Contribution of NAD 2D-NMR in liquid crystals to the determination of hydrogen isotope profile of methyl groups in miliacin. (15th January 2016) - Berdagué, Philippe
Abstract: The hydrogen isotopic composition ( δD or ( D / H ) value) of molecular biomarkers preserved in sedimentary archives is increasingly used to provide clues about the evolution of past climatic conditions. The rationale is that intact biomarkers retain isotopic information related to the climatic conditions that prevailed at the time of their synthesis. Some of these biomarkers may be degraded during diagenesis, however. The extent to which these degradations alter the original δD value of the source biomarker is presently debated and the capacity to resolve this question by determination of compound-specific δD values alone is limited. The "bulk" or "global" δD value of any molecule is in fact a composite of δD values at each site within this molecule ( δD i or ( D / H ) i with i = number of hydrogen/deuterium atoms in the considered molecule). Determination of this site-specific δD i value in biomarkers could not only yield outstanding paleoenvironmental information but also help forecast the impacts of diagenesis and define essential steps in biosynthetic pathways. This task is analytically challenging. Here, we examined the capabilities of natural abundance deuterium 2D-NMR (NAD 2D-NMR) using homopolypeptide liquid crystals as an NMR solvent to: (i) analyze the NAD spectra of biomakers; (ii) determine the site-specific distribution of hydrogen in the nine methyl groups ( δD Me i with i = 23–31) of miliacin, a pentacyclic triterpene of the amyrin family and key biomarker for broomcorn millet in sedimentary archives. Relative ( D / H )Me i values were established by anisotropic NAD 2D-NMR. Then absolute δD Me i values were obtained by determining δD Me i value of the methoxy group of miliacin using two independent approaches: isotropic NAD NMR (SNIF-NMR ™ ) and GC–irMS. The resulting isotope profile for miliacin shows, for the first time, large variations in δD Me i values that can directly be explained by biosynthetic processes. This approach has also the potential to permit predicting fractionations during pentacyclic triterpene diagenesis.