Sugars Question?

[StefBall]

So I was wondering what constitutes an alpha vs beta anomer? Is it the fact that it the anomeric hydroxyl is on the opposite side of some reference carbon (making it alpha), or is it the fact that the anomeric hydroxyl is oriented axially (making it alpha)? See my picture from Kaplan below.

Which of the isomers shown in figure 2 are a-anomers?

---

Members don't see this ad.

 

[stevvo111]

I thought if it was just up then it was alpha, and down was beta... pretty sure it has nothing to do with whether it is equatorial or axial.


^anyone back me up on that?

in the pic I think 2 and 5 are alpha, while 3 and 4 are beta.

the carbon you should look at is the acetal one...which is next to the -0- bond, so for this the anomeric carbon would be the one attached to OH and CH3OH, but this is the first time I've seen something (ch2-OH) other than H attached to the anomeric carbon, I suppose it happens though.

 

[MTHFR]

I thought if it was just up then it was alpha, and down was beta... pretty sure it has nothing to do with whether it is equatorial or axial.


^anyone back me up on that?

in the pic I think 2 and 5 are alpha, while 3 and 4 are beta.

the carbon you should look at is the acetal one...which is next to the -0- bond, so for this the anomeric carbon would be the one attached to OH and CH3OH, but this is the first time I've seen something (ch2-OH) other than H attached to the anomeric carbon, I suppose it happens though.

No. Beta ("bet-UP") is typically up (when drawn in the way furanoses, pyranoses, and other cyclic sugars are typically drawn); alpha is down. Formally, it is a beta anomer if the anomeric hydroxyl is cis with the last carbon. It's alpha if the anomeric hydroxyl is trans with the last carbon.

That said, I'm having trouble with understanding what I'm looking at in your picture. In II, I believe that is beta since it is cis, although the fact that it is in the boat conformation might change things. In III, I believe it's alpha since the it is trans. Again, since it's in the boat conformation, I'm not entirely sure. Maybe somebody smarter than me can figure out what is what in your picture.

 

[FeinMS]

I've never seen a fructose in its pyranose form (IV and V)
II is beta because C6 (the one on the top left) is cis with anomeric OH
III is alpha because C6 is trans with anomeric OH.

According to wikipedia, it says IV is alpha and V is beta IDK how to tell because C6 is not sticking out anywhere in pyranose form..

 

[Schenker]

You guys are right. In the beta anomer, the anomeric hydroxyl and the C6 group are cis. In the alpha anomer, they are trans.

With some mental rotation, this is how you can identify anomeric forms of fructose when in its pyranose form: Imagine the hydrolysis product of the ether, without changing the conformation of the sugar (i.e., replace C6-O-C2 with C6-OH HO-C2). Rotate C5 (along the C4-C5 bond) until the hydroxyl attached to C5 is across from the HO-C2 you produced by mental hydrolysis. Form an ether from those two (now adjacent) hydroxyls (i.e., make C5-OH HO-C2 into C5-O-C2). Now, you'll have fructose in its furanose form. Identify the anomer.