], aspartic acid by its cyclisation product, 2,5-pyrrolidine dione [11,12] and lysine by the N-methyl, 2-aminohex-5-enoic acid methyl ester [12,16]. 5-Oxoproline, methyl ester is often detected in these pyrolysates and interpreted as originating from glutamic acid [12] or from aspartic acid [11,[13][14][15][16]30]. Similarly, 2,5-pyrrolidinedione is commonly identified in TMAH pyrolysates [11,12,18,19,30] and is sometimes assigned to aspartic acid [11,12].…”

“…The occurrence of indole derivatives is also generally considered as indicative of tryptophan [18,19,29]. Additional peaks, although not identified but characterized by a base peak at m/z 116 were attributed to amino acid derivatives [13][14][15].…”

“…Due to the polar nature of the natural organic matter, Curiepoint pyrolysis was performed in the presence of TMAH. In several cases, methylated homologues of amino acids could be detected through this technique, although in quite low abundance in recent [11,12] and ancient sediments [13][14][15], in peat humic acid [16,17] and in natural waters [6,18,19]. Moreover, only a limited number of amino acids could be recognised in these pyrolysates.…”

“…However, Curie-point pyrolysis of soil and water organic matter only scarcely releases nitrogen-containing products, including various heterocyclic products [5,6]. In contrast, solid state 15 N NMR revealed that nitrogen mostly occurs as amide functions in soils [7], recent sediments [8] and waters [9], and a proteinaceous nature is often proposed for natural organic nitrogen. However, hydrolyses only release 10-60% of the total organic nitrogen in soils and sediments [10].…”

Formation of sulfur and nitrogen cross-linked macromolecules under aqueous conditions

“…In such an environment we are likely to find high amounts of ammonia/ amines from the mineralization by microorganisms. There is evidence for the presence of amino acids during the formation of sulfur rich kerogen (Riboulleau et al, 2002;del Rio et al, 2004). These authors reported on the preservation of amino acids (mainly glycine) by the sulfur crosslinked matrix of the kerogen.…”