alpha beta linkage

[pepocho]

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[DrknoSDN]

Without overly explaining at the risk of saying something incorrectly, I believe it is just the direction (up or down) the group attached to carbon one is pointing.
Lactose both are O and OH groups on the anomeric carbons are pointed up (beta) and the connection is between carbon 1 and 4.
Maltose both the O and OH anomeric carbon groups are pointed down (alpha), and connection is between carbon 1 and 4.
Sucrose is considered alpha 1, beta 2 because The left ring is down from carbon 1 (alpha 1), while the right ring does not have an anomeric carbon and could be numbered either direction but arbitrarily the lower number 2 is used instead of 4.

 

[Gauss44]

This may help:

Also remember to count carbons starting at the anomeric carbon (the carbon directly bonded to 2 oxygens) for 6 member rings like glucose. And watch for ketone sugars like the fructose ring in your top diagram because in those, the anomeric carbon is carbon #2 (you start counting at the carbon above it, which is #1).

 

[quicheduty]

Without overly explaining at the risk of saying something incorrectly, I believe it is just the direction (up or down) the group attached to carbon one is pointing.
Lactose both are O and OH groups on the anomeric carbons are pointed up (beta) and the connection is between carbon 1 and 4.
Maltose both the O and OH anomeric carbon groups are pointed down (alpha), and connection is between carbon 1 and 4.
Sucrose is considered alpha 1, beta 2 because The left ring is down from carbon 1 (alpha 1), while the right ring does not have an anomeric carbon and could be numbered either direction but arbitrarily the lower number 2 is used instead of 4.

Just a few corrections on sucrose: C2 of sucrose is the anomeric carbon and the naming is not arbitrary, but based off the straight chain (where C1 is the carbon 'above' the ketone of sucrose)

Looks like @Gauss44 beat me to it!

 

[pepocho]

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[DrknoSDN]

Just a few corrections on sucrose: C2 of sucrose is the anomeric carbon and the naming is not arbitrary, but based off the straight chain (where C1 is the carbon 'above' the ketone of sucrose)

Looks like @Gauss44 beat me to it!

Ahh I didn't know that about the Anomeric carbon being C2. I actually checked wiki before posting too.

http://en.wikipedia.org/wiki/Sucrose
"What is notable about sucrose is that, unlike most disaccharides, the glycosidic bond is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the nonreducing end of the other. This linkage inhibits further bonding to other saccharide units. Since it contains no anomeric hydroxyl groups, it is classified as a non-reducing sugar."

 

[Czarcasm]

So, it depends on the perspective of which you are looking at it? that is why sucrose has both alpha and beta, because you can start with either one? Thank you all. Crap, this has been bothering for days now.

Yeah, it's not really that difficult though.

Just follow these steps:
1. Locate anomeric carbon -OH and the last carbon substituent
2. For that sugar ring, do these point in the same direction (beta), or trAns/opposite (alpha)

If it's a dissaccharide, it may or may not be linked by two anomeric carbons (as it is in sucrose) depnding on the linkage. You may have for example, an alpha 1,4 linkage. (carbon 4 is not anomeric).

 

[quicheduty]

So, it depends on the perspective of which you are looking at it? that is why sucrose has both alpha and beta, because you can start with either one? Thank you all. Crap, this has been bothering for days now.

Good question! For sugars, you generally name the whole sugar ending with the reducing sugar (that is, the sugar that has an open anomeric -OH ). However, sucrose is a non-reducing sugar as both of its anomeric carbons are linked. Sucrose has both alpha and beta based on the stereochem of the specific monomer carbons. In this case you would "read" the monosaccharides from left to right if the name is a1-b2 because you know the left sugar is alpha (trans) and the right is beta (cis). However, for naming purposes, because both are non-reducing sugars, sucrose can also be called b2-a1 (if you read from right to left).

Ahh I didn't know that about the Anomeric carbon being C2. I actually checked wiki before posting too.

http://en.wikipedia.org/wiki/Sucrose
"What is notable about sucrose is that, unlike most disaccharides, the glycosidic bond is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the nonreducing end of the other. This linkage inhibits further bonding to other saccharide units. Since it contains no anomeric hydroxyl groups, it is classified as a non-reducing sugar."

Maybe what's throwing you off is that part--yes, sucrose is a non-reducing sugar because it does not have a free anomeric -OH group to be reduced. Sucrose cannot be "extended." However, fructose still contains an anomeric carbon, which is a carbon bonded to 2 O's.

 

[DrknoSDN]

Yeah, i found another thread on the topic. Thank You.
http://forums.studentdoctor.net/threads/alpha-beta-for-sucrose.920515/