In a typical chemistry or biochemistry laboratory setting, Lab 9: Peptides and Proteins generally focuses on the chemical properties, identification, and denaturation of these essential biomolecules.
Lab 9: Peptides and Proteins
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1. What is a peptide
bond?
A peptide bond forms between the carboxylic acid of one amino acid and
the amino group of the next amino acid. Loss of H2O.
2. How does the primary
structure of proteins dif-
fer from the secondary
structure?
Primary - The peptide bonds that join one amino acid to the next.
Secondary - Includes the alpha helix formed by coiling of the peptide
chain and a beta pleated sheet structure formed between protein strands.
Triple helix.
3. Define protein Composed of molecular building blocks called amino acids. If more than
50 amino acids are in the peptide chain that has biological activity, it is a
protein.
4. Identify major roles of
proteins in the body.
Proteins make up many important features in the body including skin,
muscle, cartilage, hair, fingernails, enzymes, and hormones.
5. Protein shape and
bonding at primary.
Linking two or more amino acids by peptide bonds. Di, tried, tetra, penta
and poly - peptides
6. Protein shape and
bonding at secondary.
Alpha helix coiling of the peptide and beta pleated sheet formed between
protein strands.
Hydrogen bonding of the C=O group and the NH group in the next turn.
Alpha helix - Think of a spiral staircase
Beta pleated sheet - Think of a pleated dress or folding a piece of paper
into a fan
7. Protein shape and
bonding at tertiary.
Interactions between the side groups such as ionic bonds or salt bridges,
disulfide bonds, and hydrophilic interactions give the protein a compact
shape. Evident in spherical shape of globular proteins
8. Two or more tertiary units. Two or more polypeptide chains or subunits
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Lab 9: Peptides and Proteins
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Protein shape and
bonding at quaternary.
9. Describe the types
of bonds broken and
structures disrupted
when a protein un-
dergoes denaturation
by: temperature, pH,
heavy metals, and or-
ganic molecules.
Temperature: Hydrogen bonds; hydrophobic interactions between non
polar R groups.
pH: Hydrogen bonds between polar R groups; salt bridges
Heavy metals: Disulfide bonds in proteins by forming ionic bonds
Organic molecules: Hydrophobic interactions
10. Identify and explain the
color changes associ-
ated with positive and
negative tests of Buiret
test
The test is positive for a peptide or protein with two or more peptide
bonds.
The blue color of a basic solution of Cu2+ turns to a violet color when a tri
peptide or larger peptide is present.
Individual amino acids, and dipeptides do not react with the reagent, and
the solution will remain blue (negative)
11. Identify and explain the
color changes associ-
ated with positive and
negative tests of Ninhy-
drin test
The test is used to detect amino acids and most proteins.
In the test, most amino acids produce a blue-violet color.
Proline and hydroxyproline give a yellow color.
12. Identify and explain the
color changes associ-
ated with positive and
negative tests of Xan-
thoproteic test
The test is specific for amino acids that contain an aromatic ring.
Concentrated nitric acid reacts with the side chains of tyrosine and tryp-
tophan to give nitro-substituted benzene rings that appear as yellow-col-
ored products.
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Lab 9: Peptides and Proteins
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13. Why are heat and al-
cohol used to disinfect
medical equipment?
They both kill bacteria by denaturation which disrupts the interactions that
stabilize the secondary, tertiary, and quaternary structure.
14. Why is milk given to
someone who acciden-
tally ingests a heavy
metal ion such as silver
or mercury?
The heavy metal ions will act on the proteins in the milk rather than the
body. The milk will act as a buffer.
15. How does a change in
pH affect the structural
levels of a protein?
The change in pH disrupts the bonds that hold the tertiary structure
together.
16. After working with
HNO3, a student no-
ticed that she had a yel-
low spot on her hand.
What might be the rea-
son?
The nitric acid from the xanthoproteic test got on her hand. When nitric
acid oxidizes it creates a yellow colored product.
17. Which samples gave a
negative biuret test?
Why?
Glycine and Tyrosine because biuret tests determine if a sample has
multiple peptide bonds.
If the color is purple then this means that the sample has multiple bonds
and is negative
18. What functional group
gives a positive test in
the xanthoproteic test?
Aromatic ring
19. What tests could you
use to determine
whether an unlabeled
Biuret test
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