THE CARBOHYDRATES AND THEIK METABOLISM 237 



Special Properties of Monosaccharides. The general properties and 

 reactions of the monosaccharides have just been presented and it remains 

 to point out properties of the individual carbohydrates that are of special 

 interest biologically. 



Hexoses. Only two hexoses are found naturally as such, d-glucose and 

 d-fructose; d-glucose, the most common monosaccharide occurring in na- 

 ture, is found in most plant and animal tissues. Commercially it is ob- 

 tained by hydrolyzing starch with dilute acid. This glucose is a mixture of 

 a- and ^-glucose and is called y-glucose. It is readily purified by one 

 crystallization from glacial acetic acid and washing with alcohol. From 

 aqueous solution it crystallizes with one molecule of water. This form 

 melts at 86 C. The anhydrous form, obtained by crystallization from 

 aqueous solution at high temperature, melts at 146 C. One hundred 

 parts of water dissolve 81.7 parts of anhydrous glucose at 15 C., while in 

 alcohol it is. rather insoluble. It is insoluble in ether and almost insoluble 

 in acetone. Its aqueous solutions are neutral and are not. electrolytes. 



When heated to 170 it darkens and gives off much water, leaving 

 in the residue a deliquescent substance, glucosan, which can be converted 

 to glucose by boiling with water or acids. It is not sweet nor does it 

 undergo fermentation. It is dextrorotatory. 



Methyl Glucosides. a-Methyl glucoside was first obtained by E. 

 Fischer, by dissolving glucose in acetone-free anhydrous methyl alcohol, 

 containing 0.25 per cent hydrogen chlorid, heating it under pressure, dis- 

 tilling off the alcohol and obtaining the crystals from the residual solution. 

 Both the a- and (3-methyl glucosides are found in this reaction, the equilib- 

 rated mixture containing 77 per cent of the o-form. 



a-Methyl glucoside forms rhombic crystals melting at 165 C., easily 

 soluble in water, difficultly soluble in cold alcohol, practically insoluble in 

 ether. Its specific rotation is -f- 157 and does not show mutarotation. 

 It does not reduce, does not form hydrazones, nor exhibit any aldehydic 

 properties and is therefore believed to exist in the y-lactone form only. 



GEL O CH 



HC OIL 



CH 2 OH 

 a-Methyl Glucoside 



(3-Methyl Glucoside 



