[Rayan]

hello,
in drawing the structure of amylose for example, w have an alpha (1-4) glycosidic bond, so the o bond is pointing downward. and in cellulose  it's also alfa(1-4) but in drawing it, we draw it beta with respect to 1 and alpha with respect to 4 which make the structure flipped over.
would anyone explain to me when do we draw the structure flipped over and when we dont, i cant figure it out.
thanks

[Albert]

Cellulose is a polymer of BETA-CELLOBIOSE: this means you have a beta-1,4- linkage between two molecules of glucose.

The stablest conformation for beta-1,4 bonds is 180° rotated. So, the resulting polymer looks like a beta-sheet.

[Rayan]

i just can't get it.
how do i tell that the most stable is the 180 degree?
i mean, in the exam they would just ask us to draw an amylose molecule for example. how do i tell when the bonds are up and when are they down?

[Albert]

(ALPHA-)amylose is a polymer of maltose (alpha-1,4 bonds) and is 25% of starch.

Cellulose is just a polymer of cellobiose (beta - 1,4 bonds).

If you're asked to draw an amylose, I'd draw a series of molecules of maltose.

[Yggdrasil]

Whenever people draw carbohydrate's on paper they're not really trying to correctly reproduce the 3-D structure.  There are many different ways of representing alpha (1->4) and beta (1->4) linkages.   For example, here are different representations of a beta (1->4) linkage:

http://opbs.okstate.edu/~Blair/Bioch2344/Chapter10/CPT10FIGS/cellobiose.gif
http://www.guidobauersachs.de/oc/betagly.gif
http://tonga.usip.edu/gmoyna/biochem341/disaccharide6.gif

In terms of which one best represents the correct 3D structure of cellbiose, the thrid one is best, but all three are accurate representations of cellbiose.  The other two have their merits, however, becaues they preserve the glucose units in their standard Haworth projections, which makes them easier to identify to those who haven't worked much with carbohydrates.  As long as you show the correct connectivity and stereochemistry, I think you'll be fine no matter what projection you draw (and it is probably best to get used to seeing many different projections of a molecule, as issues like this pop up in all of chemistry and biochemistry).

[Yggdrasil]

Another note.  I just reread the OP and saw that there might also be some confusion regarding the definition of alpha and beta.  Alpha and beta do not necessarily represent up and down.  Rather, alpha and beta refer to the position of the anomeric OH group (in glucose, the OH on carbon #1) with respect to the CH₂OH group  (in glucose, carbon #6).  If the OH is anti to the CH₂OH group, the OH is in the alpha form.  If the OH is cis to the CH₂OH group, the OH is in the beta form.  In glucose, alpha and beta only apply to the anomeric carbon (carbon #1).  To say another carbon is in the alpha or beta postion (like saying carbon #4 is in the alpha position) is wrong since carbons 2-6 are incapable of mutorotation (conversion between alpha and beta).

[Albert]

You're right, but amylose has alpha-1,4 bonds...and nothing to do with cellulose.

[Rayan]

hey,
thanks but none of you guys have answered my question yet.
http://www.guidobauersachs.de/oc/betagly.gif
in the above beta (1-4) linkage, we drew it flipped over. can we draw both bonds up,
that is:
|^-O-|
i dont know if u understood what i wanted to say..

[Albert]

As shown below, if you have to draw a disaccharide like cellobiose, which contains a BETA - 1,4 linkage, you don't have to flip over anything.

On the other hand, if you are asked to draw a chain of cellobiose in a molecule of cellulose you must flip it over: in this context, for the reason I told you, cellobiose has this particular aspect.

That's it. Cellbiose changes its 'aspect' only when it represents a monomer in a linear polymer, like cellulose. Linear means, in this context, beta-sheet structure.

For what concerns amylose, I repeat: it is just a polymer of maltose with alpha - 1,4 linkages, which are not altered (flipped over).

[Rayan]

i finally got that.
thanks