Isotope discrimination

The natural cycles of the bioelements carbon, oxygen, hydrogen, nitrogen and sulphur) are subjected to various discrimination effects, such as thermodynamic isotope effects during water evaporation and condensation or isotope equilibration between water and CO2. On the other hand, the processes of photosynthesis and secondary plant metabolism are characterised by kinetic isotope effects, caused by defined enzyme-catalysed reactions [46]. 

---

Komer, Ch., Farquhar, G.D. and Roksandic, Z. 1988 A global survey of carbon isotope discrimination in plants from high altitude. Oecologia 74 623-632. 

A second kind of model is briefly treated, based on isotopic mass balance arguments, and it is shown that large isotopic discrimination during methano-genesis in ruminants may account for data trends when comparing herbivores and carnivores. A third class of model is sketched at the level of biochemical flows, where some fundamental points are made concerning points where the isotopic composition of metabolites may be altered. The relevance of this to nitrogen isotopic enrichment is considered. 

As mentioned in the Introduction, it is necessary but not sufficient for isotopically discriminating reactions to occur if isofractionation is to be observed in any particular body component. Whether the isofractionation incurred in a particular reaction in the pathway leads to a measurable effect in a body pool component depends on the pattern or topology of the metabolic fluxes. Three basic cases are considered here for illustrative purposes. 

Carbon isotope discrimination by leaves with the C3 pathway of photosynthesis is related to pj/pa (Farquhar et al., 1982), through... 

Isotope discrimination and water-use efficiency If we rearrange Equation 6 and substitute for pjp. in Equation 4, we obtain a negative dependence of IV on A ... 

Fig. 5. Water-use efficiency (carbon basis) v. average carbon isotope discrimination in the whole plant, r= -0.88. Open symbols represent well-watered plants and closed symbols represent plants that were droughted. Tifton 8 A, Florunner A, VB187 and +, Chico are cultivars of peanut (Arachis hypogaea). (From Flubick etal., 1986).

Condon, A.G., Richards, R.A. Farquhar, G.D. (1987). Carbon isotope discrimination is positively correlated with grain yield and dry matter production in field-grown wheat. Crop Science, 27, 996-1001. 

Evans, J.R., Sharkey, T.D., Berry, J.A. Farquhar, G.D. (1986). Carbon isotope discrimination measured concurrently with gas exchange to investigate CO2 diffusion in leaves of higher plants. Australian Journal of Plant Physiology 13,281-92. 

Francey, R.J., Gifford, R.M., Sharkey, T.D., Weir, B. (1985). Physiological influences on carbon isotope discrimination in huon pine (Lagarostrobes frank-linii). Oecologia, 66, 211-18. 

Hubick, K.T. Farquhar, G.D. (1989). Genetic variation of transpiration efficiency among barley genotypes is negatively correlated with carbon isotope discrimination. Plant, Cell and Environment (in press). 

Hubick, K.T., Farquhar, G.D. Shorter,R. (1986). Correlation between water-use efficiency and carbon isotope discrimination in diverse peanut (Arachis) germplasm. Australian Journal of Plant Physiology, 13, 803-16. 

Masle, J. Farquhar, G.D. (1988). Effects of soil strength on the relation of water use efficiency and growth to carbon isotope discrimination in wheat seedlings. Plant Physiology, 6,147-55. 

Farquhar, G.D., O Leary, M.H. Berry, J. A. (1982). On the relationship between carbon isotope discrimination and the intercellular carbon dioxide concentration in leaves. Australian Journal of Plant Physiology, 9, 121-37. 

Agren GI, Bosatta E, Balesdent J (1996) Isotope discrimination during decomposition of organic matter a theoretical analysis. Soil Sci Soc Am J 60 1121-1126 Balesdent J (1987) The turnover of soil organic fractions estimated by radiocarbon dating. Sci Total Environ 62 405-408... 

Keywords Energy production 13C fractionation maintenance 13C isotopic discrimination... 

Clay et al. (2006) proposed an approach to account for isotopic discrimination that occurs during relic carbon mineralization. This approach is based on the equation... 

Extension of this research was then delayed until two of us had returned to New Zealand. One of the first jobs of Tasker, a PhD student from Brisbane, was to use 3H-labeling methods to examine epimerization. At that time we were under the impression that reprotonation of carbanion intermediates would be diffusion controlled, as was subsequently reported for phenylglycine in phosphate buffers (18), so that no isotopic discrimination against 3H would be present. But then Wautier et al. reported (6) the complete loss of chirality in [Co(tren)(AA-(S)-AA OMe)]3+, prepared by treating [Co(tren)((S)-AAOMe)]3+ (AA = Ala, Leu) generated in situ in MeOH with (S)-AA OMe (AA = Leu, His, Ala, Val), although their reaction times and temperatures (1-15 h sometimes 35-50°C) and other experimental conditions ([Co(III)] —0.1 M L/V-ethylmorpholine salt of p-toluene sul-... 

As for all trace-level analyses, sample preparation and handling are of crucial importance. In addition to all the usual problems of GC-MS, measurements of isotope ratios must ensure that none of these steps introduce any isotope discrimination. Any chemical reactions, including conversion of the organic sample molecules to the simple gases which are those actually analyzed, must be quantitative (100% conversion) to avoid kinetic isotope effects [627]. Until relatively recently, all gas IRMS experiments employed a dual-inlet system to permit switching between sample and standard C02 contained in two bellows containers. The pressures in the two bellows are adjusted to be equal and,... 

Blackmer AM, Bremner JM (1977) Nitrogen isotope discrimination in denitrification of nitrate in soils. Soil Biol Biochem 9 73-77... 

---