26 CARBOHYDRATES 



the same configuration as those on the right in the open-chain formulas 

 with carbon 1 on top. 



The ring structures shown above are of the 1,5-oxide, or pyranose type, 

 and are formed by aldohexoses, aldopentoses, and ketohexoses. Oc- 

 casionally a 1,4-oxide or furanose ring is formed, as for example in the 

 fructose component of sucrose (see p. 44). 



Pentoses 



There is no clear-cut evidence to show that pentoses occur free in 

 plants. No free pentoses or characteristic derivatives of free pentoses 

 have ever been isolated from seeds or green plants, or from any other 

 natural source. Qualitative tests and quantitative data which were 

 formerly attributed to free pentoses are now thought to be due to other 

 compounds, such as glucuronic acid. A good test for pentoses depends 

 on their conversion into furfuraldehyde by heating with fairly concen- 

 trated solutions of mineral acids: 



H^ /OH H 



HOCH "^0 H* HC=^ ^0 



I I — -* I I -}-3H,0 



HC C-CH2OH ^ HC=C— CHO 



H 

 H 



An aldopentose Furfuraldehyde 



(furanose ring form) 



This product produces a brilliant rose-red color when warmed with 

 aniline acetate, and therefore indicates the presence of pentoses. Hex- 

 uronic acids also give this test (p. 39) but hexoses do not, since they are 

 converted by the acid treatment into levulinic acid, CH3COCH2CH2COOH, 

 which gives no color with aniline acetate. 



Anhydrides of some of the pentoses are very abundant in plant materials, 

 however, and therefore the corresponding pentoses can be easily prepared. 



jy-Xylose. Xylose is sometimes called wood sugar, as it can be made 

 readily from wood, straw, seed hulls, and other fibrous materials. It is 

 easily prepared from corn cobs by hydrolysis and crystallization. Corn 

 cobs contain about 35 per cent of pentosans and yield about 12-15 g. of 

 xylose per 100 g. of cob. The pure sugar sells for about $25 per pound, 

 largely because there is not enough demand for it to make large-scale 

 production worth while. It has been estimated that on a large scale it 

 could be made for 5 cents per pound. Its use is limited almost entirely 

 to bacteriological laboratories, where it is of considerable aid in the 

 classification of bacteria. 



-L-Arabinose. This pentose is found in the form of complex poly- 

 saccharides in wheat and rye brans, in pectins, and in gummy materials 



