15 2 Fischer Projections of Monosaccharides Glucose galactose

15. 2 Fischer Projections of Monosaccharides Glucose, galactose, and fructose are the most important monosaccharides. Learning Goal Identify and draw the D or L configurations of the Fischer projections for common monosaccharides. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Fischer Projections A Fischer projection, used to represent carbohydrates, • places the aldehyde group (most oxidized) at the top. • shows the — H and — OH groups on the horizontal intersecting line. • places the achiral — CH 2 OH group at the bottom. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

D and L Notations In a Fischer projection, the — OH group on the • chiral carbon farthest from the carbonyl group determines an L or D isomer. • left is assigned the letter L. • right is assigned the letter D. Core Chemistry Skill Identifying D- and L-Fischer Projections General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

D and L Isomers: Monosaccharides General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Important Monosaccharides D-Glucose, D-galactose, and D-fructose are the most important monosaccharides. They are all hexoses with the molecular formula C 6 H 12 O 6 and are isomers of each other. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Monosaccharides: D-Glucose • • is the most common hexose. is found in fruits, vegetables, corn syrup, and honey. is also known as dextrose and blood sugar in the body. is a building block of the disaccharides sucrose and lactose and of polysaccharides such as cellulose and glycogen. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Monosaccharides: D-Galactose D-galactose, an aldohexose with the formula C 6 H 12 O 6, • is obtained from the disaccharide lactose, found in milk. • is important in the cellular membranes of the brain and nervous system. In a condition called galactosemia, • the enzyme needed to convert D-galactose to D-glucose is missing. • galactose accumulates in the blood and tissue, which can lead to cataracts, mental retardation, failure to thrive, and liver disease. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Monosaccharides: D-Fructose • is a ketohexose with the formula C 6 H 12 O 6. • is the sweetest of the carbohydrates, twice as sweet as sucrose (table sugar). • is obtained as one of the hydrolysis products of sucrose. High-fructose corn syrup (HFCS) is a sweetener that is produced by using an enzyme to break down sucrose to glucose and fructose. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Guide to Identifying D- or LMonosaccharides General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Study Check Identify the following structure of a monosaccharide as D- or Lribulose. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Solution Identify the following structure of a monosaccharide as D- or Lribulose. ANALYZE THE PROBLEM Given Need ribulose identify as D- or LFischer projection STEP 1 Number the carbon chain starting at the top of the Fischer projection. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Study Check Identify the following structure of a monosaccharide as D- or Lribulose. STEP 2 Locate the chiral carbon farthest from the top of the Fischer projection. STEP 3 Identify the position of the — OH group as D- or L-. The — OH group is drawn on the right of carbon 4, making it D-ribulose. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Chemistry Link to Health: Hyperglycemia and Hypoglycemia In the body, • glucose has a normal blood level of 70– 90 mg/d. L. • a glucose tolerance test measures blood glucose for several hours after ingesting glucose. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Chemistry Link to Health: Hyperglycemia and Hypoglycemia Diabetes mellitus can cause hyperglycemia, which • occurs when the pancreas is unable to produce sufficient quantities of insulin. • allows glucose levels in the body fluids to rise as high as 350 mg/d. L of plasma. Symptoms of diabetes include • thirst and excessive urination. • increased appetite and weight loss. In older adults, diabetes is sometimes a consequence of excessive weight gain. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Chemistry Link to Health: Hyperglycemia and Hypoglycemia When a person is hypoglycemic, • the blood glucose level rises and then decreases rapidly to levels as low as 40 mg/d. L. • low blood sugar may occur as a result of an overproduction of insulin by the pancreas. • symptoms may appear, such as dizziness, general weakness, and muscle tremors. • a diet may be prescribed that consists of several small meals high in protein and low in carbohydrates. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Chemistry Link to Health: Glucose Testing • Normally, blood glucose flows through the kidneys and is reabsorbed into the bloodstream. • When the blood level exceeds about 160 mg of glucose/d. L of blood, the kidneys cannot reabsorb all of the glucose, and it spills over into the urine, a condition known as glucosuria. The color of the dye o-methylaniline on a test strip determines the glucose level in urine. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Chemistry Link to Health: Glucose Testing • A symptom of diabetes mellitus is a high level of glucose in the urine, which can be identified using Benedict’s test. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Study Check In the following Fischer projection, identify the monosaccharide as a D or an L isomer. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Solution In the following Fischer projection, identify the monosaccharide as a D or an L isomer. — OH is on the left; it is an L isomer. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.