CARBOHYDRATES 



17 



The names of the common aldehyde sugars have been used to derive prefixes de- 

 scriptive of hydroxylation patterns in less common carbohydrates. For example, sedo- 

 heptulose may be named as D-a/^ro-heptulose to indicate: 



a. It belongs to the D-series (i. e. the highest numbered asymmetric carbon is 

 written to the right in the open chain structure). 



b. It has four asymmetric hydroxylated carbons with the same configuration as 

 the four in altrose. 



c. It has seven carbons. 



d. It is a keto sugar. 



This prefix system is not likely to supplant the common names of already known 

 sugars, but it is useful in naming new or little known sugars or their derivatives. The 

 ending "-ulose" is used to designate keto sugars. (Although not all keto sugars are named 

 in accordance with this rule. ) Structures of some of the keto sugars are given in Figure 

 2-4. 



Some of the most widespread sugars contain non -hydroxylated carbons and are named 

 as deoxy sugars. Some illustrations are given in Figure 2-5. 



A few of the monosaccharides are found free in plant saps. Glucose is almost uni- 

 versally found in this way. Others which occur free are fructose in many fruits, D-maniio- 

 heptulose and D-glycero-D-maiiiio-octulose in avocados (Persea aiuericana) (2), sedo- 

 heptulose in succulent plants, and rhamnose in poison ivy (Rhus radicans) . Many more of 

 the monosaccharides do not occur as free sugars but are very common as esters, polymers, 

 glycosides and other derivatives to be discussed below. 



Some rare monosaccharides have branched carbon chains. For instance, apiose 

 occurs as the glycoside apiin of parsley (Petroseliniuu hor tense), and hamamelose is 

 found as an ester in the tannin of witch hazel (Hauiamelis virginiaiia) . 



HC=0 



HC=0 HOCHo-COH 



I ^ I 



HCOH HCOH 



I I 



HOCH^-COH HCOH 



CH2OH CHjOH 



apiose hamamelose 



GLYCOSIDES AND OTHER ETHERS 



As hydroxyl compounds the carbohydrates are capable of forming ethers with other 

 alcohols. Most important of these are the glycosides, which have the ether group linked 

 to the anomeric carbon atom. Ethers involving the other carbon atoms are important as 

 synthetic compounds and for studies of carbohydrate structure, but they are rare in nature. 

 3-O-methyl-D-galactose is found in the hydrolysis products of slippery elm (Ulmtis fulva) 

 mucilage. Other methyl ethers are found in more complex carbohydrates. 



