Carbohydrates, where they are synonym of saccharide, are found throughout nature. Carbohydrates are polyhydroxy aldehydes and ketones that form essential structural elements in nature, confer individuality to biological surfaces, and are vital intermediates in the production and storage of energy in biological systems. When one or more atoms of carbohydrates are replaced by the stable isotopes of that atom, that is stable isotope-labeled carbohydrates. Carbohydrates contain multiple carbon, hydrogen, and oxygen atoms, and sometimes nitrogen atoms, making possible a wide range of different isotopomers of any given carbohydrates that include ¹³C, ²H, ¹⁸O, and ¹⁵N stable isotopes. Carbohydrates can be singly (site-selective), multiply, or uniformly labeled in a given isotope. More than one type of isotope can be present in a labeled carbohydrate ^[1].

Synthesis

Take the synthesis method of stable isotope-labeled monosaccharides as example. Because monosaccharides are the smallest carbohydrates and the basic molecular units of various carbohydrates like disaccharides and polysaccharides.

Chemical and enzymic methods are often considered for the preparation of monosaccharide isotopomers that are singly and multiply labeled with ¹³C, ²H, ¹⁸O, and ¹⁵N isotopes. The attractiveness of enzymic approaches lies in their normally high yields and specificity in comparison to chemical methods. However, the enzyme-catalyzed reactions are not always superior to chemical method. There are, and will continue to be, efficient chemical reactions that are easy to perform on large scales and give excellent yields. Two primary isotope insertion reactions in chemical method are cyanohydrin reduction (CR) and molybdate-catalyzed epimerization (MCE). Thus, optimal synthetic protocols for the synthesis of simple stable isotope-labeled monosaccharides and their assembly into oligomers (for example, oligosaccharides, oligonucleotides) will frequently be composed of integrated chemical and enzymic reactions.

Fig. 1 The scheme of use of cyanohydrin reduction to introduce carbon, hydrogen, and oxygen isotopes into aldoses.