Biochem: Sugar nameology

[Kingfish]

I am having a lot of trouble with determining whether a cyclical sugar is alpha or beta from fischer projections. I'm not even really comortable with assigning D or L. For starters, how are the carbons in a fischer projection numbered, is it the same as Iupac rules? I am confused about how to find the highest numbered asymmetric carbon. My ochem prof never taught us fischer projections because he didn't think we would see them again (whoops) and regardless its been a year since I've had any organic whatsoever. My text is really lacking on this subject. Any help would be much appreciated (tips, tricks, mnemonics)

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[Quix]

O-Chem = voodoo witchcraft. Sacrifice a chicken.

 

[DropkickMurphy]

nameology

My first thought was Willem Dafoe's character (Agent Smecker) on Boondock Saints

 

[Quix]

Okay, serious answer - I'd suggest looking at books like Organic Chemistry as a Second Language (specifically second semester topics, when carbohydrates are generally covered, although First Semester would be useful for Fischer projections). It's been nearly a year since I last considered O.Chem, and I'm in denial about it until I buckle down and pick up a biochem textbook, so I don't want to give you bad information. Just check the books mentioned and you should feel more comfortable.

 

[Kingfish]

I'd rather sacrifice a chicken. (and yes, boondock saints is where I got the thread title)

Seriously, theres no easy way to figure out what the highest numbered symmetric carbon is?

 

[melissainsd]

I PMed you Kingfish...

 

[deleted103644]

It's a lot easier to understand if you look at things in their linear structure. I can't for the life of me remember this stuff now, but when I did do it I came up with a very geeky way to memorize the sugars. If you look at the linear representation, you basically have a bunch of OH groups that can go on the left or the right. Call one of those orientations 1 and the other 0, and then for all of the different sugars, you've just got a number between 0 and 2^n in binary. Then all you have to do is remember the orientation you're counting from, and one number for each sugar.

 

[deleted103644]

Sorry, that tip is for memorizing sugar names if you know how fischer projections work. In that case, D sugars could be any odd number and L sugars any even number (or less than 2^n-1/>= to it). This has probably just confused you even more, so I apologize.

 

[armybound]

I'm not sure if I'm understanding the question corrently, but carbon 1 is the carbonyl end, whereas the last carbon is the one farthest from that carbon. so on the ends of the chain, one carbon is double-bonded to oxygen and the other end is not. the double-bonded end is carbon 1.

the carbon next to the last one (carbon 5 of a hexose) determines L or D. if the OH is on the left, it's L, if it's right, it's D.

http://www.chem.ucalgary.ca/courses/351/Carey5th/////Ch25/ch25-4.html

in the Fischer picture, notice that the carbon on top has the C=O, that's carbon 1. Carbon 5 has the OH pointing to the right, so that's a D orientation

 

[Kingfish]

I'm not sure if I'm understanding the question corrently, but carbon 1 is the carbonyl end, whereas the last carbon is the one farthest from that carbon. so on the ends of the chain, one carbon is double-bonded to oxygen and the other end is not. the double-bonded end is carbon 1.

the carbon next to the last one (carbon 5 of a hexose) determines L or D. if the OH is on the left, it's L, if it's right, it's D.

http://www.chem.ucalgary.ca/courses/351/Carey5th/////Ch25/ch25-4.html

in the Fischer picture, notice that the carbon on top has the C=O, that's carbon 1. Carbon 5 has the OH pointing to the right, so that's a D orientation

You got it! I was confused because my book states C1 as being the highest carbon in one example (because it has the highest priority I assume) rather than C6 (which is the highest # obviously). Thanks a ton.

 

[armybound]

You got it! I was confused because my book states C1 as being the highest carbon in one example (because it has the highest priority I assume) rather than C6 (which is the highest # obviously). Thanks a ton.

glad you've got it.

and you can't determine alpha or beta from Fischer projections because C1 loses its double bond to go alpha/beta, and you can't predict it from the Fischer projection

(I think)

 

[Wizard of Oz]

The numbering should be such that the most oxidized carbon (either the ketone or the aldehyde) has the lowest number, i.e. it's either C-1 (aldose) or C-2 (ketose). Go from there.

You can just memorize the structures straight if you don't want to learn the conventions. Just beware that some profs are sneaky and flip the Haworth so that C-6 dangles under the ring. This will reverse the alpha/beta anomer convention.

 

[Kingfish]

glad you've got it.

and you can't determine alpha or beta from Fischer projections because C1 loses its double bond to go alpha/beta, and you can't predict it from the Fischer projection

(I think)

Not according to my prof. If the hydroxy group on the anomeric carbon is on the same side as the oxygen in the cyclical structure of the fischer projection, it is alpha for d molecules and beta for l molecules. This is so dry and boring.

 

[bretticus]

I believe the word you're looking for is "nomenclature."

 

[Kingfish]

The numbering should be such that the most oxidized carbon (either the ketone or the aldehyde) has the lowest number, i.e. it's either C-1 (aldose) or C-2 (ketose). Go from there.

You can just memorize the structures straight if you don't want to learn the conventions. Just beware that some profs are sneaky and flip the Haworth so that C-6 dangles under the ring. This will reverse the alpha/beta anomer convention.

Yea, he warned us to be able to recognize what happens if he flips it. Thanks for the heads up though, I almost forgot.

 

[Kingfish]

I believe the word you're looking for is "nomenclature."

Actually, I believe its nameoism. But I'm not much of a wordologist, so I'll leave it to the experts.

/put your books down and go watch boondock saints, you'll thank me for it.

 

[booji]

D/L refers to sterochem at anomeric carbon. Numbering usually starts at aldolic or ketonic carbon, and alpha/beta depends on whether OH group is pointing up or down.

 

[Falco2525]

D/L refers to sterochem at anomeric carbon. Numbering usually starts at aldolic or ketonic carbon, and alpha/beta depends on whether OH group is pointing up or down.

exactly!

 

[geogil]

PM OP, I have some great stuff from orgo that will help you out. pdf's etc..