Hemiacetal formation is the mechanism by which pentoses and hexoses assume their cyclic forms. Nucleophilic attack by typically the #4 or #5 carbon's hydroxyl group onto the carbonyl group (the anomeric carbon) of the noncyclic form leads to hemiacetal formation. Another nucleophile can act out the second part of the acetal formation mechanism upon the anomeric carbon forming a mixed acetal, termed a glycoside. The nucleophile may be adenine, guanine, cytosine, thymine, or uracil, in which case the glycoside product is a nucleoside, a fundamental building block of the nucleic acids DNA and RNA. If the nucleophile is another sugar, the process leads to the formation of a disaccharide (or polysaccharide). In other words, disaccharides are glycosides with the nucleophile being another sugar molecule.
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