How do we assign alpha/beta status in polysaccharides?

In polysachharides like sucrose or any other disachhaharides, how is it that we assign whether the glycosidic bond is alpha/beta ?

For example the images here: http://forums.studentdoctor.net/showthread.php?t=898328 (kindly refer to the images of the original question, both of them)

How does one decide which glycosidic bonding is alpha and which is beta ? What if one of the monosachharides is alpha and one is beta constituting the disachharides ? How do you know that in the final acetal formation which sachharide acted as nucleophile and which acted as electrophile after seeing the disaccharide , take for example sucrose , glucose/sucrose (?) which acted as nucleophile any why ?

2 Answers

Here is an explanation for alpha- and beta- glucose. What it doesn't say is that the alpha-glucose can convert to beta-glucose and back again. Once the linkage is formed, it cannot change from alpha to beta.

Alpha linkage has the oxygen (on the aldehyde or ketone) below the ring and the beta has it above the ring.

Down the page, it shows beta-Maltose. The label beta (for the right most oxygen) is unimportant since the link has to be alpha for it to be maltose. A beta link would result in a molecule of cellobiose.

In beta you can't form a glycosidic bond; you have to remove a water molecule from the $\beta$-glucose to make a glycosidic bond which is (0). so drawn out alpha and beta the $\ce{-OH}$ in alpha on carbon one points below and on the adjacent carbon points below.

So water can be removed and leave behind an O hence the downward linkage you can't do that on beta so we say the adjacent beta molecule is rotated 180 degrees to get the 1-4 linkage

• A little punctuation goes a long way . . . You're not texting. – It's Over Mar 5 '16 at 19:33