n-Alkane 13C/12C indicates differential metabolic controls of fatty lipid chain extension in C3 and C4 grasses

Fundamental differences in metabolic control of fatty acids chain extension are reflected in the contrasting carbon isotopic composition profiles of C3 and C4 grasses.

State-of-the-Art of Analytical Techniques to Determine Food Fraud in Olive Oils

The benefits of the food industry compared to other sectors are much lower, which is why producers are tempted to commit fraud. Although it is a bad practice committed with a wide variety of foods, it is worth noting the case of olive oil because it is a product of great value and with a high percentage of fraud. It is for all these reasons that the authenticity of olive oil has become a major problem for producers, consumers, and legislators. To avoid such fraud, it is necessary to develop analytical techniques to detect them. In this review, we performed a complete analysis about the available instrumentation used in olive fraud which comprised spectroscopic and spectrometric methodology and analyte separation techniques such as liquid chromatography and gas chromatography. Additionally, other methodology including protein-based biomolecular techniques and analytical approaches like metabolomic, hhyperspectral imaging and chemometrics are discussed.

Goat performance on pure Andropogon gayanus pastures or associated with legumes

The objective of this study was to evaluate the grazing of goats in a grass monoculture system and in intercropping systems of grass + legumes. A randomized block design was adopted, with the treatments arranged in a split-plots scheme. The plots consisted of three cropping systems: monoculture-Andropogon gayanus cv. Planaltina; mixture I-A. gayanus cv. Planaltina + Stylosanthes cv. Campo Grande; and mixture II-A. gayanus cv. Planaltina + Calopogonium mucunoides. The subplots consisted of two grazing cycles. The highest (P = 0.04) total forage mass (TFM) was recorded in the mixture I (A. gayanus cv. Planaltina + Stylosanthes. cv. Campo Grande), which was 2.6 ± 0.1 tons DM/ha. The crude protein (CP) was lower in the monoculture which also showed the highest content of neutral detergent fiber (NDF). The grazing time in the monoculture was the longest (8.23 ± 1.14 h). The goats used a longer time for rumination (P < 0.01) in the mixture I. The lowest (P < 0.01) bite rate was found in mixture II in comparison to the other cropping systems. The bite rate was higher (P < 0.01) in grazing cycle II than in all the other cropping systems. In mixture I, the Stylosanthes cv. Campo Grande, and in mixture II, the C. mucunoides presented the lowest (P < 0.01) δ¹³C value in the forage. The highest forage intake occurred in the mixture systems (P < 0.01) in comparison to the monoculture. The mixture pastures presented better results for forage mass, nutritive value, and intake in comparison to the monoculture.

Intramolecular 13C analysis of tree rings provides multiple plant ecophysiology signals covering decades

Measurements of carbon isotope contents of plant organic matter provide important information in diverse fields such as plant breeding, ecophysiology, biogeochemistry and paleoclimatology. They are currently based on ¹³C/¹²C ratios of specific, whole metabolites, but we show here that intramolecular ratios provide higher resolution information. In the glucose units of tree-ring cellulose of 12 tree species, we detected large differences in ¹³C/¹²C ratios (>10‰) among carbon atoms, which provide isotopically distinct inputs to major global C pools, including wood and soil organic matter. Thus, considering position-specific differences can improve characterisation of soil-to-atmosphere carbon fluxes and soil metabolism. In a Pinus nigra tree-ring archive formed from 1961 to 1995, we found novel ¹³C signals, and show that intramolecular analysis enables more comprehensive and precise signal extraction from tree rings, and thus higher resolution reconstruction of plants’ responses to climate change. Moreover, we propose an ecophysiological mechanism for the introduction of a ¹³C signal, which links an environmental shift to the triggered metabolic shift and its intramolecular ¹³C signature. In conclusion, intramolecular ¹³C analyses can provide valuable new information about long-term metabolic dynamics for numerous applications.

On the contributions of photorespiration and compartmentation to the contrasting intramolecular 2H profiles of C3 and C4 plant sugars

Compartmentation of C₄ photosynthetic biochemistry into bundle sheath (BS) and mesophyll (M) cells, and photorespiration in C₃ plants is predicted to have hydrogen isotopic consequences for metabolites at both molecular and site-specific levels. Molecular-level evidence was recently reported (Zhou et al., 2016), but evidence at the site-specific level is still lacking. We propose that such evidence exists in the contrasting ²H distribution profiles of glucose samples from naturally grown C₃, C₄ and CAM plants: photorespiration contributes to the relative ²H enrichment in H⁵ and relative ²H depletion in H¹ & H⁶ (the average of the two pro-chiral Hs and in particular H^6,pro-R) in C₃ glucose, while ²H-enriched C₃ mesophyll cellular (chloroplastic) water most likely contributes to the enrichment at H⁴; export of (transferable hydrogen atoms of) NADPH from C₄ mesophyll cells to bundle sheath cells (via the malate shuttle) and incorporation of ²H-relatively unenriched BS cellular water contribute to the relative depletion of H⁴ & H⁵ respectively; shuttling of triose-phosphates (PGA: phosphoglycerate dand DHAP: dihydroacetone phosphate) between C₄ bundle sheath and mesophyll cells contributes to the relative enrichment in H¹ & H⁶ (in particular H^6,pro-R) in C₄ glucose.

The isotopic structures of geological organic compounds

Organic compounds are ubiquitous in the Earth's surface, sediments and many rocks, and preserve records of geological, geochemical and biological history; they are also critical natural resources and major environmental pollutants. The naturally occurring stable isotopes of volatile elements (D, 13C, 15N, 17,18O, 33,34,36S) provide one way of studying the origin, evolution and migration of geological organic compounds. The study of bulk stable isotope compositions (i.e. averaged across all possible molecular isotopic forms) is well established and widely practised, but frequently results in non-unique interpretations. Increasingly, researchers are reading the organic isotopic record with greater depth and specificity by characterizing stable isotope ‘structures’ – the proportions of site-specific and multiply substituted isotopologues that contribute to the total rare-isotope inventory of each compound. Most of the technologies for measuring stable isotope structures of organic molecules have been only recently developed and to date have been applied only in an exploratory way. Nevertheless, recent advances have demonstrated that molecular isotopic structures provide distinctive records of biosynthetic origins, conditions and mechanisms of chemical transformation during burial, and forensic fingerprints of exceptional specificity. This paper provides a review of this young field, which is organized to follow the evolution of molecular isotopic structure from biosynthesis, through diagenesis, catagenesis and metamorphism.

Natural Isotope Abundance in Metabolites: Techniques and Kinetic Isotope Effect Measurement in Plant, Animal, and Human Tissues

The natural isotope abundance in bulk organic matter or tissues is not a sufficient base to investigate physiological properties, biosynthetic mechanisms, and nutrition sources of biological systems. In fact, isotope effects in metabolism lead to a heterogeneous distribution of ²H, ¹⁸O, ¹³C, and ¹⁵N isotopes in metabolites. Therefore, compound-specific isotopic analysis (CSIA) is crucial to biological and medical applications of stable isotopes. Here, we review methods to implement CSIA for ¹⁵N and ¹³C from plant, animal, and human samples and discuss technical solutions that have been used for the conversion to CO₂ and N₂ for IRMS analysis, derivatization and isotope effect measurements. It appears that despite the flexibility of instruments used for CSIA, there is no universal method simply because the chemical nature of metabolites of interest varies considerably. Also, CSIA methods are often limited by isotope effects in sample preparation or the addition of atoms from the derivatizing reagents, and this implies that corrections must be made to calculate a proper δ-value. Therefore, CSIA has an enormous potential for biomedical applications, but its utilization requires precautions for its successful application.

The new face of isotopic NMR at natural abundance

The most widely used method for isotope analysis at natural abundance is isotope ratio monitoring by Mass Spectrometry (irm-MS) which provides bulk isotopic composition in ² H, ¹³ C, ¹⁵ N, ¹⁸ O or ³⁴ S. However, in the 1980s, the direct access to Site-specific Natural Isotope Fractionation by Nuclear Magnetic Resonance (SNIF-NMR^TM ) was immediately recognized as a powerful technique to authenticate the origin of natural or synthetic products. The initial - and still most popular - application consisted in detecting the chaptalization of wines by irm-² H NMR. The approach has been extended to a wide range of methodologies over the last decade, paving the way to a wide range of applications, not only in the field of authentication but also to study metabolism. In particular, the emerging irm-¹³ C NMR approach delivers direct access to position-specific ¹³ C isotope content at natural abundance. After highlighting the application scope of irm-NMR (² H and ¹³ C), this article describes the major improvements which made possible to reach the required accuracy of 1‰ (0.1%) in irm-¹³ C NMR. The last part of the manuscript summarizes the different steps to perform isotope analysis as a function of the sample properties (concentration, peak overlap) and the kind of targeted isotopic information (authentication, affiliation). Copyright © 2016 John Wiley & Sons, Ltd.

Variability in the carbon isotope composition of individual amino acids in plant proteins from different sources: 1 Leaves

The natural carbon isotope composition of individual amino acids from plant leaf proteins has been measured to establish potential sources of variability. The plant leaves studied, taken from a range of plant groups (forbs, trees, grasses, and freshwater aquatic plants), showed no significant influence of either season or environment (water and light availability) on their Δδ(13)C values. Plant groups did, however, differ in carbon isotope composition, although no consistent differences were identified at the species level. A discriminant analysis model was constructed which allowed leaves from (1) nettles, (2) Pooideae, (3) other Poales, (4) trees and (5) freshwater higher plants to be distinguished from each other on the basis of their natural abundance (13)C/(12)C ratios of individual amino acids. Differences in carbon isotope composition are known to be retained, to some extent, in the tissues of their consumers, and hence an understanding of compound-specific variation in (13)C/(12)C fractional abundance in plants has the potential to provide dietary insights of value in archaeological and ecological studies.

Analytical method for simultaneous determination of bulk and intramolecular (13) C-isotope compositions of acetic acid

RATIONALE

Headspace solid-phase microextraction (HS-SPME) combined with gas chromatography/pyrolysis-gas chromatography/combustion-isotope ratio mass spectrometry (GC/Py-GC/C-IRMS) was developed for the simultaneous determination of the intramolecular and molecular carbon-isotopic composition (δ(13) C value) of acetic acid.

METHODS

The δ(13) C values of carboxyl and methyl carbon were standardized using calibration curves constructed from the regression between the measured δ(13) C values and the δ(13) C values of working standards determined in a previous study. We applied this developed HS-SPME-GC/Py-GC/C-IRMS technique to commercial vinegars.

RESULTS

In one injection analysis, the bulk and intramolecular δ(13) C values of pure acetic acid standards can be obtained. The repeatability (1σ) of the bulk δ(13) C values is within ±0.4‰, and that of the δ(13) Ccarboxyl and δ(13) Cmethyl values is within ±0.6‰. The intramolecular δ(13) C values of acetic acid in vinegars exhibit a similar pattern. The average Δδ value (δ(13) CCOOH - δ(13) CCH3 ) is 4.3 ± 2.0‰.

CONCLUSIONS

The approach presented herein for the molecular and intramolecular δ(13) C determination of acetic acid avoids switching between configuration systems and thereby reduces systematic errors. It is expected to be useful for examining isotope fractionation associated with processes related to organic acid (bio)transformations.

Nonstatistical 13C distribution during carbon transfer from glucose to ethanol during fermentation is determined by the catabolic pathway exploited

During the anaerobic fermentation of glucose to ethanol, the three micro-organisms Saccharomyces cerevisiae, Zymomonas mobilis, and Leuconostoc mesenteroides exploit, respectively, the Embden-Meyerhof-Parnas, the Entner-Doudoroff, and the reductive pentose phosphate pathways. Thus, the atoms incorporated into ethanol do not have the same affiliation to the atomic positions in glucose. The isotopic fractionation occurring in each pathway at both the methylene and methyl positions of ethanol has been investigated by isotopic quantitative (13)C NMR spectrometry with the aim of observing whether an isotope redistribution characteristic of the enzymes active in each pathway can be measured. First, it is found that each pathway has a unique isotope redistribution signature. Second, for the methylene group, a significant apparent kinetic isotope effect is only found in the reductive pentose phosphate pathway. Third, the apparent kinetic isotope effects related to the methyl group are more pronounced than for the methylene group. These findings can (i) be related to known kinetic isotope effects of some of the enzymes concerned and (ii) give indicators as to which steps in the pathways are likely to be influencing the final isotopic composition in the ethanol.

Multi-factorial in vivo stable isotope fractionation: causes, correlations, consequences and applications

Many physical and chemical processes in living systems are accompanied by isotope fractionation on H, C, N, O and S. Although kinetic or thermodynamic isotope effects are always the basis, their in vivo manifestation is often modulated by secondary influences. These include metabolic branching events or metabolite channeling, metabolite pool sizes, reaction mechanisms, anatomical properties and compartmentation of plants and animals, and climatological or environmental conditions. In the present contribution, the fundamentals of isotope effects and their manifestation under in vivo conditions are outlined. The knowledge about and the understanding of these interferences provide a potent tool for the reconstruction of physiological events in plants and animals, their geographical origin, the history of bulk biomass and the biosynthesis of defined representatives. It allows the use of isotope characteristics of biomass for the elucidation of biochemical pathways and reaction mechanisms and for the reconstruction of climatic, physiological, ecological and environmental conditions during biosynthesis. Thus, it can be used for the origin and authenticity control of food, the study of ecosystems and animal physiology, the reconstruction of present and prehistoric nutrition chains and paleaoclimatological conditions. This is demonstrated by the outline of fundamental and application-orientated examples for all bio-elements. The aim of the review is to inform (advanced) students from various disciplines about the whole potential and the scope of stable isotope characteristics and fractionations and to provide them with a comprehensive introduction to the literature on fundamental aspects and applications.

Intramolecular 13C pattern in hexoses from autotrophic and heterotrophic C3 plant tissues

The stable carbon isotope (13)C is used as a universal tracer in plant eco-physiology and studies of carbon exchange between vegetation and atmosphere. Photosynthesis fractionates against (13)CO(2) so that source sugars (photosynthates) are on average (13)C depleted by 20‰ compared with atmospheric CO(2). The carbon isotope distribution within sugars has been shown to be heterogeneous, with relatively (13)C-enriched and (13)C-depleted C-atom positions. The (13)C pattern within sugars is the cornerstone of (13)C distribution in plants, because all metabolites inherit the (13)C abundance in their specific precursor C-atom positions. However, the intramolecular isotope pattern in source leaf glucose and the isotope fractionation associated with key enzymes involved in sugar interconversions are currently unknown. To gain insight into these, we have analyzed the intramolecular isotope composition in source leaf transient starch, grain storage starch, and root storage sucrose and measured the site-specific isotope fractionation associated with the invertase (EC 3.2.1.26) and glucose isomerase (EC 5.3.1.5) reactions. When these data are integrated into a simple steady-state model of plant isotopic fluxes, the enzyme-dependent fractionations satisfactorily predict the observed intramolecular patterns. These results demonstrate that glucose and sucrose metabolism is the primary determinant of the (13)C abundance in source and sink tissue and is, therefore, of fundamental importance to the interpretation of plant isotopic signals.

Comparison of IRMS and NMR spectrometry for the determination of intramolecular 13C isotope composition: application to ethanol

Isotopic (13)C NMR is a relatively recent technique which allows the determination of intramolecular (13)C isotope composition at natural abundance. It has been used in various scientific fields such as authentication, counterfeiting or plant metabolism. Although its precision has already been evaluated, the determination of its trueness remains still challenging. To deal with that issue, a comparison with another normalized technique must be achieved. In this work, we compare the intramolecular (13)C isotope distribution of ethanol from different origins obtained using both Isotope Ratio Mass Spectrometry (IRMS) and Nuclear Magnetic Resonance (NMR) spectrometry techniques. The IRMS approach consists of the oxidation of ethanol to acetic acid followed by the degradation of the latter for the analysis of each fragments formed. We show here that the oxidation of ethanol to acetic acid does not bring any significant error on the determination of the site-specific δ(13)C (δ(13)C(i)) of ethanol using the IRMS approach. The difference between the data obtained for 16 samples from different origins using IRMS and NMR approaches is not statistically significant and remains below 0.3‰. These results are encouraging for the future studies using isotopic NMR, especially in combination with the IRMS approach.