18.5 References 859Table 18.44.Isotope discrimination in primary photosynthetic CO 2 binding
Plant group CO 2 Acceptor δ(^13 C) value Foods
‰
C 3 -Plant D-Ribulose-1,5-bis-phosphate −32 to−24 Wheat, rice, oats, rye, potatoes, barley, batata,
carboxylase (RuPB-C) soybean, orange, sugar beet, grapes
C 4 -Plant Phosphoenolpyruvate −16 to−10 Corn, millet, sugar cane
carboxylase (PEP-C)
CAM-Planta RuBP-C/REP-C −30 to−12 Pineapples, vanilla,cactaceae, agave
aCAM: Crassulacean acid metabolism.
plant (Table 18.44). The discrimination in C 3 -
plants is the greatest and is caused by the kin-
etic isotope effect in the reaction catalyzed by
ribulose-1,5-biphosphate carboxylase. It is con-
siderably less in C 4 -plants. CAM plants occupy
an intermediate position (Table 18.44) because
the C 3 -ortheC 4 -path is taken depending on the
growth conditions.
The large differences in the masses of
(^1) H
2 O,
(^2) H (^1) HO, and (^2) H
2 O result in consider-
able thermodynamic isotope effects on phase
transitions. On evaporation, deuterium (^2 H)
correspondingly decreases in the volatile phase,
so that surface-, ground-, and rain-water contains
less^2 H than the oceans. The^2 H enrichment
in the oceans is greatest at the equator and
decreases with increasing latitude because the
amount of water evaporating depends on the
temperature.
The hydrogen of plant foods comes from precip-
itation and from the ground-water in that particu-
lar location. Therefore, plants of the same type of
photosynthesis, which are cultivated at different
places, differ in theirδ(^2 H) values. Kinetic iso-
tope effects in plant metabolism, which due to the
mass difference^2 H/^1 H are much higher than in
the case of^13 C/^12 C, also have an effect on the
δ(^2 H) values.
For isotope analysis, the sample is subjected to
catalytic combustion to give CO 2 and H 2 O. Af-
ter drying, the^13 C/^12 C ratio in CO 2 is determined
by mass spectrometry. The^2 H/^1 H ratio is deter-
mined in hydrogen, which is formed by reduc-
ing the water obtained from catalytic combustion.
The^2 H/^1 H ratio can change by^2 H/^1 H exchange,
e. g., as undergone by OH groups. Therefore, such
groups are eliminated before combustion. For ex-
ample, only theδ(^2 H) values of the CH-skeleton
in carbohydrates are determined after conversion
to the nitrate ester.
Table 18.45.δ(^13 C) andδ(^2 H) values for orange juice
and sugar of different origins
Food δ(^13 C)(‰) δ(^2 H)(‰)
Orange juice, freeze-dried − 25. 6 ± 0 .8n.a.
Sucrose isolated from − 25. 5 ± 2. 5 − 22 ± 10
orange juice
Beet sugar − 25. 6 ± 1. 0 − 135 ± 25
Cane sugar − 11. 5 ± 0. 5 − 50 ± 20
Glucose-fructose syrup − 10. 8 ± 0. 9 − 31
(corn)
n.a.: not analyzed.
Sweetening orange juice with cane sugar or
glucose-fructose syrup from corn starch lowers
theδ(^13 C) value of sugar, which is− 25 .5‰ in
the native juice (Table 18.45). On the other hand,
the addition of beet sugar (C 3 -plant) can be rec-
ognized only via theδ(^2 H) value. The addition of
synthetic products from petrochemicals (δ(^13 C):
− 27 ±5‰) to foods from C 3 -plants cannot be
detected via theδ(^13 C) value, but via theδ(^2 H)
value in many cases.
Apart from the global^13 Cand^2 H contents of
food constituents, the intramolecular distributions
of these isotopes are typical of the origin and,
therefore, of great analytical importance. They
can be measured after chemical decomposition
of the substance or with^13 Cor^2 H NMR spec-
troscopy (example in 5.5.1.5).
18.5 References
Bell, E.A., Charlwood, B.Y. (Eds.): Secondary plant
products. Springer-Verlag: Berlin. 1980
Berger, R.G., Shaw, P.E., Latrasse, A., Winterhalter,
P: Fruits I–IV. In: Volatile compounds in foods and