[Kalibasa]

The definitions in my book are a little vague, and while I think I pieced together what they mean, I did want to double-check these assumptions! If they're right you can just tell me that, quick and easy, but if I got anything wrong I may need a little explanation, esp. on the second part...

1) Acetals and Ketals vs. Glycosides- From the way my book phrased it, it sounds like acetals and ketals are formed from an open-chain hemiacetal (which is not as common), while glycosides are basically the same process but formed from a cyclic hemiacetal. Is this right- is it that simple?

2) I'm confused on how mutarotation differs from isomerization reactions. While mutarotation seems to mostly deal with shifts between the alpha and beta forms, my book says that mutarotation can also lead to both pyranose and furanose ring structures, even if they're in small amounts. I thought that that would count as an isomerization- does it? (Specifically, I'm looking at glucose- would a glucofuranose be a different isomer than a glucopyranose? If so, why would this be mutarotation and not isomerization?)

Thank you!

[renge ishyo]

1. Acetals and Ketals have two R groups attached to the originally carbonyl carbon. They can be converted back into the carbonyl under acidic consitions, but NOT basic conditions because there is no H to abstract from the oxygens. In contrast, hemiacetals can be coverted back to carbonyls in both acidic and basic conditions because one of the groups attached is OH instead of OR. Hence, in the slightly basic pH of the body it is the hemiacetal that can open and close freely and not the other two.

2. Actually, all those different "anomers" of glucose are considered the same compound (technically) because the configuration of the carbons remain the same. It is a gray area like conformers...which are also not considered isomers even though the molecule in that case can have different properties depending on how the bonds are being rotated. Mutarotation describes what will happen if say you painstaking crystallize and isolate B-glucose crystals. If you place it in solution and observe the optical activity of solution you will notice it change for awhile before settling out to its value. It does this because once B-glucose is placed in solution it is interconverted to all its other forms each of which have a different optical activity.

[Kalibasa]

Those were good answers, thanks!, but while I better understand the difference between acetals/ketals and hemiacetals, I still don't see how glycosides fit into the picture.

[renge ishyo]

A glycoside can be created when you replace the "H" on the oxygen attached to the anomeric carbon of the ring sugar with an R group attached to this oxygen. Hence glucose all by itself can be opened and closed in acid or base by removing the H on the oxygen of its anomeric carbon (it is a hemiacetal in the ring form). In contrast if you replace this H with an R group on the ringed glucose in this same position you have created a glycoside, now it can only be opened in acidic solution and not basic (because now the anomeric carbon forms a ketal or acetal in this situation).

This is what makes glucose polymers so stable in the body. Originally you have two separate 6 membered glucose molecules both with hemicacetal groups at the anomeric carbon that can open and close. If you react one sugar by taking its anomeric OH carbon and react it with the OH group at the 4 position of another (with the release of water) now you have formed a ketal at the anomeric carbon for this link. This cannot be opened in the basic environment of the blood anymore. Hence you started with two separate hemiacetals and when you combined them you ended up with only one hemiacetal group left...the other was transformed into a glycoside (and the bond formed between its anomeric carbon and the 4 OH group on the other is called a glycosidic bond). See an illustration in your text for glycogen formation or cellulose formation to see how this occurs.