Epimeric and anomeric carbon11/3/2023 ![]() ![]() The structural isomers have an equal molecular formula, but the connections or their order is different. The isomers have an equal number of atoms of each chemical element, but different arrangements of these atoms, and therefore – different properties. We will examine the effects of these differences more closely in Section 16.7 “Polysaccharides” and when we discuss enzyme specificity in Chapter 18 “Amino Acids, Proteins, and Enzymes”, Section 18.5 “Enzymes”.The isomers are molecules with equal molecular formula, but with a different structure (structural isomerism) or different spatial orientation (stereoisomerism). This explains why we can get energy from the starch in potatoes and other plants but not from cellulose, even though both starch and cellulose are polysaccharides composed of glucose molecules linked together. The difference between the α and the β forms of sugars may seem trivial, but such structural differences are often crucial in biochemical reactions. Any group written to the right in a Fischer projection appears below the plane of the ring in a Haworth projection, and any group written to the left in a Fischer projection appears above the plane in a Haworth projection. The structure is simplified to show only the functional groups attached to the carbon atoms. The molecules are drawn as planar hexagons with a darkened edge representing the side facing toward the viewer. In Figure 16.6 “Monosaccharides”, and elsewhere in this book, the cyclic forms of sugars are depicted using a convention first suggested by Walter N. Thus, all the molecules may eventually react, even though very little free aldehyde is present at a time. ![]() As the small amount of free aldehyde is used up in a reaction, there is a shift in the equilibrium to yield more aldehyde. The observed rotation of this solution is +52.7°.Įven though only a small percentage of the molecules are in the open-chain aldehyde form at any time, the solution will nevertheless exhibit the characteristic reactions of an aldehyde. At equilibrium, the mixture consists of about 36% α-D-glucose, 64% β-D-glucose, and less than 0.02% of the open-chain aldehyde form. The opening and closing repeats continuously in an ongoing interconversion between anomeric forms and is referred to as mutarotation (Latin mutare, meaning “to change”). You can start with a pure crystalline sample of glucose consisting entirely of either anomer, but as soon as the molecules dissolve in water, they open to form the carbonyl group and then reclose to form either the α or the β anomer. When the sample is dissolved in water, however, a mixture is soon produced containing both anomers as well as the straight-chain form, in dynamic equilibrium (part (a) of Figure 16.6 “Monosaccharides”). The α form melts at 146☌ and has a specific rotation of +112°, while the β form melts at 150☌ and has a specific rotation of +18.7°. It is possible to obtain a sample of crystalline glucose in which all the molecules have the α structure or all have the β structure. (Note: In the middle diagram for (b), the O that’s just floating to the right should be double-bonded to C2. The interconversion between the forms is known as mutarotation, which is shown for D-glucose (a) and D-fructose (b). Figure 16.6 Monosaccharides. In an aqueous solution, monosaccharides exist as an equilibrium mixture of three forms.
0 Comments
Leave a Reply.AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |