Lecture 4 More Sugars: Disaccharides and Rings Wood Chemistry PSE 406 Lecture 4 More Sugars: Disaccharides and Rings PSE 406 - Lecture 4

Hemiacetal Formation A hemiacetal is a compound having the C(OH)OR group that results from the reaction of an aldehyde with one mole of an alcohol. Sugar molecules contain an aldehyde group and lots of alcohol groups. PSE 406 - Lecture 4

Hemiacetal Formation in Sugars (1) Five and six carbon sugars will undergo internal hemiacetal formation resulting in the formation of either 5 or 6 membered rings. Larger or smaller rings are too strained or thermodynamically unstable. In solution, a sugar will exist as in both of these ring forms as well as in the open chain form. Six membered rings are known as pyranose units. Five membered rings are furanose units. Notes Pyranose ring Furanose ring PSE 406 - Lecture 4

Hemiacetal Formation in Sugars (2) β-D-glucopyranose α-D-glucopyranose PSE 406 - Lecture 4

Important Monosaccharide Projections There are several ways to draw a sugar molecule. The simplest method is to draw it in the open chain form as in the Fisher formula. We will also draw the molecule in the ring form as shown above by the Haworth formula and the chair configuration. The trick is to be able to convert a sugar between these formulations. We will learn this soon. D-glucose α-D-glucopyranose α-D-glucopyranose Fisher Haworth Chair Configuration Notes PSE 406 - Lecture 4

Conformation: The Chair a=axial e=equatorial Chair β-D-glucopyranose The most stable form of a sugar in a six membered ring is in the chair formation. The substituent groups on the molecule are either in an axial or equatorial position depending on what gives the most stable molecule. PSE 406 - Lecture 4 a D glucopyranose

Conformation: Other Forms We won’t be worrying about these forms. PSE 406 - Lecture 4

The Haworth Projection This is a very common way that wood chemists draw sugars in the ring form. In this projection, the thick bonds are perpendicular to the plane of the paper Substituents are parallel to the plane of the paper and lie either above or below the plane of the molecule PSE 406 - Lecture 4

Formation of Anomers Ring closure creates new chiral center at C1; new pair C1 epimers termed anomers. For D sugars, the following is true: alpha form: OH below the ring beta form: OH above the ring For L sugars, the opposite is true. a-D-glucopyranose b-D-glucopyranose PSE 406 - Lecture 4

Nomenclature O H  -D-xylopyranose O C H  -D-xylofuranose O H  2  -D-xylofuranose O H  -D-xylopyranose O C H 2  -D-xylofuranose PSE 406 - Lecture 4

Mutarotation (1) Notes PSE 406 - Lecture 4 If the molecule has a migratory hydrogen , the ring can open up-the carbonyl group can rotate before ring closing, and the other anomer will form. The inter conversion of the alpha and beta anomers via the carbonyl from is called mutarotation. As discussed earlier, sugars take several forms in solution. This diagram shows this process known as mutarotation. When solutions of sugars are analyzed using chromatography (discussed later), all five forms of the sugars can be found. The amount of each form in solution varies significantly depending on the sugar. The open form is the least stable and is hardly found. A single sugar molecule in solution will continuously change its form in solution. PSE 406 - Lecture 4

Mutarotation (2) Note: the cyclic form must contain a hemi-form in order to have a migratory hydrogen and therefore be capable of ring-opening (that is, mutarotation). PSE 406 - Lecture 4

Fischer to Haworth In order to convert a Fischer projection of a sugar to a Haworth project, certain rules must be followed in order to maintain correct orientation of the molecule. Remember, the horizontal bonds in the Fischer projection (solid wedges) are out of the plane and the vertical wedges (broken wedges) are into the plane. PSE 406 - Lecture 4

Fischer to Haworth Modify the Fischer formula so that all of the ring atoms lie along a vertical line. When rotating substituents around a chiral carbon, to keep the correct configuration, after a 90 rotation, the opposite substituents must be switched. Proceed around the Haworth formula placing the groups on the left above the hexagon and those on the right below the plane. PSE 406 - Lecture 4

Monosaccharides D Versus L, Part II PSE 406 - Lecture 4

Linkages Between Sugars In the tree, monosaccharides are linked through enzymatic processes. Linkage proceeds through a dehydration process (loss of H2O). The bond between the glucoses units in cellobiose is a glycosidic bond. Once linked, the glucose unit on the left is no longer a hemiacetal, it is now an acetal. Cellobiose 4-O-(b-D-Glucopyranosyl)-D-Glucopyranose Cellobiose is disaccharide produced from the partial hydrolysis of cellulose. It is not naturally found in wood. The squiggly bond on C1 above means it can be either a or b. Cellobiose, the 1,4 linkage is beta not alpha. We have 2-glucopyranose units. Cellobiose can be hyrolysed only with beta-glucosidase (very specific for beta linkage) not alpha. Bioconversion. PSE 406 - Lecture 4

Linkages Between Sugars The glycosidic linkage between the glucose units is between the b C1 of one glucose to C4 of the other glucose. So this bond is a b(14) linkage. The glucose molecules in the drawing on the right are opposite to make drawing the b(14) linkage easy. Cellobiose 4-O-(b-D-Glucopyranosyl)-D-Glucopyranose PSE 406 - Lecture 4

More Disaccharides Neither of these sugars are found in wood. Sucrose occurs naturally in sugarcane and in sugar beets. Maltose is formed through the degradation of starch. Sucrose a-D-Glucopyranosyl-b-D-Fructofuranoside Maltose 4-O-(a-D-Glucopyranosyl)-D-Glucopyranose PSE 406 - Lecture 4

Reducing End Groups Sugars containing aldehydes and ketones are referred to as reducing sugars. These sugars will reduce copper in a specific reducing substances test. The aldehyde or ketone groups are oxidized by the copper. Sucrose is not a reducing substance Fehling’s Reaction: deep blue solution to red orange precipitate. PSE 406 - Lecture 4