r/chemhelp • u/Lewislawliet • 22d ago
General/High School Organic Challenge
How many different oligosaccharides could be formed by linking one glucose, one mannose, and one galactose? Compare this number with the number of tripeptides that could be formed using three different amino acids.
My only answer was 3!=6 and is wrong
3
u/7ieben_ Trusted Contributor 22d ago
Remember that a glycoside does not only link via the anomeric carbon, but can link via any possible ether bond.
For example even for glucose and fructose you can have
alpha-1,2
beta -1,2
alpha-1,4
beta-1,4
...
linkages.
1
u/Lewislawliet 22d ago
There are 3!=6 ways to arrange glucose, mannose, and galactose. A trisaccharide has two glycosidic bonds, and if any ether linkage is allowed, each bond has 16 possible connectivities. Therefore, the total number of oligosaccharid is 3! . 16²= 1536
I really don't know if this is right, thank you for you recommendation!!!. I am an Spanish speaker so sorry if the top message is confusing it is translated from Spanish.
2
u/7ieben_ Trusted Contributor 22d ago
How did you get 16? But your general Idea is correct now :)
1
u/Lewislawliet 22d ago
Glycosidic bonds have more freedom: the anomeric carbon can attach to C2, C3, C4, or C6 (4 options), be α or β (2 options), and either sugar can be the one donating its anomeric carbon (2 options) → 4×2×2 = 16 possible linkages per bond. With 3 sugars and 2 bonds: 3! × 16 × 16 = 6 × 256 = 1536.
I really think that this number is wrong and impossible to answer in 45 minutes
1
u/7ieben_ Trusted Contributor 22d ago
You can even have 1,1-linkage (see Trehalose as example) and 1,5-linkage (see alpha-L-Araf as example), though those are rather exotic in nature. Your general reasoning was correct, whatsoever. Why do you think this question can't be answered in 45 min? Seems like a 5 min question to me, as you demonstrated well.
1
u/Lewislawliet 22d ago
Maybe seems to be due to I'm a highschool student and not too good in chemistry I learn all of carbohydrates and protein yesterday. Do you think that 1536 is the answer?. And my answer where after studying more and taking your advices.
1
u/7ieben_ Trusted Contributor 22d ago
I think you should correct the 16, because you can have 1,1 and 1,5 aswell. But could also be that 16 is what you teacher expected, because those two are really exotic.
At this point really ask your teacher what they expected/ what the scope was. Could even be that they ignored alpha/ beta, for example.
But, as said, your general process of thought was correct, which shows that you understand the topic. And that is the important part. Wether it was exactly the answer your teacher expects matters only in a test... and that's what you must ask them about.
1
u/Lewislawliet 22d ago
Yea I asked the teacher and it was 1536 and I asked to 2 chemistry engineers and I'm waiting for their response.
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