1. What happens to glucose or galactose when the Cu2 in Benedict’s is reduced?
The glucose and galactose both monosaccharaides are oxidized. The cyclic structures of both sugars are opened resorting to an aldehyde group at the end of the aliphatic compound. The aldehyde group is the one that reduces benedict’s solution and oxidizes the sugars.
2. Would you expect fructose or glucose to form a red color rapidly with Seliwanoff’s reagent?
Fructose is a ketose therefore, a deep red color will form quickly with Seliwanoff reagent, while glucose an aldose with form a light pink color slowly in the same reagent.
3. Why don’t all the disaccharides undergo fermentation with yeast?
Yeast contains certain enzymes that ferment monosaccharaides and disaccharides forming ethanol and carbon dioxide. However, not all enzymes in yeast can ferment all sugars. In the case of disaccharides, there are no enzymes in yeast that can ferment Maltose.
4. How can the iodine test be used to distinguish between amylose and glycogen?
Amylose traps iodine molecules forming a blue black complex, while glycogen traps iodine molecules forming a reddish-purple color.
5. Results of Carbohydrate tests
Glucose is tested by benedicts solution where the solution is reduced forming a red to green precipitate (Cu+) from a blue solution (Cu2+)
Fructose can be identified by the Saliwanoff’s reagent which determines Ketoses. Therefore, in the presence of fructose, the reagent forms a deep red color urgently.
Sucrose is a disaccharide that cannot be reduced in benedict’s solution. Therefore, benedict’s solution would remain blue if reacted with sucrose.
Lactose gives negative results in fermentation test where in the presence of yeast there is no ethanol and bubbles of carbon dioxide formed.
Starch reacts with iodine to form complexes forming colors such as blue-black, reddish-purple, and red-brown colors
Questions and Problems
Q.1 From the results in part A, list the sugars that are reducing sugars and those that are not.
Reducing sugars:
Glucose, Galactose, Fructose, Maltose, and Lactose
Nonreducing sugars:
Sucrose, starch, glycogen, and cellulose
Q.2 What sugars above are ketoses?
Fructose
Q.3 What sugars above give a positive fermentation test?
Glucose, Fructose, Maltose, and Sucrose
Q.4 Which carbohydrates above give a blue-black color in the iodine test?
Amylose (a starch)
Q.5 What carbohydrate(s) would have the following test results?
a. Produces a reddish-orange solid with Benedict’s and a red color with Seliwanoff's reagent in 1 minute
Fructose
b. Gives a color change with Benedict's test, a light orange color with Seliwanoff's reagent after 5 minutes, and produces no bubbles during fermentation
Galactose and Lactose
c. Gives no color change with Benedict's or Seliwanoff's test, but turns a blue-black color with iodine reagent
Amylose (Starch)
Q.7 When starch undergoes hydrolysis, one of its products is glucose. What would you expect to happen if the iodine test was done before and after hydrolysis of starch?
Iodine test before hydrolysis produces dark colors or blue black color of the sample, but after hydrolysis the color of iodine is noticed which is yellow signifying no color change of iodine in sample.
PEPTIDES AND PROTEINS
1. What is a peptide bond?
A bond formed after the linkage of a carboxylic acid from an amino acid and an amino group from the next amino acid with the release of water.
2. How does the primary structure of proteins differ from the secondary structure?
Primary structure entails the link (peptide bond) between several amino acids, while a secondary structure is an α-helix formed after coiling of a peptide chain (primary structure).
A. Peptide Bonds
A.1 Structure of glycylserine
Structure of serylglycine
A.2 Hydrolysis of serylglycine
(NH3+)Serine-Glycine (OH) + H20 --> (NH3+)Serine(OH) + NH3+(Glycine)(OH)
Serylglycine is a dipeptide that breaks during hydrolysis to form serine and glycine as shown in the above equation. The products are amino acid ions instead of amino acids.
C. Denaturation of Proteins
Observations of Egg Albumin Explanation
Heat
White precipitate forms on heating
Heat increases the motion of the atoms and disrupts the hydrogen bonds and the hydrophobic (nonpolar) attractions.
Acid
White precipitate forms
Acids disrupt the ionic bonds between amino acids with acidic and basic side groups causing coagulation.
Base
White precipitate forms
Bases disrupt the ionic bonds between amino acids with acidic and basic side groups causing coagulation.
Alcohol
White precipitate forms
They are organic solvents which, destroys hydrogen bonds resulting in coagulation.
QUESTIONS AND PROBLEMS
Q.1 Why are heat and alcohol used to disinfect medical equipment?
Heat is used to destroy the shape of proteins. Germs have bodies that are protein in nature therefore when subjected to temperatures that are above 500C their shape is destroyed resulting to sterilized equipment. The same is true with alcohol which destroys the hydrogen bonds in proteins resulting to their denaturing. Therefore, the protein structures in germs would be destroyed resulting in them being killed.
Q.2 Why is milk given to someone who accidentally ingests a heavy metal ion such as silver or mercury?
Milk forms complexes with the heavy metals in the body which can then be easily excreted in the body. The complexes are also less harmful in the human body.
D. Isolation of Casein (Milk Protein)
Questions and Problems
1. Describe the appearance and odor of the precipitate.
The precipitate is sticky white curds with a light rancid smell
2. Find another household product and approximately compare the amount of the precipitate formed to the amount of household product (i.e. the precipitate may be the size of 2 Tic-Tacs, 4 grains of rice, etc.).
Soya beans are another source of casein although very minute quantities are obtained.
Q.4 How does a change in pH affect the structural levels of a protein?
Acids and bases which result in low and high pH respectively alter peptide bonds occurring between acidic and basic side groups. This damages permanently the protein structures.
E. Color Tests for Proteins: Given their structures, predict the color changes and whether the tests would be positive or negative for the samples below:
Sample Biuret Test
The test determines the number of peptide bonds present in a sample. If the bond is one or two then the test becomes negative with Cu2+ ions not changing the color (blue). Above three peptide bonds result in changing color of Cu2+ from blue to violet color—a positive test.
Ninhydrin Test and Xanthoproteic Test
Nunhydrin test determines amino acids and some proteins. Positive test for amino acids is a blue-violet color while proline and hydroxyproline gives a yellow color.
Xanthoproteic test is a specific test to determine amino acids having an aromatic ring. Concentrated Nitric acid reacts with aromatic rings in amino acids to give a nitro-substituted ring that are yellow in appearance—a positive test.
Glycine
A biuret test would be suitable for testing glycine producing a negative test where the biuret solution remaining blue. This is because glycine has one peptide bond.
Tyrosine
The amino acid has an aromatic ring in its structure hence the Xanthoproteic test would be concrete. A positive test would result in a yellow appearance of the sample.
Egg albumin (egg protein)
Biuret test would be suitable for testing egg protein since it contains many peptide bonds. A positive test would be found a violet color.
Casein (milk protein)
Biuret test would also be suitable for testing casein since it contains many peptide bonds. A positive test would be found a violet color.
Q.5 After working with HNO3, a student noticed that she had a yellow spot on her hand. What might be the reason?
HNO3 is very corrosive and damages the skin and eyes of living organisms. The nitrate bonds with the amino acids present in cells producing a yellow color.
Q.6 Which samples above give a negative biuret test? Why?
Samples that have two peptides bonded together or a single one give a negative test. Glycine is an example which has one unit of amino acid.
Q.7 What functional group gives a positive test in the xanthoproteic test?
Samples having an aromatic (Benzene) ring produce a positive test of brown color of the sample. For example Tyrosine has a benzene ring in its structure.
Q.8 What tests could you use to determine whether an unlabelled test tube contained an amino acid or a protein?
A biuret test is effective in differentiating an amino acid from a protein. An amino acid would yield a negative test of blue color while a protein having many peptide bonds would yield a positive test of a violet color.
