18O pattern and biosynthesis of natural plant products

Oxygen atoms in plant products originate from CO2, H2O and O-2, precursors with quite different delta O-18 values. Furthermore their incorporation by different reactions implies isotope effects. On this base the resulting non-statistical O-18 distributions in natural compounds are discussed. The delta O-18 value of cellulose is correlated to that of the leaf water, and the observed O-18 enrichment (similar to + 27 parts per thousand) is generally attributed to an equilibrium isotope effect between carbonyl groups and water. However, as soluble and heterotrophically synthezised carbohydrates show other correlations, a non-statistical O-18 distribution - originating from individual biosynthetic reactions - is postulated for carbohydrates. Similarly, the delta O-18 values of organic acids, carbonyl compounds, alcohols and esters indicate water-correlated, but individual O-18 abundances (e.g. O from acyl groups similar to + 19% above water), depending upon origin and biosyntheses. Alcoholic groups introduced by monooxygenase reactions, e.g. in sterols and phenols, show delta O-18 values near + 5 parts per thousand, in agreement with an assumed isotope fractionation factor of similar to1.02 on the reaction with atmospheric oxygen (delta O-18 = +23.5 parts per thousand). Correspondingly, a "thermodynamically ordered isotope distribution" is only observed for oxygen in some functional groups correlated to an origin from CO2 and H2O, not from O-2. The individual isotopic increments of functional groups permit the prediction of global delta O-18 values of natural compounds on the basis of their biosynthesis. (C) 2001 Published by Elsevier Science Ltd.