Structural Biochemistry/Carbohydrates - Wikibooks, open books for an open world
They originate as products from carbon dioxide and water by photosynthesis, The most commonly known ones are perhaps glucose and fructose. .. *** Deoxyribose (component of DNA)-The principal difference between RNA and DNA N-glycosidic bond involves the bonding of the anomeric carbon of a sugar and the. As noted here, the formulas of many carbohydrates can be written as carbon hydrates, . The importance of these relationships may be seen in the array of aldose D-Fructose, the sweetest of the common natural sugars, is for example As with the furanose ring, the anomeric carbon is placed on the right with the ring. Another difference is that in glucose, the anomeric carbon is the first carbon, whereas in fructose, the anomeric carbon is the second carbon. The anomeric.
But it really does just follow suit with the Haworth projection as far as the substituents being above or below the ring.
So let me just kinda keep filling in the substituents here. I'll number them off, again, just so you can kind of see that there's some consistency here. So we've got one, two, three, four, five, six.
And again this three-carbon right here is the only one with the hydroxyl group pointing up. And I guess I better change the color of our one-carbon, to keep that consistent, as well. Now I didn't indicate the position of the anomeric carbon's hydroxyl group yet, because I think it makes more sense to show it in this diagram. Remember that the original nucleophilic attack by the oxygen way back over here, that could've created two different products: So that last hydroxyl group can actually be in two different positions.
One one hand, the hydroxyl group would be cis to the last carbon in the equatorial position.
Carbohydrates - cyclic structures and anomers
So it'd be cis to this last carbon over here. And it's in a equatorial position. And we call this the beta anomer.
Then on the other hand, I guess, it can be trans to that last carbon group, which would place it in the axial position down here. So I guess it could be down here in the axial position, and we call it the alpha anomer when the hydroxyl group is in the axial.
And I kinda remember that a little bit easier: And I guess I've also heard that if fishies are down in the sea and birds are up in there air. So if that helps you keep them straight, you might be able to use that also. Now you gotta remember that what caused this ring to close in the first place, was some amount of acid or base. And the amount of acid and base in water is actually kinda capable of doing that. Because that's what facilitated this ring-closing process in the first place.
And in water, the ring can actually open and close spontaneously. And when it opens up, the C1 and C2 bond right here can actually rotate, and when it closes again you can form either the alpha or the beta product.
So this thing is constantly opening and closing to form the two different products.
25.5 Cyclic Structures of Monosaccharides: Anomers
And we call that process, where it opens, and rotates, and closes again, mutarotation. So this thing is mutarotating in the water at all times. And the outcome is that we end up with both configurations, the beta and the alpha, in the equilibrium concentration. So for glucose, that's gonna be about 36 percent alpha, and about 64 percent beta. And the reason that the alpha configuration is less favored in equilibrium for glucose, is because the transpositioning of the hydroxyl group creates some stearic hindrance.
As with the furanose ring, the anomeric carbon is placed on the right with the ring oxygen to the back of the edgewise view. These Haworth formulas are convenient for displaying stereochemical relationships, but do not represent the true shape of the molecules.
We know that these molecules are actually puckered in a fashion we call a chair conformation.
Carbohydrates - cyclic structures and anomers (video) | Khan Academy
Examples of four typical pyranose structures are shown below, both as Haworth projections and as the more representative chair conformers. The anomeric carbons are colored red.
The size of the cyclic hemiacetal ring adopted by a given sugar is not constant, but may vary with substituents and other structural features. Aldolhexoses usually form pyranose rings and their pentose homologs tend to prefer the furanose form, but there are many counter examples. The formation of acetal derivatives illustrates how subtle changes may alter this selectivity. A pyranose structure for D-glucose is drawn in the rose-shaded box on the left.
Acetal derivatives have been prepared by acid-catalyzed reactions with benzaldehyde and acetone. As a rule, benzaldehyde forms six-membered cyclic acetals, whereas acetone prefers to form five-membered acetals.
Anomer - Wikipedia
The top equation shows the formation and some reactions of the 4,6-O-benzylidene acetal, a commonly employed protective group.
A methyl glycoside derivative of this compound see below leaves the C-2 and C-3 hydroxyl groups exposed to reactions such as the periodic acid cleavage, shown as the last step.
The formation of an isopropylidene acetal at C-1 and C-2, center structure, leaves the C-3 hydroxyl as the only unprotected function. Selective oxidation to a ketone is then possible. Finally, direct di-O-isopropylidene derivatization of glucose by reaction with excess acetone results in a change to a furanose structure in which the C-3 hydroxyl is again unprotected.
The six-membered pyranose ring is not actually planar, as Haworth perspectives suggest, but tends to assume either the "boat" or the "chair" conformation Fig. We shall see that the specific three-dimensional conformations of the monosaccharide units are important in determining the biological properties and functions of some polysaccharides. Organisms Contain a Variety of Hexose Derivatives In addition to simple hexoses such as glucose, galactose, and mannose, there are a number of derivatives in which a hydroxyl group in the parent compound is replaced with another substituent, or a carbon atom is oxidized to a carboxylic acid Fig.
We have already encountered some of these derivatives as components of glycolipids see Figs. In glucosamine, galactosamine, and mannosamine, the hydroxyl at C-2 of the parent compound is replaced with an amino group. The amino group may be condensed with acetic acid, as in N -acetylglucosamine Fig. This glucosamine derivative is part of many structural polymers, including those of the bacterial cell wall. Bacterial cell walls also contain another derivative of glucosamine in which the three-carbon carboxylic acid lactic acid is ether-linked to the oxygen at C-3 of N -acetylglucosamine to form N-acetylmuramic acid Fig.
The substitution of a hydrogen for the hydroxyl group at C-6 of galactose or mannose produces fucose or rhamnose, respectively Fig. When the carbonyl aldehyde carbon of glucose is oxidized to a carboxylic acid, gluconic acid is produced; other aldoses yield other aldonic acids. Oxidation of the carbon at the other end of the carbon chain C-6 of glucose, galactose, or mannose forms the corresponding uronic acid; glucuronic, galacturonic, or mannuronic acids.
Both aldonic and uronic acids form stable intramolecular esters, called lactones Fig. In addition to these hexose derivatives, one ninecarbon acidic sugar deserves mention. N-acetylneuraminic acid sialic acida nine-carbon derivative of N-acetylmannosamine, is a component of many glycoproteins and glycolipids in higher animals.
The car boxylic acid groups of these sugar derivatives are ionized at pH 7, and the compounds are therefore correctly named as carboxylatesglucuronate, galacturonate, etc.
In the synthesis and metabolism of carbohydrates, the intermediates are very often not the sugars themselves, but their phosphorylated derivatives. Condensation of phosphoric acid with one of the hydroxyl groups of a sugar forms a phosphate ester, as in glucosephosphate Fig.
Sugar phosphates are relatively stable at neutral pH, and bear a negative charge. One effect of sugar phosphorylation within cells is to prevent the diffusion of the sugar out of the cell; highly charged molecules do not, in general, cross biological membranes without specific transport systems Chapter Phosphorylation also activates sugars for subsequent chemical transformation.