Molecular level isotopic techniques

Fructose syrup. In addition to the glucose/fructose syrups mentioned above, a fructose syrup has been produced using inulin as a source. Inulin is the fructose analogue of starch, and the chicory root is the standard source for commercial hydrolysis. Fructose syrups are usually too expensive for routine use in beverage production but they have been employed where a particular claim is to be made for fructose. They have also been used for the adulteration of fruit juices as they are chemically difficult to detect. Detection is possible at the sub-molecular level by techniques such as stable isotope ratio measurement. Fructose is also manufactured using sucrose as a starting material. 

However, photoinduced proton transfer is still not well understood at the molecular level. In terms of its study, it has significant experimental advantages over electron transfer. It can be detected through vibrational spectroscopic techniques, can diffract X-rays and may undergo isotope exchange, thus permitting studies of kinetic isotope effects. 

Finally, some strategies are described which are employed to interpret the infrared difference spectra at a molecular level. The first goal is the identification of bands caused by the protein and cofactors (chromophore, substrate, etc.). In the case of retinal proteins in which the retinal chromophore can be removed and replaced by an artificial one, the method of isotope labeling has provided detailed information. Not only does it help to identify the chromophore bands, but it also enables their assignment to specific normal modes from which structural information can be derived. Of course, a similar technique should be applicable to quinones... 

Deuterium nmr spectroscopy has been utilized for the last decade to determine large (primary deuterium) KIEs in reactions with isotopes present at the natural abundance level (Pascal et al., 1984,1986 Zhang, 1988). A great advantage of this approach is that labelled materials do not have to be synthesized. Neither is there any need for selective degradation procedures, which are often necessary to produce the molecules of low mass, e.g. C02, acceptable for isotope ratio mass spectrometry. Moreover, the KIEs for several positions can be determined from one sample. However, until quite recently the relatively low precision of the nmr integrations that are used for the quantitative assessment of the amount of deuterium at specific molecular sites has limited the applicability of this technique for determining small (secondary deuterium) KIEs. 

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