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Roland A Werner

Publications and source records attributed to Roland A Werner.

9 recordsLinked to original sources
PubMed2006

The prediction of isotopic patterns in phenylpropanoids from their precursors and the mechanism of the NIH-shift: basis of the isotopic characteristics of natural aromatic compounds.

The theoretical 2H-distribution in the aromatic ring of phenylpropanoids can be predicted from that of their precursors--erythrose-4-phosphate, phosphoenolpyruvate and NADPH--and by invoking the mechanism of the NIH-shift and implied deuterium isotope effects. For each position in the non-oxygenated ring, the predicted natural 2H-abundance is in excellent agreement with experimental data obtained from quantitative 2H NMR-measurements on natural compounds, especially concerning the relative 2H-abundances p > o > or = m. For the p-hydroxylated derivatives, the experimentally determined 2H-abundance sequence order m > o can also be deduced, assuming an anisotropic migration (intramolecular isotope effect) of the p-hydrogen atom to the two differently 2H-substituted m-positions during the NIH-shift (intramolecular hydrogen transfer) and an in vivo deuterium kinetic isotope effect of approximately 1.20 on the final hydrogen elimination from the proposed ketodiene intermediate. The predicted 2H-distribution pattern of methyl salicylate 10, a representative of an o-hydroxylated natural compound, is in excellent agreement with that reported from 2H NMR analyses. However, for salicyl alcohol, minor differences between the theoretical and experimentally determined pattern are found that cannot yet be satisfactorily explained. On the other hand, a very good agreement is found between the theoretical and experimental pattern of coumarin, provided a deuterium kinetic isotope effect of approximately 1.30 is assumed for the elimination of the H-atoms from the ketodiene intermediate. The secondary m-hydroxylation of p-coumaric acid in the biosynthesis of vanillin seems to proceed without large isotope effects. Parallel differences are also observed for the 18O-kinetic isotope effects on the corresponding monooxygenase-catalysed reactions. The results demonstrate convincingly that the mechanisms of these general reactions of the phenylpropanoid biosynthetic pathway are identical and follow general principles. Small observed differences between the 2H-patterns of individual natural aromatic compounds originating from the same hydroxylation type can therefore be assigned to differences of the patterns of the precursors, the extent and the orientation of the hydrogen migration, and the kinetic isotope effect on the final hydrogen elimination. The evidence for the existence of general systematic rules governing isotopic patterns in the shikimic acid pathway and its subsequent reactions is further supported by the recently reported 13C-distribution pattern of vanillin, which is also in agreement with that predicted from the precursors. Hence, it is apparent that the systematics of the isotope patterns of phenylpropanoids are in line with the generally accepted biosynthetic reactions in the shikimic acid pathway and that this knowledge can strengthen their value as an essential support for the distinction of natural and synthetic aromatic compounds.

Benzaldehydes↗
PubMed2004

Short-term variations in delta(13)C of ecosystem respiration reveals link between assimilation and respiration in a deciduous forest.

We present a comprehensive dataset of hourly, daily, and monthly measurements of carbon isotope measurements of CO(2) in canopy air from a temperate deciduous forest with the aim to identify the relevance of short-term variations in the isotopic signature of ecosystem respiration (delta(13)C(R)) and to understand its underlying physiological processes. We show that during daytime low vertical mixing inside the canopy can lead to decoupling of the air in the lower and upper canopy layer resulting in large spatial variation of delta(13)C in CO(2) of canopy air. Intercept of Keeling Plots also showed large temporal variation (3.8 per thousand) over the course of the day demonstrating that intercepts can differ between day and night and suggesting that choosing the right time for sampling is essential to capture the isotopic signature of ecosystem respiration (delta(13)C(R)). delta(13)C(R) as obtained from night-time measurements showed large variation of up to 2.65 per thousand on a day-to-day basis, which was similar to the observed variation of delta(13)C(R) over the seasonal cycle (3.08 per thousand). This highlights the importance of short-term physiological processes within ecosystems for the isotopic composition of CO(2) in the atmosphere, not reflected by bulk plant and soil organic samples. At daily and monthly time scales, delta(13)C(R) increased with increasing ratio of vapour pressure deficit to photosynthetically active radiation, measured 4-5 days before. This suggests that ecosystem respiration was isotopically linked to assimilation. Furthermore, assimilates recently fixed in the canopy seem to form a labile carbon pool with a short mean residence time that is respired back to the atmosphere after 4-5 days.

Air↗
PubMed2004

Pythons metabolize prey to fuel the response to feeding.

We investigated the energy source fuelling the post-feeding metabolic upregulation (specific dynamic action, SDA) in pythons (Python regius). Our goal was to distinguish between two alternatives: (i) snakes fuel SDA by metabolizing energy depots from their tissues; or (ii) snakes fuel SDA by metabolizing their prey. To characterize the postprandial response of pythons we used transcutaneous ultrasonography to measure organ-size changes and respirometry to record oxygen consumption. To discriminate unequivocally between the two hypotheses, we enriched mice (= prey) with the stable isotope of carbon (13C). For two weeks after feeding we quantified the CO2 exhaled by pythons and determined its isotopic 13C/12C signature. Ultrasonography and respirometry showed typical postprandial responses in pythons. After feeding, the isotope ratio of the exhaled breath changed rapidly to values that characterized enriched mouse tissue, followed by a very slow change towards less enriched values over a period of two weeks after feeding. We conclude that pythons metabolize their prey to fuel SDA. The slowly declining delta13C values indicate that less enriched tissues (bone, cartilage and collagen) from the mouse become available after several days of digestion.

Analysis of Variance↗
PubMed2004

Is the isotopic composition of nitrous oxide an indicator for its origin from nitrification or denitrification? A theoretical approach from referred data and microbiological and enzyme kinetic aspects.

Literature data on the isotopic composition of nitrous oxide indicate a general predominance of the alpha-15N-isotopomer and a parallel 18O-enrichment in N2O from nitrification and denitrification, respectively. As the kinetic isotope effects on any individual reactions of the two processes lead to depletions of the heavy isotopes of nitrogen and oxygen in the products, the observed enrichments could mainly be caused by enzymatic reduction of NO, provided it occurs via a symmetric intermediate like hyponitrite; infrared data are in favour of large differences between the binding constants of the isotopomers and isotopologues of this compound. As a matter of fact one of the mechanisms discussed for the nitric oxide reductase from certain microorganisms implies the parallel binding of two NO molecules and the formation of a symmetrical intermediate, while that of the enzyme from other microorganisms reduces NO in a sequential mechanism. In addition, isotope effects on the reduction of N2O to N2 must contribute to the observed isotope characteristics of N2O, especially in context with denitrification. Therefore, the known enzymatic reaction pathways suggest that the alpha-15N-isotopomer preference and the 18O-signature of the produced N2O is not essentially characteristic for its origin from nitrification or denitrification, respectively, but rather from the involved population of microorganisms and the type of their nitric oxide reductases. This has to be confirmed experimentally.

Bacteria↗
PubMed2004

Biosynthesis of gallic acid in Rhus typhina: discrimination between alternative pathways from natural oxygen isotope abundance.

The biosynthetic pathway of gallic acid in leaves of Rhus typhina is studied by oxygen isotope ratio mass spectrometry at natural oxygen isotope abundance. The observed delta18O-values of gallic acid indicate an 18O-enrichment of the phenolic oxygen atoms of more than 30 per thousand above that of the leaf water. This enrichment implies biogenetical equivalence with oxygen atoms of carbohydrates but not with oxygen atoms introduced by monooxygenase activation of molecular oxygen. It can be concluded that all phenolic oxygen atoms of gallic acid are retained from the carbohydrate-derived precursor 5-dehydroshikimate. This supports that gallic acid is synthesized entirely or predominantly by dehydrogenation of 5-dehydroshikimate.

Gallic Acid↗
PubMed2003

The online 18O/16O analysis: development and application.

Oxygen is a globally important element, in inorganic compounds it is involved in key structural compounds of the earth, and it is providing functional groups for most organic compounds. Oxygen isotope ratio analyses on these compounds provide information on the oxygen sources and (formation) reactions in which the compound was involved. Therefore generally applicable analytical methods for the delta18O determination are highly desirable. After a description of online high temperature "carbon reduction" methods for the conversion of oxygen in the compounds mentioned into CO and discussion of problems involved, especially in analysis of inorganic substances like carbonates and silicates, selected examples for the application of these carbon reduction methods will be presented. The well-known correlation of delta18O values of cellulose (in tree rings) to that of (leaf) water is used since the 1970's as climate indicator. The observed enrichment in 18O in cellulose by about 27% in comparison to leaf water is attributed to an oxygen equilibrium isotope effect between carbonyl groups and water. But not all carbohydrates show this "general"18O enrichment relative to water. By means of a recently published model a non-statistical oxygen isotope distribution in carbohydrates is postulated. Oxygen isotope discrimination has been recognized long ago as an important principle of authenticity evaluation in food and beverages. Though many basic mechanisms of isotope discrimination in nature are known, the corresponding application of data is often based only on empirical observations in this field. The fact that oxygen in natural organic plant material is derived from three sources (CO2, H2O, O2) with distinct differences in and that their incorporation by different reactions implies oxygen isotope effects of different sizes, is therefore of special importance, and it is attempted by means of individual isotopic increments of functional groups to predict global delta18O values of natural and nature-identical compounds. Vice versa delta18O values can also give hints on the elucidation of biosynthetic pathways.

Cellulose↗
PubMed2002

The in vivo nitrogen isotope discrimination among organic plant compounds.

The bulk delta 15 N-value of plant (leaf) biomass is determined by that of the inorganic primary nitrogen sources NO(3)(-), NH(4)(+) and N(2), and by isotope discriminations on their uptake or assimilation. NH(4)(+) from these is transferred into "organic N" mainly by the glutamine synthetase reaction. The involved kinetic nitrogen isotope effect does not become manifest, because the turnover is quantitative. From the product glutamine any further conversion proceeds in a "closed system", where kinetic isotope effects become only efficient in connection with metabolic branching. The central and most important corresponding process is the GOGAT-reaction, involved in the de novo nitrogen binding and in recycling processes like the phenylpropanoid biosynthesis and photorespiration. The reaction yields relatively 15N-depleted glutamate and remaining glutamine, source of 15N-enriched amide-N in heteroaromatic compounds. Glutamate provides nitrogen for all amino acids and some other compounds with different 15N-abundances. An isotope equilibration is not connected to transamination; the relative delta 15 N-value of individual amino acids is determined by their metabolic tasks. Relative to the bulk delta 15 N-value of the plant cell, proteins are generally 15N-enriched, secondary products like chlorophyll, lipids, amino sugars and alkaloids are depleted in 15N. Global delta 15 N-values and 15N-patterns of compounds with several N-atoms can be calculated from those of their precursors and isotope discriminations in their biosyntheses.

Biological Transport↗