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Ribose and its related compound, deoxyribose, are the building blocks of the backbone chains in nucleic acids, better known as DNA and RNA. Ribose is used in RNA and deoxyribose is used in DNA. The deoxy- designation refers to the lack of an alcohol, -OH, group as will be shown in detail further down. Ribose and deoxyribose are classified as monosaccharides, aldoses, pentoses, and are reducing sugars.
Ring Structure for Ribose
The chair form of ribose follows a similar pattern as that for glucose with one exception. Since ribose has an aldehyde functional group, the ring closure occurs at carbon # 1, which is the same as glucose. See the graphic on the left. The exception is that ribose is a pentose, five carbons. Therefore a five membered ring is formed. The -OH on carbon #4 is converted into the ether linkage to close the ring with carbon #1. This makes a 5 member ring - four carbons and one oxygen.
Steps in the ring closure (hemiacetal synthesis)
- The electrons on the alcohol oxygen are used to bond the carbon #1 to make an ether (red oxygen atom).
- The hydrogen (green) is transferred to the carbonyl oxygen (green) to make a new alcohol group (green).
The chair structures are always written with the orientation depicted above to avoid confusion. Carbon # 1 is now called the anomeric carbon and is the center of a hemiacetal functional group. A carbon that has both an ether oxygen and an alcohol group is a hemiacetal.
Compare Ribose and Deoxyribose Structures
The presence or absence of the -OH group on carbon (#2) is an important distinction between ribose and deoxyribose. Ribose has an alcohol at carbon # 2, while deoxyribose does not have the alcohol group. See red -OH and H in the structures below. The Beta position is defined as the -OH being on the same side of the ring as the C # 6. In the ring structure this results in a upward projection. The Alpha position is defined as the -OH being on the opposite side of the ring as the C # 6. In the ring structure this results in a downward projection. The alpha and beta label is not applied to any other carbon - only the anomeric carbon, in this case # 1.
Charles Ophardt (Professor Emeritus, Elmhurst College); Virtual Chembook