Glucose Muta-rotation

Figure 1.20 Possible transition states in the catalysis of tetramethyl glucose muta-rotation by 2-pyridone, and calculated transition state for formic acid catalysis of 2-hydroxytetrahydropyran ring opening. In the calculated transition state, the proton is largely transferred to the endocyclic oxygen, the endocyclic C-O bond has started to break, but the hydroxyl proton is not transferred to the catalyst, i.e. the reaction is concerted but not synchronous.

The situation just discussed for glucose is described as a simple muta-rotation. In such cases, as already indicated (see Figure 3 for example),... 

It was originally proposed by Lowry, and later argued again by Swain, that since the function of the acid (or base) in these reactions is to facilitate the shift of a proton from one part of the substrate molecule to another (i.e., a prototropic shift), that a mechanism that involves simultaneous removal and addition of a proton might provide the most rapid path. In such case, the reactions should be expected to involve the simultaneous attack of acid and base on the proper parts of the molecule. In the muta-rotation of a-D-glucose the process can be pictured as... 

Bentley and Bhate140 conducted a meritorious study of the muta-rotations of D-glucose and D-galactose in the presence and absence of the enzyme mutarotase from Penicillium notatum under a variety of conditions. For measurements of the water-catalyzed and mutarotase-catalyzed reactions at 23-24°, they found the following... 

If optically active substances undergo chemical conversion (e.g.. sucrose in acid solution), the specific rotation becomes time dependent (muta-rotation). For example, when dextrorotatory sucrose reacts until chemical equilibrium is reached, a levorotatory mixture of glucose and fructose is formed (inversion of sucrose). 

Although the acetylated galactose hydrazones probably have acyclic structures, the hydrazones with free hydroxyls may exist in the ring forms. In solution, the sugar hydrazones show complex mutarotations which pass through a maximum or minimum 208 210, 211). The failure of the muta-rotation equation to follow the first-order equation indicates that three or more substances take part in the equilibrium. Three isomeric glucose eiO. H. Jacobi, Ann. 272, 170 (1892). 

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