Biochemistry Carbohydrate Lecture 3 PDF

Biochemistry Carbohydrate A tetrahedral carbon atom is represented in a Fischer projection by two crossed lines. The horizontal lines represent bonds coming out of the page, and the vertical lines represent bonds going into the page. Thus, (R)-glyceraldehyde, the simplest monosacchar...

Biochemistry Carbohydrate A tetrahedral carbon atom is represented in a Fischer projection by two crossed lines. The horizontal lines represent bonds coming out of the page, and the vertical lines represent bonds going into the page. Thus, (R)-glyceraldehyde, the simplest monosaccharide, is represented as shown in Figure 14.1. Carbohydrates with more than one chirality center are shown in Fischer projections by stacking the centers on top of one another, with the carbonyl carbon at or near the top. Glucose, for example, has four chirality centers stacked on top of one another in a Fischer projection. Such representations don’t, however, give an accurate picture of a molecule’s true three-dimensional conformation, which is curled around on itself like a bracelet. Worked Example 14.2 Drawing a Fischer Projection Convert the following tetrahedral representation of (R)-butan-2-ol into a Fischer projection: Strategy Orient the molecule so that two horizontal bonds are facing you and two vertical bonds are receding away from you. Then press the molecule flat into the paper, indicating the chirality center as the intersection of two crossed lines. Worked Example 14.3 Interpreting a Fischer Projection Convert the following Fischer projection of lactic acid into a tetrahedral representation, and indicate whether the molecule is (R) or (S): Strategy Place a carbon atom at the intersection of the two crossed lines, and imagine that the two horizontal bonds are coming toward you and the two vertical bonds are receding away from you. The projection represents (R)-lactic acid. Problem 14.4 Convert the following Fischer projections into tetrahedral representations, and assign R or S stereochemistry to each: S R S Problem 14.5 Redraw the following molecule as a Fischer projection, and assign R or S configuration to the chirality center (yellow-green - Cl):  14.3 D,L Sugars Chiral molecules are optically active; that is, they rotate the plane of polarized light. The convention for designating D and L isomers was originally based on the optical properties of glyceraldehyde. Glyceraldehyde, the simplest aldose, has only one chirality center and thus has two enantiomeric (mirror-image) forms. Only the dextrorotatory enantiomer occurs naturally, however. That is, a sample of naturally occurring glyceraldehyde placed in a polarimeter rotates plane- polarized light in a clockwise direction, denoted (+). Since (+)-glyceraldehyde has been found to have an R configuration at C2, it can be represented as in Figure 14.2. For historical reasons dating from long before the adoption of the R,S system, (R)-(+)-glyceraldehyde is also referred to as D-glyceraldehyde (D for dextrorotatory).The other enantiomer, (S)-(+)- glyceraldehyde, is known as L-glyceraldehyde (L for levorotatory). Because of the way that monosaccharides are synthesized in nature, glucose, fructose, ribose, and most other naturally occurring monosaccharides have the same R stereochemical configuration as D-glyceraldehyde at the chirality center farthest from the carbonyl group. In Fischer projections, therefore, most naturally occurring sugars have the -OH group at the bottom chirality center pointing to the right (Figure 14.2). Such compounds are referred to as D sugars. In contrast to D sugars, all L sugars have an S configuration at the lowest chirality center, with the bottom -OH group pointing to the left in Fischer projections. Thus, an L sugar is the mirror image (enantiomer) of the corresponding D sugar and has the opposite configuration at all chirality centers. Note that the D and L notations have no relation to the direction in which a given sugar rotates plane-polarized light. A D sugar may be either dextrorotatory or levorotatory. The prefix D indicates only that the stereochemistry of the lowest chirality center is the same as that of D-glyceraldehyde and that the -OH group points to the right when the molecule is drawn in the standard way in a Fischer projection. Note also that the D,L system of carbohydrate nomenclature describes the configuration at only one chirality center and says nothing about the configuration of other chirality centers that may be present. Worked Example 14.4 Drawing the Fischer Projection of an Enantiomer Look at the Fischer projection of D-fructose in Figure 14.2, and draw a Fischer projection of L-fructose. Strategy Since L-fructose is the enantiomer of D-fructose, simply take the structure of D-fructose and reverse the confi guration at every chirality center.  L (S or D (R or D (R or -) Problem 14.7 Draw the enantiomers of+) +) the carbohydrates shown in Problem 14.6, and identify each as a D sugar or an L sugar. Problem 14.7 Draw the enantiomers of the carbohydrates shown in Problem 14.6, and identify each as a D sugar or an L sugar. D L L