14
$\begingroup$

What's the difference between $\alpha$-D-glucose and $\beta$-D-glucose? Are they isomers? Or do they differ in their elemental composition?

$\endgroup$
  • $\begingroup$ beta D-glucose units makes up the structure of cellulose polysaccharides while alpha D-glucose units makes up the structure of polysaccharides starch. $\endgroup$ – user9873 Nov 18 '14 at 14:12
26
$\begingroup$

$\alpha$-D-glucose and $\beta$-D-glucose are stereoisomers - they differ in the 3-dimensional configuration of atoms/groups at one or more positions.

$\alpha$-D-glucose

enter image description here

$\beta$-D-glucose

enter image description here

Note that the structures are almost identical, except that in the $\alpha$ form, the $\ce{OH}$ group on the far right is down, and, in the $\beta$ form, the $\ce{OH}$ group on the far right is up.

More specifically, they are a class of stereoisomer called an anomer. Anomers are capable of interconverting in solution. All cyclic structures of monosaccharides exhibit anomeric $\alpha$ (down) and $\beta$ (up) versions. These differences occur at the anomeric acetal carbon (the only carbon with two $\ce{C-O}$ bonds.

These two forms exist because all monosaccharides also have an open-chain form with one fewer stereocenter. When the chain closes to the cyclic structure, the aldehyde or ketone carbon becomes a stereocenter, and it can do so in either configuration. One configuration is preferred ($\beta$), but both exist.

Open chain form of glucose:

enter image description here

In the presence of acid or base (although water can fulfill this role if need be), the two anomers interconvert through the open form until dynamic equilibrium is established. The mechanism below starts with $\alpha$ in the upper left and finishes with $\beta$ in the lower right. The open-chain form is in the middle.

enter image description here

$\endgroup$
  • 4
    $\begingroup$ Just to add, in the L-configuration, the situation is reversed, since you draw the CH2OH below the ring in the Haworth projection. In both cases, the structure is ß when the relevant groups are on the same side of the ring, and α when they're on opposite sides. Had a question on a test that hinged on that understanding. Understanding the concept of the "anomeric carbon" and the "reference carbon" is also helpful. $\endgroup$ – user5434 Nov 19 '14 at 19:01
  • $\begingroup$ It's worth noting that while $\beta$-glucose is more stable, the stability of $\alpha$-glucose is higher than one would expect it to be (anomeric effect) and also that the stabilities are reversed for mannose, i.e. $\alpha$-mannose is more stable than $\beta$-mannose. $\endgroup$ – Jan Jun 10 '15 at 0:11

Not the answer you're looking for? Browse other questions tagged or ask your own question.