Dear Tim,

according to my knowledge sucrose is a non-reducing disaccharide
(Fehling reaction negative) and does therefore not exhibit mutarotation.
This means that both monosaccharides are linked by a glycoside link
between C1 of glucose and C2 of fructose. That means that the anomeric C
atoms of both monosaccharides are linked and so both components of the
sucrose molecule should not show any "aldehyde group character". Oxygen
in aldehyde groups is most prominent for oxygen isotope exchange
reaction (the oxygen isotope exchange of ketones is normally less
pronounced).
It should therefore be expected that sucrose should not show any oxygen
isotope exchange reaction under physiological regime (pH around 7,
moderate temperature and pressure).
Invertase enzyme and low pH can lead to a breakdown of sucrose to
glucose and fructose which both can exchange the O atoms of their C=O
groups with water.
Consequently, Sternberg et al (1986) found no oxygen isotope exchange
when autoclaving sucrose with water of different isotopic composition
(-15 and +100; Plant Physiol. 82, 423ff).

--> Your finding is very interesting and could have consequences for
standardising in d18O determination by on-line carbon reduction systems.
Could you please give aditional information
What are slight differences? How do you treat your samples after
freeze-drying? ....

Open for discussion:
Could it be that the (low, but existing; sorry I cannot find additional
data at the moment) hygroscopicity of the sucrose could influence your
sucrose samples?
It is expectable that freeze-dried hygroscopic sample will take up water
from the atmosphere, e.g. in the autosampler or any storage place not
kept under absolute dry atmosphere. This will lead to a sort of "oxygen
isotope equilibrium" of the water in the sugar to the water in the lab air.
An uptake or loss of any water (e.g. a change in humidity) into or out
of the sugar will possibly lead to a disturbance of this "equilibrium"
for a "more or less" short time. An (re-)adjusting of this "equilibrium"
between dried sugar and more or less humid air will probably also take
some time. The adjustment time could be influenced by the dissolving and
re-drying  process of  "granulated" sugar (by changing of surface area?).

Greetings from sunny Zurich + best regards

Roland



Heaton, Timothy HE wrote:

>Can sucrose undergo O-isotope exchange with water? In preliminary tests we are finding slight differences in d18O between IAEA CH-6 weighed out into capsules as a solid powder, and the same IAEA CH-6 dissolved in water and pipetted into capsules. All cases used well-homogenised powder and thorough freeze-drying.
>
>Tim H.E. Heaton
>
>NERC Isotope Geosciences Laboratory
>British Geological Survey
>Keyworth, Nottingham NG12 5GG, England
>(www.bgs.ac.uk/nigl/index.htm)
>
>Tel. +44(0)115 936 3401
>Email: [log in to unmask]
>
>
>
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