The Inversion of Cane Sugar

 

Monday's Molecule #46 was the chemical reaction shown above. This is an historically important reaction that contributed to our understanding of catalysis.

The reaction shows the hydrolysis of sucrose to glucose and fructose in an acid solution. The reason why this was such an important reaction 100 years ago is because it is accompanied by a change in the direction of rotation of polarized light. Sucrose is an optically active compound, which means that it causes polarized light to rotate when you shine it through a solution of sucrose. The rotation is measured in a polarimeter. In the case of sucrose, light is rotated to the right. We call this dextrorotatory or "d" (lower case) from the latin prefix for "turning to the right". In modern chemical terminology it would be (+)-sucrose.

In the presence of acid, sucrose breaks down into D-glucose D-fructose. Both of the sugars are optically active. The "D" forms (Upper case "D" or small caps) are identified by the orientation of the CH₂OH group (red oval) in the ring structures shown above. In both cases the group is above the plane of the sugar so these are the "D" forms of the molecule. In L-glucose and L-fructose those groups would be below the plane of the sugar ring and all the -OH groups would be flipped as well.

Now here's the tricky part. The original determination of the "D" and "L" structures was related to the optical rotation properties. It was thought that all carbohydrates with the "D" configuration were dextrorotary ("d") and that's why they were named "D". The "L" forms were thought to be laevorotary (Latin: "turning to the left). However, it turns out that this assumption isn't correct and D-glucose and D-fructose are good examples. They both have the "D" configuration but D-glucose rotates the plane of polarized light slightly to the right and D-fructose rotates it strongly to the left. The way to identify this property in modern terms is D(+)-glucose and D(-)-fructose. There's a good description of these properties on the Biochemical Howlers website.

That's all very interesting but why was it important back in 1900? It was important because if you treated a solution of sucrose with acid it "inverted" polarized light from rotating to the right to rotating to the left because D-fructose affected rotation more strongly than D-glucose. This meant that you could follow the reaction in real time by carrying it out in a test tube placed in a polarimeter. This was one of the few reactions of this sort that were amenable to kinetic studies back then.

Many workers discovered that the rate of the reaction was increased by increasing acid concentrations and it led to detailed studies of reaction kinetics with large molecules. (There were plenty of inorganic reactions that could be followed by watching changes in color.) An modern example is shown in the accompanying figure from Shalaev et al. (2000).

As you can see, the kinetics of the reaction are easy to follow and the results lead to simple mathematical interpretations of the rate and extent of the reaction. It was experiments like this that led to a theory of catalysis in the early 1900's and the awarding of a Nobel Prize to Wilhelm Ostwald in 1909.

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Shalaev, E.Y., Lu, Q., Shalaeva, M. and Zografi, G. (2000) Acid-catalyzed inversion of sucrose in the amorphous state at very low levels of residual water. Pharm. Res. 17:366-70.